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The larval development of Mytilus edulis L.

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We, the Committee appointed to supervise
the work of Margaret E, Miller, a candidate
for the Master’s Degree of this University,
have done soj have given her the regular oral
and written examinations on her research and
other work, and on her general knowledge of
Zoology, and find her properly qualified for
admission to this degree.
We, therefore, recommend that the Master’s
Degree be conferred on her at the June Convoca­
tion, 1940.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
R eproduced with perm ission o f the copyright owner. F urther reproduction prohibited w itho ut perm ission.
THE LARVAL DEVELOPMENT
OF
MYTILUS EDULIS L.
by
Margaret E. Miller
Thesis presented in partial fulfillment
of the requirements for the degree of
Master of Arts
at
The University of Western Ontario
May 1940
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UMI Number: EC54079
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TABLE OF CONTENTS
Page
Introduction................
1
Chapter 1
Literature in R e v i e w .........
4
Chapter S
Distribution and Spawning ..................
11
Chapter 5
Fertilization, Cleavage, and Development
to Metamorphosis
......
45
Chapter 4
Settlement..........
48
Chapter 5
Growth Rates
Summary
..............
.........................
59
65
Bibliography.....................
Acknowledgements ........................
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66
69
INTRODUCTION
The present investigation of the development of Mvtllus
edulis (i,.) is concerned primarily with the larval growth rates
in relation to temperature.
In certain localities these pro­
lific molluses may seriously foul oyster spat collectors. Hence
a method of predicting the time of their settlement would be of
particular value to oyster farmers.
The immediate problem was
undertaken in an attempt to meet this economic need.
The data for this thesis was collected during the summers
of 1938-1939 in the vicinity of the Marine Biolft^ot/ Station at
Ellerslie, Prince Edward Island.
This station is situated on
the Bideford river, a small stream which enters Malpeque bay.
Due to exceptionally static hydrographie conditions in
Malpeque bay, temperature is believed to be the major controll­
ing factor in the rate of development of bivalve larvae.
The
bay is well isolated from the gulf by protecting sandbars and
relatively little fresh water flows from the small flat island
to the shallow inlets and estuaries.
Since the bay and estu-
arial waters are very similar, the ebb and flow of the tides
result simply in an interchange of waters of much the same
character.
At temperatures above approximately 9°C. hydro-
graphic records show (1929) that the pH varies between 7.1 and
8.3 and the salinity between 22 and 30 parts per mille.
Such
slight variations in salinity are of no significance in Mytilus
development, since this bivalve is very tolerant to wide ranges,
and may be found in salt, brackish or nearly fresh waters
(White 1937).
Water currents are slight and inconsequential.
1
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Strong winds blowing across the shallow bays prove of far more
significance in water disturbance.
Field (1928), Stafford (1910), Cole (1936) and others
state that detritus and the smaller plankton.
are utilized
by Itytilus and other Lamellibranchs as food.
Blegvad (1914)
regards detritus as the principal mollusean food and the value
of live phytoplanidbon
as very slight.
Since plankton
was
abundant in the summers of 1938 and 1939 at Malpeque Bay, food
need not be considered as a limiting factor in bivalve develop­
ment.
Temperature, the main factor to be dealt with here is
controlled chiefly by the time of day, and by the daily varia­
tion in solar radiation.
Malpeque Bay attains high water tem­
peratures and has a rapid seasonal rise to those temperatures.
For this reason, as it has been pointed out by several authors,
(Dawson 1875, Stafford 1910, Medcof 1939) the fauna at Malpeque
latitude 46.5 is more characteristic of Cape Cod, latitude 42.
The absence of large areas of mud flats or sand bars
prevent
any abrupt changes in water temperature which could be induced
by sunlight absorption.
An unusual tidal cycle is observed along the Prince Edward
Island coast, explained (Tide Tables for Charlottetown and
Rustico, P.E.I. and Pictou, N.S. 1939) by a building up of anamollstlc, synodic and declination factors and resulting in
what may be classed as ,!mixedw tides.
When the declination of
the moon is approaching high, the tide shows a pronounced range
only once in the day.
When the declination is at its greatest,
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there is only one low and one high water a day, with a range
south
in Malpeque hay of about 4 feet.
When the moon is^of the equa­
tor the usual semidiurnal tide is apparent with a range of
about 2 feet.
The Investigation, although fundamentally concerned with
growth rates, of necessity has included a study of the whole
larval or free swimming stage of the mussel up to metamorphosis
and attachment.
Due to the specific hydrographic conditions
under which the observed larvae completed their development,
variations from the descriptions of previous investigators,
(Field 1921, White 1937) may be attributed to regional differ­
ences.
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CHAPTER I
REVIEW OF THE LITERATURE
Mytilus edulis (L.) is one of five marine species of
the Family Mytilidae, found on our Eastern coast.
It ranges
from the Arctic Ocean to Cape Hatteras and is a gregarious
littoral species which is attached by byssus threads, to
rocky shores, boat hulls and wharfs.
The anatomy of the mussel was first described by de
Heede in 1683, and more completely by Poli in 1791.
Baster
(1762) gave the first accurate account of the generation
of Mytilus.
Recent and more extensive papers by Field (1921)
and White (1937) recount its life history, anatomy, physio­
logy, and histology.
The adult mussel at the completion of spawning and in
the periods between spawning bursts, builds a food reserve
the- f o r m
of
in^ glycogen, proteid, and fat.
(Daniel 1921-1922).
This
reserve food supply drops during the spawning activity.
Daniel observed that mussels even when deprived of food,
do not draw upon these reserves which are stored for the
gonads.
At 0°C., according to Gray (1923) the adult Mytilus
emerges from hibernation, so to speakj action of the cilia
is instigated, feeding and active metabolism begins.
A
proliferation and development of the gonads in preparation
for spawning follows immediately.
With the exception of
the gills, muscles and foot, practically the whole internal
structure is covered or occupied by ramifications of the
genital organs (Field 1921).
4
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The two sexes in Mytilus edulis are separate.
The only
macroscopic differences observed are the lumpy texture of the
ripe mantle in the male and the deep apricot coloration of
the female gonad.
The gonadal products are collected by five pairs of geni­
tal canals which lead from the anterior region of the mantle
and the surface of the liver, to converge to a common point
of union just below the pericardium.
These five canals spring
from the mid region of the mantle, from the posterior part
of the mantle, and from the dorsal region between the posterior
adductor musele and the dorsal body wall.
The main genital
duct thus formed on each side of the body, penetrates the inner
surface of the mantle, turns posteriorly to the ventral body
wall and continues parallel with the axis of the inner gill to
its external opening, the genital papilla, situated just in
front of the posterior adductor muscle.
(Field 1981).
The
main gonadal canals break up into many minor canals which in
turn terminate in a net work of small outgrowths, the follicles.
These follicles, and one side of many of the minor canals are
lined with germinal epithelium, and it is here that the ova and
spermatozoa develop.
It has been observed that many common bivalve molluscs
(OstrearPrytherch, 1928, T. Nelson, 1922; PectensGutsell, 1951,
Venus-Belding 1909) require a certain threshold of water tempera­
ture before the gonads are stimulated to discharge their pro­
ducts.
Thurlow Nelson (1928) has placed this critical spawn­
ing temperature for MLvtilus at 10 to 12°C.
On maturity of
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the gonads, and this threshold temperature having been reached,
the germ cells are expelled into the water.
Here the ova com­
plete the reduction division and fertilization takes place.
Only a portion of the reproductive cells present in the
follicles undergo growth to maturity at one time, since there
is insufficient space in the gonads for theircontemporariess
development.
Thus discharge of these parcels of mature germ
cells occur at stated intervals and result in what are known
as spawning ’’bursts11 (Phytherch 1928, Hopkins 1931) .
During
summers of 1938 and 1939 at Bideford, three such bursts
were recorded for Mytilus.
A lapse of nearly two weeks was
found to be necessary before another group of germ cells were
ready for spawning.
two days,
In Ostrea this period may be as short as
(ledcof 1939).
This intermittent spawning is re­
sponsible for the distinct age groups which are observed in
the larval tow samples, and which enables a 1,brood” history
to be followed until its settlement.
Field has calculated that in one spawning burst, which
may last for one-half hour, a mussel 3| inches long can set-free
25 million eggs.
Considerable local differences appear to exist in the
seasonal periods for spawning.
Thus at the Marine Biological
Laboratory, Woods Hole, Mass., spawning occurs from June to
September, at Plymouth, England from January to May, and at
Lanchester and North Wales in July, with a minor and earlier
spawning.
(White 1937).
In the next chapter of this thesis
another variation in the spawning activity will be considered,
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7
exemplified by the Bideford Mytilus.
It is generally ceded that any mussel from the beginning
of its second summer may be considered sexually mature and
capable of producing spawn.
(Field 1921).
Upon release of th4 ova into the water, the germinal
vesicle, if it has not already done so in the genital duct,
breaks down and the spindle for the first polar body appears.
Development is not extended beyond this point unless fertili­
zation occurs within the next 3 or 4 hours (Field, 1921).
(The account given here of the progressive development
of the Mytilus larva in relation to time changes is taken from
Field (1921) which was verified by White (1937) and in accord
with Wilson (1887) .
Variations in the anaieJ of development
of the Malpeque Mvtilus larva will be dealt with in a succeed­
ing chapter).
At 20°C., the ova immediately upon fertilization, extrudes
the first polar body, and about ten minutes
later the second.
A short period of quiescence follows, at the end of which the
zygote divides into a micromere and a macromere.
Cell multi­
plication now rapidly follows by the division of the micromeres
and the giving off of micromeres by the macromere.
The micro-
mere cap finally envelops the macromere, and forms the ecto­
dermal germ layer with an underlying segmentation cavity.
The
macromere at this point divides into two equal cells, the fore­
runners of the mesoderm.
At the end of about 4| to 5 hours
cilia have developed on the exposed surface of the ectoderm
and a flagellum appears at the anterior region of the organism.
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8
The larva now enters the rapidly swimming trochophore
stage which is characterized by the presence of a digestive
with
tract,A proctodaeum, stomodaeum, and the appearance of the
shell gland.
At about 69 hours the shell gland has secreted
thin limy cuticular valves which inclose the fleshy parts
of the embryo and are divided at a median dorsal hinge line
into right and left sides.
The animal is no longer cylindri­
cal but laterally compressed.
At this time there is formed
from an apical plate a contractile ciliated organ of locomo­
tion, the velum, which compensates for the loss of the ecto­
dermal cilia at the appearance of the encompassing larval shell
or prodissoconch, (named by Jackson in 1888 to be contrasted
with the calcareous adult dissoconch).
The direction of loco­
motion is vertical and not horizontal.
Tides and currents
are responsible for the migration of the larvae.
This velum
is retained throughout further larval development until it
is lost at metamorphosis and settlement.
The appearance of
the bivalve shells, the hinge line, and the velum marks the
larva
straight hinge stage. The/is now 68.5 microns in height and
5 days old.
Stafford (1910) describes it as colorless, trans­
parent and quite rounded except for the hinge line.
At about 260 microns in height or about 6 weeks, the larva
shows a well developed anterior and posterior adductor muscle,
liver, anterior and posterior retractor muscles of the velup,*
oesophagus, stomach, and the newly formed prismatic cell or
dissoconch.
Somewhat later there appears the foot, four
branchial filaments, an eye, a pedal ganglion, the byssus
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gland,a definite rectum and anus and a kidney.
otm bo
entering the straight-hinge stage.
It is now
At the same time the de­
veloping mussel has undergone a progressive shape change from
the symmetrically circular and straight-hinge outline to the
more triangular ovate form of the umbo
stage.
Growth of the
prodissoconch takes place more rapidly in the ventral and
posterior directions and
protuberances of the prodissoconch
appears above the hinge line as the umbones.
The transition to the adult or metamorphosis and attach­
ment is accompanied by various changes in the structure of the
organism.
lost obvious of these changes is the development of the
A.
a
calcareous dissoconch from the limy glandular secretion at
the centre of each mantle lobe.
Deposition of the calcareous
matter procefds until at metamorphosis it begins to extend
beyond the limits of the prodissoconch.
The velum, or loco­
motor organ of the larva degenerates and disappears.
At
this time development of the outer branchial gill commences.
At about 1.6 mm. there are 25 such filaments in the inner
gill and 15 in the outer.
Pelagic dissoconchs of Mytilus edulis have frequently been
observed and reported by Nelson (1928).
He explains their
ability to float to the presence of bubbles of oxygen which
collect in the posterior part of the pallial cavity of mussels
which have not been fortunate enough to find a place of attach­
ment.
Nelson also described dissoconchs adhering by their
feet in a negative geotrople manner to the surface-tension film.
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10
The succeeding chapters of this thesis are based upon
experimental data and deal in more detail with the life his­
tory of the Bideford Mytilus.
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CHAPTER 2
DISTRIBUTION AND SPAWNING
1.
Distribution
Examination of random samples of adult Mytilus from
thickly populated mussel beds of Bideford river and vicinity,
revealed a slight predominance of the three year old organisms.
This is in accord with the findings of Mossop (1921),
In
deeper water, however, the greater percentage were consider­
ably older, of approximately five years, while in the inter­
tidal zone the predominant age was two years.
The deeper
water mussels were from thriving beds covered by about 8 feet
of water at half tide.
Mussels settling in the intertidal
zone at Bideford are sometimes prevented from surviving until
they attain an average mussel age, possibly as suggested by
ical
Dr. Needier, director, Ellerslie Biolog:/ Station, due to
their exposure and death during a severe winter.
shallow waters the ice 4ns frequently forma
while the surface layer
melts,
In the
near the bottom,
in the sun.
Mussels are
frozen into the ice and separated from their beds by new layers
which form underneath.
Through the surface melting, masses of
them are gradually left exposed on the top ice layer.
2.
Spawning
Thurlow Nelson (1928) states that the critical tempera­
tures for the spawning of various Bamellibranchs fall into
groups differing by reason of fundamental processes which
control vital phenomena in the organisms considered.
11
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As it
IE
has been pointed out in Chapter 1, he placed this temperature
at 10 to 1S°C. for Mytilus edulis.
Experiments were carried out at Bideford in 1939 in an
attempt to arrive at the spawning threshold for the Mytilus
of this region.
All temperatures maintained in the experiments
were controlled by a constant temperature chamber, in the sub­
basement of the laboratory.
Experiment I.
Determination of Critical Spawning Tem­
perature under Conditions of Slowly Rising Temperature.
On June 16, 16 adult sexually mature mussels were taken
from water of a temperature of 18°C. and placed in separate
finger bowls containing sea water.
An attempt was made to
preserve the oxygen content of the water by the introduction
of Chaetomorpha (sp.$, a filamentous green alga.
The finger
bowls were placed in a water bath for temperature regulation
and gradually, over a period of 12 hours, lowered to 8°C.
The
mussels were checked for spawning during the previous 12 hours
and were found 100# negative.
The temperature was then raised
1°C. every 12 hours and examined for spawning following each
degree rise.
Those mussels which spawned only slightly were
detected by microscopic examination of the water, while those
recorded as Mspawned out” completely clouded the surrounding
water with genital products.
The results of Experiment I (a) were as follows;
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13
NUMBER OF MYTILUS- -16
Date
Time
Temp.oc.
Number
Snawned
Sex
Degree of
Spawn
June 17
8.30 A.M.
10-11°C.
0
-
-
June 17
8.30 P.M.
11-12°C.
0
-
-
June 18
8.30 A.M,
12-13°C.
1
1 f.
Spawned out
June 18
8.30 P.M.
13-14°C.
1
1 m.
Spawned out
June 19
8.30 A.M.
14-15°C.
0
-
-
June 19
8.30 P.M.
15-16°C.
0
-
-
June 20
8.30 A.M.
16-17°C.
2
2 f.
June 20
8.30 P.M.
17-18°C.
3
June 21
8.30 A.M.
18-19°C.
1
1 f.
20-21°C•
0
-
-
Sex
Degree of
Snawn
June 21-22
-
2m., If.
Slight
Slight
Slight
Experiment 1(b)
NUMBER OF UTILUS- -13
Date
Time
Temp.°C.
Number
Snawned
-
June 22
9 A.M.
10-11°C.
0
June 22
9 P.M.
11-12°C.
3
2f,, lm.
June 23
9 A.M.
12-13°C.
1
1 f.
June 23
9 P.M.
13-14°C.
0
-
-
June 24
9 A.M.
14-15°C.
0
-
-
June 24
9 P.M.
15-16°C.
2
2 m.
June 25
9 A.M.
16-17°C.
0
-
June 25
9 P.M.
17-18°C.
2
June 26
9 A.M.
18-19°C.
0
-
19°C.
3
June 26-29
If., lm.
lm., If.,
spawned out
1 f. slight
Spawned out
slight
-
slight
lm., 2f.
-
slight
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14
In Experiment I(a) only 8 mussels out ©f 16 spawned, and
in Experiment 1(b) 10 out of IS.
It is interesting to note
that under low temperature conditions, where spawning took
place, it was, with one exception, complete, but at the higher
temperatures the spawning was only slight.
This may be indi­
cative of a forced spawning under somewhat adverse conditions.
It is, however, the lower temperatures which are significant
in determining the spawning threshold.
Experiment 11(a).
Determination of Critical Spawning
Temperature under Conditions of Rapidly Rising Temperature.
It has been demonstrated that adult oysters, if ripe, are
be stimulated to spawn in the event of a sudden rise in tempera­
ture, even though that temperature does not quite reach 20°C.,
the normal spawning threshold for oysters.
Stafford 1910, Prytherch 1928).
(Nelson 1928,
In this experiment the possi­
bility of such a stimulation being positive for Mytilus was
investigated.
On June 22, 16 adult Mytilus. over a period of
two days, were cooled to 7.5°C.
and raided to 10°C.
for spawning during this time were negative.
Checks
The temperature
of the water bath was then raised from 10° to S0°C. over a
period of 8 hours or about 24 minutes to each degree rise in
temperature.
Only one mussel spawned— a male at 24.5°C.
At
about gO°C. adductor muscles were relaxed and did not resume
contraction when removed to normal conditions.
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15
Experiment 11(1))*Degree of
Spawn
Date
Temp.QC.
Number
Spawned
Sex
June 24
10-11°C.
4
4 m.
Spawned out
11-18.8°C.
2
2 m.
Spawned out
11.8-13°C.
1
1 f.
Spawned out
13-28°C.
0
-
-
On first trial Experiment II gave no significant results.
Upon second trial seven out of 14 spawned, but none at the
higher temperatures.
The length of time to which mussels are
subjected to higher temperatures may thus control the slight
or forced spawning evidenced in Experiment I.
Experiment III(a).
Determination of Critical Spawning
Temperature under Conditions of Slowly Falling Temperature.
Oysters have been observed to respond to the stimulus
of change of temperature only when that change is an increase
(Medcof 1939).
This experiment was carried out to study the
validity of the same phenomena in regard to Mvtilus.
On
June 24, 14 mussels were placed in finger bowls of sea water
at a temperature of 17°C. and allowed to remain for 24 hours.
The temperature was then lowered 1°C. every 12 hours to 10°C.
Only two mussels discharged germ cells.
At 13 to 12°C. a fe­
male spawned out, and at 12 to 11°C. one male spawned out.
On repeating the experiment only one female out of 16 mussels
spawned at 11°C.
Spawning under swiftly falling temperatures was checked
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16
by the examination of the cooled mussels in preparation for
Experiment I.
They were found in all cases to be negative
for spawning.
The apparent reluctance of the mussels to spawn under
conditions of a falling temperature might indicate that
Mytilus shows a tendency similar to that of Ostrea in response
to the stimulus of temperature change.
This is evidenced by
the limited number of spawnings in Experiment III.
The dura­
tion of exposure to the falling temperatures as it nears the
spawning threshold may be a controlling factor.
These experiments collectively imply that under labora­
tory conditions, male mussels spawn between 11 and 12°G. and
female mussels slightly higher or between IS and 13°C.
DISCUSSION
Any inferences which might be drawn from the results of
these experiments, beyond the establishment of a slightly
higher spawning threshold for Bideford Mvtilus than that
found by Nelson for New Jersey, may be considered doubtful.
The number of individuals used in each experiment were
limited by the number of finger bowls available.
The results
would also have been more authentic had the temperature in
the first two experiments been carried up to 25°C.> a point
frequently reached during the summer in the bay water.
Examination of actual spawning times observed in the
Mytilus population for 1939 will be considered shortly.
It,
however, showed no definite correlation with temperature, but
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17
seemed rather to depend upon the state of maturity of the
gonads.
The fact that 44 out of 73 Mytilus failed to spawn
while subjected to experimentation may be due to the immatu­
rity of the gonads.
In Qstrea a period of rest following
the apparent maturity of the gonads is recognized (Nelson
1928).
Field also describes a resting period in Mytilus of
about two weeks, in which the germ cells although apparently
mature, are actually inert.
From histological studies, such
an extended resting period is not indicated in the Bideford
Mytilus but however short, it could still be significant.
The specimens chosen to be used in these spawning experiments
were observed to be sexually mature, in fact, every specimen
microscopically examined from June 1 to September 8, possessed
active sperm and apparently mature ova.
Yet at the time dur­
ing which the experiments were conducted, the ova perhaps
slower to mature than the sperms could have been existing in
a resting phase.
However, this idea is weakened by the fact
for
that at about the same time and/several days previous to the
experiments, artificial fertilization was successfully carried
out.
It is to be noted that 16 mussels Which t!spawned out"
did so at the lower temperatures and the nslight spawnings”
of the remaining 13 occurred at the higher temperature levels.
This slight nforced” spawning could be attributed to a general
fouling of the water in which the mussels had existed for
several days. (^mperabureT^iF this be true, may not be con­
sidered the only variant capable of bringing about spawning.
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18
Field has quoted several cases in which mechanical mistreat­
ment will instigate spawning.
At Bideford 8 mussels were ex­
posed on June 26 to direct sunlight for 3 hours in the absence
of water, and were observed to spawn within 10 minutes follow­
ing restoration to normal conditions.
The sun, however, may
have merely hastened the maturation of the ova, for the whole
population was nearing the spawning which took place on June
29.
On the other hand, some doubt may be thrown on th4 res­
ponsibility of adverse conditions for spawning at the higher
temperatures, for although mussels unspawned at the comple­
tion of each experiment were allowed to remain under those
conditions for several days, no subsequent spawnings occurred,
slight or complete, forced or spontaneous.
It is considered,
moreover, that under adverse environmental conditions, the
viscera of mussels will suffer at the expense of the gonads
(Daniel 1921-1922).
Mussels,as it is shown in various ac­
counts in the literature, are a very hardy 'lamellibranch.
Dodgson (1928) found that the temperature range for ordinary
activity extended from 0° to 26°G.
They live normally over
a month in sterile water, can survive 24 days in the hot sun
and movement of the adductors is possible after 40 days of
anaerobic conditions.
Coulthard (1929) showed that maximum
growth is displayed in 50$ sunlight and that darkness is less
harmful than full sunlight, indicating that in this respect
the subbasement was valid as an experimental location.
The experimental conditions, although considered under
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19
the circumstances not to be toxic, were nevertheless sub­
normal.
Conditions more closely approaching those of their
natural environment would have to be introduced to prevent
the confusing of other variants with the one under considera­
tion, namely, that of temperature.
In nature slight spawnings were not very evident in the
tow samples from the bay.
Odd straight hinge larvae were
found at various times but they were never observed to develop
to the umbo stage.
This pertains to the period subsequent to
the settlement of the major broods.
Thus, the factors which
prevented further mass spawning may have influenced also the
slight spawnings.
The cooling of the mussels in the foregoing experiments
to 8°C. followed by an elevation to higher temperatures was
intended to simulate somewhat the spring temperature condi­
tions and to provide the stimulation for spawning when the
temperature reached the spawning threshold.
Spawning of
Mytilus at the higher temperatures is obviously linked up
with factors other than temperature, or the superficial
mature appearance of the gonads, otherwise the mussels taken
from the water at 18°C. and while they were subjected to a
falling temperature between 18 and 12°C., would have been
spawning.
For the practical purposes of this investigation, how­
ever, it has been enough to show that, pending the previous
ripening of the germ cells, spawning of Mytilus edulis at
BIdeford cannot take place until the temperature rises from
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20
11 to 13°C.
It may be noted here that exposure of adult Mytilus to
a temperature of S0°C. for one hour brought about permanent
injury to the organism.
This temperature was preceded by a
longer exposure at somewhat lower temperatures.
Coulthard
(1929) observed m&rtality after 2 days at 30°C., and Henderson
(1929) reported a ciliary lethal temperature of 40.8°G. with
a 10°C. temperature rise every 5 minutes.
Duration of ex­
posure to high temperatures, fouling of the water and previous
exposure to adverse conditions will obviously influence the
lethal temperature.
3.
Histological study of the development of Mytilus gonads
in 1939.
Correlated with the actual spawning dates and tem­
peratures.
Periods of adult Mytilus spawning.
The date on which a Mytilus mass spawning occurs is
modal
calculated by a deduction of 4 days from the first/appearance
of the larvae in the water samples. * The youngest straight
hinge stages taken by a
height.
18 tow net are about S 3 f* in
Their age at that stage, 4 days, was ascertained
from the results of artificial fertilization experiments
carried out in the laboratory.
This time for development to the straight hinge stage
is admittedly not applicable to all Mytilus broods, for lab­
oratory conditions obviously cannot approximate the natural
surroundings of the bay.
It is, however, the most reliable
method available, especially when it Is corroborated by
gonadal examination.
In Ostrea virgin!ca where temperature
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21
is a major stimulus to spawning (Prytherch, 1928) a spawning
period may be fairly definitely set, when a new larval brood
makes its appearance in the water, at the time of the last
temperature rise.
At Bideford the critical spawning tempera­
ture for Mytilus is usually exceeded before the mussels are
ready to spawn, and sudden temperature rises cannot always
be correlated with spawning bursts.
The guiding factor of
temperature is thus eliminated.
The four-day period of trochophore development may be
considered a maximum estimate.
There are reasons to believe
that adverse laboratory conditions retard their growth. This
will be discussed in the outline of the fertilization experi­
ments given in Chapter S.
To make more reliable calculations of spawning dates,
the period of trochophore development would have to be deter­
mined in relation to varying temperatures.
The age of the
larvae could also be checked in these young stages by the
examination of water samples taken with a smaller meshed net.
In 1938 and 1939, spawning of the Mvtilus population
occurred simultaneously at the laboratory landing stqge-*and at station 2001 (Figure 2).
The dates were estimated to be:
1938
Date
June 1 (approximately)
1939
Average Bottom Temperature
in oe.
15.6
June 1?
12.8
May 30
12.8
% t a g e w on this and subsequent pages refers to the region
of the river immediately surrounding the Biological Laboratory.
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22
Date
1959
Average Bottom Temperature
in oc.
June 19
16.0
June 29
17.6
In 1958 work was not started sufficiently early in the
season to obtain records for the first Mytilus brood.
All
spawnings are recorded at a time of temperature rise.
No
conclusions could be drawn, however, for, at this time of
year the temperature graph exhibits a steady rise (Graphs 1
and 2).
Previous to May 50, 1959, the temperature, on two
occasions, May 16 and May 28, had reached the mussel spawning
threshold.
The absence of gonadal activity at these times
must be attributed to immaturity of the germ cells.
Histological Study
The histology of the development of the Mytilus gonad
was studied from specimens collected at stated intervals
throughout the early spring and summer of 1959.
tional samples were taken in March 1940.
Two addi­
The specimens were
chosen from beds of varying depths and regions of the Bideford
district.
These included the following: stage, at about a
seven foot depth; stage, near shorf; Totten Bed; Outside
Totten Bed; Muddigger Point; and station 2001.
(Figure 2)
Comparison showed little variation in either locality or
depth of bed, other than that which could be accounted for by
individual differences in specimens from the same sample. De­
tailed examinations were, therefore, limited to stage collec­
tions from beds at about a 7 foot depth.
The mussels were preserved in formalin or F.A.A. ,-*■the
•^■F.A.A. s Formalin, acetic acid, alcohol.
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23
mantles removed, rolled and tied.
In a few instances parti­
cularly ripe full gonads prevented rolling of the mantles.
The tissues were then dehydrated, cleared in toluol and em­
bedded in paraffin.
in thickness.
parison.
Cross sections were cut 8 to 10 microns
Coronal sections were also prepared for com­
The sections were stained with Harris’ haematoxylin
and counterstained with triosin.
In Mvtilus the sexes are separate.
The g©rm cells are
developed by a proliferation of the germinal epithelium which
lines the follicles and one side of the minor genital canals
of the gonad.
They are first observed as small separate cells
contained within the bounds of the germinal epithelium, and
develop as protruberances upon it, the expansion being direc­
ted toward the lumen.
Growth is maintained at the expense of
the interstitial cells of the mantle which contribute nourish­
ment through the epithelium.
Early development proceeds with
the base of the germ cells attached to the parent epithelium.
Contact is later severed and ultimate maturity is attained
while the cell is suspended within the follicular lumen. Pro­
liferation of the germinal epithelium takes place continuously
throughout the early spring and summer but is most intensive
previous to the first spawning of the season.
The male germ cells pass their spermatogonia and sperma­
tocyte stageswithin the confines of the parent epithelium
and break their attachment during early spermatid development.
The axial filaments are observed shortly after this de­
tachment.
In the development of the mature spermatozoa, there
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84
is a gradual decrease in the cytoplasm, accompanied by a
shrinking and concentration of the nuclear material.
The
cells adhere in chains arranged in progressive stages of
development from late spermatocyte to the segregated sperma­
tozoa occupying the centre of the cavity.
The aerosome
points toward the follicular wall while the tail extends into
the lumen and precedes the sperm in its passage down the geni­
tal ducts.
Male cells in various stages of maturity are il­
lustrated by plates 4 and d.
Multiplication of the oogonia takes place within the
female germinal epithelium.
The rest of the development
within the gonad is largely concerned with the addition of
yolk material to the oocytes.
Polar bodies are not given off
until their expulsion into the water and impregnation of the
oocyte by the sperm.
The spindle for the first polar body
may be formed in the genital duct just prior to spawning*
The oogonia when first recognized may be distinguished
from the elongated nuclei typical of the epithelium itself,
by their circular shape and prominent chromatin threads. It
was not determined whether the epithelial nuclei gave rise
to these germ cells or whether they arose by division of
primordial germ cells retained by the epithelium.
Even
the specimens taken through the ice on March 28, 1940, showed
oogonia and oocytes in various stages of growth.
The oogonia
appeared to be grouped In a sac formed by the distended epi­
thelium.
lumen.
Between these groups oocytes projected into the
The cytoplasm in the oogonia is restricted to a narrow
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25
peripheral margin.
As the cells develop and take up yolk
material, the follicles enlarge to provide space for their
attachment.
The growth of the oocyte is marked by the increase in
cytoplasm, the addition of yolk material, the appearance of
the nucleolus, the increase in transparency of the germinal
vesicle, and the widening of the vitelline membrane.
This
development is illustrated in plates 3, and 5.
The interfollicular connective tissue of the mantle, pre­
vious to spawning, and at certain periods between spawning,
consists of reticulated cells with long anastomaslng pro­
cesses.
As the germ cells mature the connective tissue dwin­
dles in volume, until Just before spawning it consists of
little more than threads of cytoplasm connecting nuclei,
surrounded by barely visible cell bodies.
The expansion of
the follicles and canals, due to the growth of the inclosed
genital products, force these thin connective tissue reti­
culations into a limited space between the adjacent follicles.
Three types of blood cells were differentiated in the
mantle tissue of Mytilus edulis.
Kollman (1908) describes
them as an evolution or maturation from one fundamental form.
The youngest, the "hyaline globules" of de Bruyne (1895) are
characterized by a large deeply staining nucleus surrounded
by a small clear cytoplasm.
which may divide.
These are the only blood cells
Since there is no lymphoid tissue in
lamellibranehs (Kollman 1908) reproduction is brought about
by the mitosis of these fundamental cells.
The nuclei of
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26
the matured forms are inactive, although they may become frag­
mented, polymorphic or dual.
The hyaline cells may diverge in two directions.
They
may enlarge into acidophilic granular leukocytes, associated
typically with nutrition, or they may become phagocytic and
function in excretion.
The eosin&philes found in the blood
stream resemble the hyaline cells, in being actively motile
and are about 1 5 in diameter.
The granules are small,
regularly distributed, and sensitive to acid stains.
In the
mantle cavity they become specialized as carriers of reserve
food.
Here the cytoplasm is impregnated with an excess of
albumin which renders them non-mobile, and thus imprisons
them in that tissue.
They are now termed by Kollman ’’cellules
rondes” (round cells) and are about 15 to 20
in diameter.
They are believed to be associated with the nourishment of the
genital tissue and possibly with additional excretory facili­
ties.
The phagocytic cells have a clear transparent cytoplasm
which is infused and concealed by excretion granules of vari­
ous sizes.
These are most prevalent following winter hiberna­
tion and, in Bideford Mytilus also in the midsummer.
At these
times the inclusions give to the cell an orange brown color.
Kollman describes a second phagocytic cell, the baso­
philic wcellules spheruleuses”.
They are typically present
in Gastropoda and are infrequently found in Mytilus connective
tissue.
In Gastropoda they are concerned with the storage of
reserve food but in the mantle tissue of Mytilus they have
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27
experimentally picked up carmine granules which would also
associate them with excretion.
1 few basophilic granules
were noted among the brown inclusions of Bideford Mytilus
phagocytes, but they were not Identified as true basophiles.
Basophiles are more typical of Anodonta and fresh water forms
where environment is believed to stimulate their formation.
In the mantle of Mytilus the hyaline cells, and the motile
eosinophiles were observed in the blood vessels and scattered
throughout the tissue.
They contributed also to certain cell
aggregations, or blood clots, formed within the lumen and near
the genital canals and follicles.
The bulk of these aggrega­
tions were made up of round cells and phagocytes, or of
phagocytes alone.
The hyaline cells are somewhat elongated and vary in
size between 8 and IS /c.
The eosinophiles are about 15 ju, in
diameter while the round cells may develop to about 20 ju.
The phagocytes as a result of the ingestion of debris are
usually about 25
in diameter.
The progressive seasonal changes observed in the Mytilus
gonads and mantle tissue will be given now in sequence.
March 28. 1940.
Ho. 1.
These specimens were collected before the ice had gone
out in the spring and presumably before normal metabolic
activity had been resumed following winter retardation.
It
is not established that feeding is completely suspended in
Bideford mussels during the winter months.
Some debris was
found in the gut of March 28 specimens and Field states that
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28
Woods Hole mussels are forming germ cells from the beginning
of the winter.
If such is not the case In Bideford then pri­
mary cell proliferation must take place in the fall before
the termperature drops for in March 28 specimens the germinal
epithelium showed the results of intensive activity.
of the ova had already advanced to about 19.4
Some
in diameter
jll
with the germinal vesicle occupying about 11.6ju.
Between
these older cells were extensive groups of oogonia, four or
five within the width of the bulging epithelium.
Intermedi­
ary stages were beginning to protrude into the follicular
lumen.
The germinal vesicle in the younger stages is very
near the free tip of the cell and exhibits numerous chromatin
granules.
Later the vesicle of the oocyte becomes clear and
almost transparent.
The sizes quoted here for developing ova are based on the
obta jn e d
average measurements of their lengths and widths,^ in an attempt
to arrive at an approximate diameter of the cell.
because crowded
considerably.
and stretched
The ova
into the lumen are distorted
Even when free within the lumen they are sub­
jected to the same influences.
It is not until they are re­
leased into the water that they become symmetrically round.
At this time, at the height of their development and previous
to impregnation by the sperm they average 77
in diameter.
While still in the follicle, each ova under examination was
followed through the serial sections and measured to find
the maximum dimension.
The figures recorded here are the
averages of a number of the most mature for any given speci­
men.
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29
The mantle tissue of the March 28 specimens is made up
of closely reticulating cells, although the amount of cyto­
plasm is considerably reduced.
This could be attributed to
a comparative metabolic inactivity during the winter.
The hyaline blood cells are predominant throughout the
tissues while many patches of phagocytes may be observed
within the follicles and in the mantle connective tissue.
In the blood vessels the youngest stage of hyaline cell com­
poses the bulk of the contents.
but in smaller numbers. -
Eosinophiles are present
Generally speaking, the vessels
which lie near the surface of the mantle tissue have surpris­
ingly few blood cells within them.
There is the possibility
that this is an artifact resulting from slow fixation.
In the sections of male gonad the average ratio of spermais
tocyte to spermatid, to spermatozoa, are as 1:5:5.
The pro­
portions of the developmental stages in sperms were determined
by average counts taken from follicles and canals of different
sizes.
Even in varying positions of the same follicle the
ratio may differ to some extent.
March 28 are uncrowded and small.
The sperm follicles on
The connective tissue reti­
culations are somewhat more loose than in the female mantle
and many of the follicles contain masses of large hyaline and
phagocytic blood cells.
The extent and prevalence of these
blood cell aggregations is to be noted to exceed their aver­
age appearance.
This may be correlated with their excretory
activities at a time when an excess of tissue metabolic pro­
ducts have accumulated.
This is in accord with Kollman (1908).
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so
March £8* 1940.
Ho. 2.
These mussels were collected from under the ice on
February 20, placed in tanks and kept at 6 to 8°C. until
March 28 when they were fixed in F.A.A.
During those S7
days considerable development has been achieved in the
gonadal tissue.
The ova reached a size of about 40
the germinal vesicle about 20 ytc .
and
This is comparable to the
gonadal development of Hvtilus taken from the water in 19S8
on May 6.
the
Most of the male cells progressed beyond^ spermatogonium
stage to
spermatids and spermatozoa .
it
■are 1:4 of spermatid to mature sperms.
The proportion#
This is comparable
to the sperm development on lay 23, 1939.
A considerable increase in the cytoplasm of the interfollicular connective tissue of both sexes is evident.
Eosinophiles are crowded along the follicular walls
in close aggregations and scattered throughout the tissue.
They are undoubtedly functioning in the building up of the
tissues.
(Hollman 1908).
but not in excess, while
Hyaline cells are still abundant
phagocytes are notably reduced
in number.
May 6. 1959.
The histological differentiation in the female gonads
of May 6 corresponds with that Just described for March 28,
Number 2, 1940.
The oocytes, which have separated into the
lumen of th8 follicles are of the same diameter.
canals and ducts are collapsed and empty.
The major
The follicles
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31
are qbout 140ju* In diameter and usually contain about 17 oo­
cytes in one field of vision.
The oocytes themselves are
still rather square and flat with wide bases of attachment.
Numerous newly formed oogonia are retained within the germinal
epith41ium between the more mature cells, but none are ob­
served where the epithelium presents a line of attachment
for the already maturing stages.
In the male gonads of May 6, the genital ducts and canals
are collapsed.
The ratio of spermatogonia to spermatid to
sperms are as 8:5:1.
Those sperms which are apparently
mature, would have to be checked for activity from living
specimens collected for the same period.
The connective tissue of the mantle in both sexes has
not as yet become very firm.
The interstitial spaces are
broad and the greatest cytoplasmic thickening surrounds the
nucleus.
Following the onset of greater metabolic activity in
the spring, it appears that a certain length of time is re­
quired to rebuild a food reserve in the connective tissue
cells.
This is linked/tJkth the presence of numerous blood
cells functioning as food carriers.
The female germ cells
seem to require these blood cells more than do the male, for
eosinophiles are more abundant in the female mantle tissue.
This is probably associated with the deposition of yolk in
the ova.
Within the follicles and canals of May 6 mussels are
large accumulations of either hyaline phagocytes or of eosino-
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32
philes.
In later specimens these show signs of deterioration
which suggest their loss of motility and subsequent imprison­
ment within the follicles.
It may be that the removal of
these disintegrating blood cells both eosinophilic and phago­
cytic, form the plug at the opening of the genital duct to
the exterior.
This plug is discharged prior to the spawning
act.
May 13. 1959.
By May 13, the oocyte dimensions have increased only
slightly, the cell body to 43jx and the vesicle to 20 ja . in
diameter.
increased.
The numbers of oocytes at this stage have, however,
The germinal vesicle has become clearer because
of the finer distribution of the chromatin granules.
The
follicles have expanded considerably to accommodate the growth
of the germ cells.
The germ cells, except for those still within the germi­
nal epithelium have all matured to spermatids and spermatozoan in ratio of 1:2.
The connective tissue of the mantle is more completely
formed than in any of the sections so far examined.
The cell
cytoplasm practically fills the interstitial spaces and is
collected in greater density around the nuclei.
The hyaline blood cells are more particularly confined
to the blood vessels while the eosinophiles are predominant
around the follicles.
Phagocytes are still in evidence but
reduced in number and limited to the lumen of the canals and
follicles.
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35
May S3. 1959.
A few of the oocytes have severed their attachment with
the germinal epithelium, and are lying free in the lumen of
the follicles.
They have not as yet reached their maximum
growth, however, and measure 5
5
occupying 33 jl*. of that diameter.
the parent epithelium
with the germinal vesicle
The greater proportion of
and younger stages are beginning to
protrude through the germinal cells to take the place of those
which have broken free.
No ova have as yet left the folli­
cles or canals for the major ducts.
There are four spermatozoa in the follicle centre to
every spermatid near the periphery.
The genital ducts are
still collapsed and unoccupied.
The mantle tissue has been congested by the enlarging
follicles, the cells decreasing in volume as the demands of
the germ cells increase.
The reticulations are open and the
processes thin.
Blood cell masses were absent from these sections.
Eosinophiles were predominant, encircling the follicles and
among the connective tissue cells.
lav 27. 1939.
The oocytes in these specimens are observed to have
reached the maximum size while present in the gonads of the
adult.
Spawning previous to this time would have been impro­
bable.
The diameter of the whole cell is about 65
germinal vesicle about 35/u.
and the
When discharged into the sea
water a certain amount of imbibition must take place for
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34
there they average 77/* in diameter.
Oocytes on May 27 have
begun to crowd into the genital ducts in preparation for
spawning.
The germinal epithelium is again exhibiting signs
of activity; while the few oocytes which in previous sections
were showing above the epithelium have now elongated into
the canal as far as apace will permit.
The ratio of mature to immature male cells has increased
to 7:1.
The follicles are very full and distended.
Sperm
cells have been present in an apparently mature form in vari­
ous proportions since the first of spring.
This gives rise
to the question of why they have not spawned before.
That
the presence of the ova in the water is necessary to provide
the stimulus for spawning is discredited by the fact that
male adults may be induced to spawn even when segregated.
There must, therefore, be a process of final development,
which may be physiological and, therefore,i a
the histologieal methods used.
not
apparent from
This probably accounts for
the "resting period®of Prytherch 1928 and Field 1921.
Active newly spawned sperm are about 53/* in length
including the tail, 46/<• and the head, 7/4
This coincides
with the measurement of the sperm in its final stage of develop­
ment within the gonad.
The connective tissue of the mantle in May 27 is typical
of its appearance in the pre-spawning state.
Growth of the
canals, follicles and ducts have depleted its cytoplasmic
content and congested it into the limited space remaining in
the mantle.
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35
The migrating blood cells are somewhat more abundant
a condition
than on May 13,/possibly accentuated by their congestion
along with the mantle tissue.
May 29. 1939.
The situation in the follicles and canals has changed
little from that of May 27.
There is, however, a greater
crowding of the genital products into the main ducts.
The
point of greatest interest is that here on the day before a
spawning,spindles for the formation of the first polar bodies
of the oocytes have not started, even in the major duct.
Field (1921) states that spindle formation may occur in the
duct after the oocyte is expelled into the water or even after
impregnation by the sperm.
It is thus of interest to compare
these specimens of May 27 with those of July 21 where the
oocytes even in the follicles exhibit maturation figures.
The preparation of the germ cells by the gonads for the
first spawning has thus taken about one month from its on­
set in the spring.
Development during that time has been
carried out under an average bottom temperature of about
11.8°C.
June 2. 1939.
There is a surprisingly small indication of the recent
spawning in the follicles.
The main ducts, however, are
very nearly empty, still enlarged but wrinkled.
During the
elapse of the past four days further maturation of younger
cells could have replaced to some extent those which left
the follicles.
Still one would be inclined to consider the
first spawning of the season as rather light.
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56
Progress in the development of oogonia to oocytes is
noted.
The proportion of spermatids to spermatozoa in the
male gonads has increased to 1:5.
and
The mantle tissue shows distinct^characteristic post­
spawning activity in that the cells have enlarged with im­
proved nutrition.
The reticular processes are firm and con­
siderably broader than on May 87 and 29.
Eosinophiles and
hyaline blood cells are abundant through the mantle tissue
and form close aggregations near the follicles.
Phagocytes
are absent from the genital canals and follicles.
June 2 specimens thus exemplify the first process in the
preparation by the gonads for a secondspawning.
This
second
preparatory period lasted 12 days whenon June 11 the next
spawning occurred.
The average temperature during this period
was 1S.5°C.
Undoubtedly a portion of the oocytes were mature before
June 11.
There must be some spawning stimulus which when ab­
sent delays the ejection of the matured elements until the
general cell mass has reached maturity.
The duct plug is
mechanically cast out by sudden valve movements of the adult,
but during the spawning act the mussel usually remains motion­
less.
Ciliary action in the canals and the pressure of the
overcrowded germ cells carry the oocytes and sperm to the
exterior.
June 9. 1959.
On June 9 the oocytes had already crowded into the geni­
tal canals and ducts, leaving the follicles uncongested and
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37
free for the further development of the newly forming oogonia.
It may be deduced then that the germinal epithelium will
commence proliferation of oogonia as soon as there is a por­
tion of the epithelial cells unoccupied by developing oocytes.
The sperms on June 9 have also started their journey
down the gonadal ducts.
There were in the follicles about
15 sperms to every spermatid.
Where there are no developing
spermatids the sperms have pulled away from the periphery.
The mantle tissue is characteristically thin-walled
and restricted as to space.
June IS. 1959.
June 12 gives a very typical picture of a "just spawned"
gonad.
The follicles and canals are wrinkled and very nearly
empty, while the germinal epithelium presents a clear study
of the newly forming germ cells.
The few oocytes lying free
in the lumen are nearly spherical because uncompressed.
The connective tissue is still widely reticulated with
a limited amount of cytoplasm surrounding the nuclei.
The
blood cells are beginning to migrate to the interstitial
spaces but have not as yet aggregated into clumps.
The gonads from June IS to June 28-29 passed through
the same stages of development as thofte followed for the second
spawning.
The temperature during the 17-day preparatory period
o
averaged 17.2 C. It is interesting to note that the third
period of gonadal development took five days longer than the
period in which maturation was achieved for the second spawn­
ing, even though the temperature average was higher.
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This
38
might be suggestive of a maximum temperature for normal
metabolism in Mytilus as well as a minimum.
For the 1939 season the brood just spawned constitutes
the final one which develops through to metamorphosis and
settlement.
The gonads, however, continue through the summer their
periodic proliferation and maturation of germ cells.
By-
July 6 the follicles and canals are again filled with sperm
and oocytes, and the germinal epithelia bulges.with newly
formed germ cells, the nuclei showing coarse granules of
chromatin.
On July 12 the oocytes and sperms have moved
into the canals and ducts.
The gonads, connective tissue
and blood vessels represent the characteristic pre-spawning
condition.
Spawning had not yet taken place on July 19.
Hyaline
blood cells and phagocytes have collected in unusual numbers
but have not penetrated the crowded follicles.
Few oogonia
or spermatogonia -are observed to be forming in the germinal
epithelium.
On July 21 spawning is still not indicated by the gonads.
The germinal epithelium, however, is beginning to prepare
another group of germ cells, the fifth.
The mantle tissue
is thickened and becoming congested with blood cells, parti­
cularly the hyaline type.
The condition of the mature sperms
within the follicles manifests little change.
The oocytes, on the other hand, in both canals and folli­
cles are clearly showing the mitotic figures for the formation
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59
of the first polar body.
Various phases of the eecentric
maturation division are observed.
It is not normal for the
oocytes to proceed beyond the spindle formation, even in the
major duct of the gonad.
This might indicate that spawn­
ing has been suppressed.
Twenty-two days have elapsed since
the last spawning, an interval considerably longer than is
recorded for the other three preparatory periods in 1939.
Unfortunately, the next collection of mussels was not
made until six days later on July 27.
The immediate fate of
these germ cells could not be learned.
On July 27, the follicles, canals, and ducts of the
gonads qre collapsed and nearly devoid of oocytes or sperms,
but newly forming germ cells are observed attached to the
epithelium.
Almost without exception there is within each
follicle and duct a large aggregation of phagocytes.
Fre­
quently they surround what are considered to be disintegrat­
ing germ cells.
The situations displayed in the gonads at that time is
illustrated in Plate 7.
A sixth brood of mature germ cells
selves for spawning by August 15.
is ' preparing them­
They also disappeared
from the gonads on September 8 leaving no indication in the
water of further larval development.
Several things could have happened in the intervals be­
tween the manifestation of maturity in the gonads and the
disappearance of the ripe germ cells from the follicles and
ducts.
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40
(1)
The mussels could have spawned, the germ cells and ova
been fertilized.
If this is so, some factor prevented their development
even as far as the straight hinge stage for no larvae were
observed in the plankton
tows.
The greatest variant at this
time of the season was temperature.
Subsequent to the spawn­
ing of the third brood on June 28-89, the temperature rose
above 20°C. and remained above that point until August 30,
averaging 28.7°C.
It is suggested, therefore, that had
zygote development started, temperature was the lethal factor
preventing its continuance.
(2)
Spawning could have occurred but impregnation of the ova
failed to take place.
This might result from inactivity of either the sperm or
the ova.
The ova fails to continue development unless ferti­
lized within four hours after expulsion into the water.
(Field 1921).
The high water temperature may have rendered
the sperms inactive.
The two cell types, sensitive perhaps
in varying degrees, to environmental influences, may have
spawned at different times resulting in a retarded
nation by the sperm.
impreg­
As has already been indicated, there
seems to be a different minimum spawning temperature in the
two sexes and there might also be a separate maximum spawn­
ing temperature.
In the light of the overdevelopment of ova
while still in the follicles, had they spawned, could pos­
sibly have been past their period of germinal activity.
(3)
Spawning did not take place at all but the germ cells
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41
were reabsorbed by the germinal epithelium and their removal
completed by the phagocytes.
Reabsorption by the epithelium was best indicated '//?
specimens collected on September 5.
That the epithelium as­
sists in the disintegration process is suggested by the ag­
gregation of germ cells at the periphery of the folliclesj
the obvious signs of their deterioration; and the comparative
lack of phagocytes within the follicles or canals during this
stage, which could have effected it.
The oocytes first ap­
pear to lose the greater part of their yolk material leaving
a skeleton of cell reticulations and most of the germinal
vesicle including the chromatin material.
In the male the
tails of the sperms seem to be absorbed, leaving, in some
cases, the middle piece.
The head appears to enlarge or
swell and the chromatin to spread out in clearer granules.
The follicles are now ready for the entrance of the phago­
cytes which remove the chromatin material.
No such phago­
cytic aggregations were observed in the canals or follicles
after the three normal spawnings in the spring.
Moreover,
the extent of the follicular blood cell masses observed on
July 27 or September 8 far exceeded those reported in the
sections from early spring specimens.
Following the temperature drop on September 1, it was
expected that a spawning of the germ elements, matured short­
ly after April 15 would have taken place.
They had perhaps
already started their process of disintegration evident on
September 5.
On September 8, the last sample collected for
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48
the season, oogonia and spermatogonia were still forming in
the epithelial tissues.
Plankton
tows were taken by Mr.
C.J. Kerswill until October 1, but up to that time no spawn­
ing at the lower fall temperatures was observed.
The reluc­
tance of adult Mytilus to spawn during falling water tempera­
tures, reported previously in Chapter 8, may explain their
inactivity at this time.
It may thus be concluded that normal gonadal activity
of the Bideford Mytilus proceeds until the water temperature
rises above 82°C.
Proliferation of the germinal epithelium
continues throughout the whole season, but development of
gonadal products is halted before the straight hinge stage.
The disappearance of the matured germ cells is effected
either by lethal factors present in the sea water, or by
their disintegration within the gonads through the action
of the epithelium and phagocytes.
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CHAPTER 3
FERTILIZATION, CLEAVAGE AND
DEVELOPMENT TO METAMORPHOSIS
(l)
Experiments on the artificial fertilization of Mytilus
ova.
cutting
Preliminary results were obtained by simply/male and
female gonads In several directions with a sharp scalpel and
dropping them into a jar of filtered sea water.
Straight
hinge stages were observed at the end of two days.
When this
proved so easily successful the experiment was repeated under
more carefully considered experimental conditions.
An at­
tempt was made to follow the directions of Ert. J . Larsen
(1937) for the artificial fertilization of mature gonadal
products.
Larsen found that excess sperms or traces of the
gonadal tissue itself present in the culture water, were
toxic to growth.
Care was thus exercised in the removal of
such fragments, in the repeated washing of the germ cells
and in the quantitative mixing of genital products*
Feeding of the developing larva does not begin until
the digestive tract is fully formed and the velum is active.
This occurs in the Malpeque mussel larva in about l| days.
There is thus a considerable period in which food and oxygen
must be supplied.
Larsen suggests methods for the culture
of the diatoms Nitzschia. Dunaliella. and Chlorella.
At-
temps to develop such food cultures were not successful.
In the hope that enough nutriment would be present for larval
development to the stage found In the tow samples, large
43
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44
hatchery jars were used, and comparatively small quantities
of germ cells were introduced.
Maturation and fertilization was not completed within
the same time in all the cells.
There is the probability
that some ova not completely ripe were forced into the cul­
ture so that development was delayed.
The stages noted and
reported were those most advanced.
Following the addition of the sperms to the ova culture,
maturation, fertilization and first cleavage took place with­
in 35 minutes.
By ten hours the germ layers had formed and
the embryo was enveloped with cilia.
trochophore stage.
fourteen hours.
This marks the first
The digestive tract appeared at about
The first indication of the shell gland
was observed at twenty-four hours.
The development of the
velum, or the swimming and feeding organ, and the covering
of the fleshy parts of the organism by the two prodissoconch
valves required an additional fourteen hours.
Up to this
time the larva has not increased in size beyond that of the
fertilized ova, its development depending on the stored nut­
riment in the egg cytoplasm.
The larva can now be said to
exist in the very first straight hinge stage.
It was about
58/a in height of a light transparent grey shade, with a very
frail looking shell.
Field reported that under his observa­
tion the Mytilus larvae required five days to reach this
stage of development.
This is to be contrasted with the 38
hours of the Bideford mussel larvae.
Field did not retain
development in the laboratory beyond this stage.
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45
From the experiments carried out at Bideford the larva
was found to reach a height of 76/<• in slightly less than
three days.
On the fourth day it had grown to 85/*.
The
first stages found in the tow samples are about 69/* and
the first mode in a growth curve usually falls at 855/*. Thus
it may be assumed that a mass spawning of the adult Mytilus
has taken place slightly less than four days previous to the
calculation of the first mode in the plankton tows.
The -eaeee*
figure was set at four days, principally because any closer
assignment of developmental time would not be significant
in the light of the usual discrepancies in measurement and
sample taking.
The four-day developmental period, although it is at
least three days less than that required for the correspond­
ing stage reported by Field, is probably a maximum determina­
tion.
Adverse laboratory conditions indicate a ^stunting” of
growth which affects the relationship of size and age.
For
it was observed that the transparent grey shade of the first
straight hinge stage, changed at 90j m to the characteristic
yellow of larvae 104jx in height taken from the hay.
The experiments recounted here were carried out at a
temperature of about 20°C.
Changes which may exist in rate
of growth in relation to temperature were not determined.
In the laboratory it was impossible to maintain condi­
tions necessary for growth after the larvae had reached a
height of 110/*.
Subsequent observations on the development
of the organism during its larval life were made from tow
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46
samples of the hay water.
Plankton
t.ows were obtained by means of a number 18
bolting silk net, towed by a power boat at a slow and con­
stant speed for ten minutes.
The contents of the tow jar
were then fixed by the addition of enough formalin to make
a
solution with the contained sea water.
The organisms
which settled to the bottom of the jar were either examined
Immediately or pipetted into vials for future observation.
Surface samples were found to give a qualitative representa­
tion of the larval population.
No attempt was made to inves­
tigate quantitative variations of Mvtilus larvae in respect
to depth, or hydrographic conditions,
The general morphological changes observed in the mussel
larvae population of Bideford river paralleled in most res­
pects the changes described by Field (1921).
The Bideford
larvae, however, passed through those stages with a greater
rapidity and arrived at metamorphosis and settlement in a
much shorter time and at a somewhat smaller size.
The end of the straight hinge stage is marked by the
development of whumps® or umbos which project upward above
each side of the hinge line.
Field describes their onset as
being accompanied by the appearance of the foot and the
fourth branchial filament at qbout 260jut in height.
The
Bldeford mussel first exhibits the umbo at about 140 jul and
the foot and fourth gill filament at 170
At this time the yellow coloring of the larva is con­
siderably deepfened and much of its transparency is lost.
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47
co/or-
The liver assumes a. deep orpnge yellow^ which clearly differ­
entiates it from the rest of the viscera.
In the early umbo
stage a slight sharpness of the anterior body line is obthis
served and/is accentuated as the organism ages. The charac­
teristic anterior shoulder slope, and the rounded deeper
posterior body line finally develops.
At settlement the
co/or
mussel larva is a deep yellow orang^, the shell outline
refractive and the umbo flatly rounded.
The length and height measurements of the larvae increase
in very nearly constant proportions from the early straight
hinge stage to settlement, the length exceeding the height by
about 34/^.
This statement is based on approximately 500
length-height measurements of the Bideford mussel larvae and
is substantiated by figures quoted by Stafford (1910) for
larvae of the same region.
Field (1921/ also gives length-
height proportions and in one instance gave measurements
where the height remained stationary at 260 Jjl while the length
increased from 274ju- to 360^.
He has not stated, however,
whether the measurements supplied are an average determina­
tion or only refer to the individuals illustrated in his
plates.
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CHAPTER 4
SETTLEMENT
Settlement of bivalves involves the metamorphosis from
the actively swimming larval form to the attached adult. The
atrophy of the typical veliger structures, the advance in
the number and complexity of the visceral organs and the
development of the adult dissoconch has been reviewed from
the literature in Chapter 1.
Data on the settlement of Mytilus larvae was obtained
by observations made every other day, of experimental bi­
valve collectors.
These consisted of squares of cemented
cardboard, cut from commercial oyster collectors and attached
by clamps at intervals of 18 inches to a vertical rod.
It
was found from experiments carried out in 1938 on the settle­
ment behavior of oyster larvae, that attachment of bivalves
is facilitated by the shielding of the attachment surfaces
from currents and water disturbances.
The collector blocks
were thus cut and fastened by the clamps in such a way that
there were two vertical sides and a horizontal surface be­
tween them.
The top and under surfaces of this horizontal
plane afforded the area which was examined.
The collectors
were hung at the end of the stage in eight feet of water
(half tide) and at station 2001 In five feet of water.
A record of the Mytilus spat fall for 1939 is presented
in tables 1 and 2.
No record is given for 1938.
necessary, therefore to rely on plankton
for settlement information.
It is
tow examinations
Using this method solely, mussel
48
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49
sets in 1938 are believed to have occurred on June 30 and
July 4, at both stage and station 2001.
In 1939 peak sets
were noted at stage and 2001 on June 20, July 1, and July
14.
It is quite probable that hydrographic conditions are
capable of influencing the level and position at which at­
tachment of bivalve larvae takes place.
From observations
in the settlement behavior of oyster speit this is indicated.
The data supplied by these tables, however, when compared
with the weather conditions on the corresponding days, dis*close no correlation for Mytilus larvae.
No preference was
shown for either the top or bottom surface, and strong winds
causing water disturbances seemed not to have discouraged
larval attachment at the shallower depths.
Obviously, other
factors may be involved and more carefully controlled experi­
ments planned with this problem in view would be essential
before a definite statement would be warranted.
Settlement Size.
The study of the ultimate height which Mytilus prodissoconcho attain during their larval period, that is the height
which marks their metamorphosis to the adult, was approached
from three angles.
First, by observation of the size at
which the larval broods disappeared from the plankton
tows,
secondly, by the measurement of the newly settled larvae in
experimental collectors and thirdly, by the measurement of
the prodissoconch shells retained on the umbos of Mytilus
adults of about 2 to 12 mm.
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50
Method 1 .
The size-frequency distribution of the last stages
of Mytilus larvae as they appear in the tow samples is re­
presented in Graphs 11 to 14 .
All individuals do not go through metamorphosis at the
same size.
Individual divergence from the average is to be
expected in any organic population.
This particular problem
is now concerned with the separation of these normally vary­
ing Individuals from the somewhat complicated picture of a
size-frequency distribution curve as the brood nears settle­
ment.
The interpretation placed upon the final distribution
curves depends upon the extent and character of the brood.
Had the spawning limited the size distribution to about three
h e a t/e
days, then settlement would,, takew place over approximately
the same period of time.
If a plankton
tow is taken short­
ly before ultimate development is achieved and the next taken
after the larvae have precipitated from the water, the modal
height would be placed at a point perhaps a day’s growth be­
hind the actual settling size had it been possible to keep a
closer check on the brood.
Under such circumstances the
discrepancy would be revealed by an examination of the ex­
perimental collectors.
On the other hand, spawning and subsequent settlement
may extend over a period of a full week.
In this case, as
the older members disappear from the tows for attachment,
the lagging individuals of the distribution curve set up a
new mode prior to their own settlement.
As the brood settles
out, the quantity of larvae in successive samples will de-
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51
crease, but each distribution curve will exhibit a mode
which represents the ultimate prodissoconch height.
The mode of a size-frequency distribution, as the brood
nears settlement, would seem to be a more reliable determina­
tion than the average.
This is exemplified by Brood I in
1959 at the stage and station 2001.
The broods in both
localities run so closely parallel that they may be considered
together, providing thus daily observations over the settle­
ment period.
The fundamental fact to be noted is that the modal
height during six successive days fell at 277ju.
According
to the experimental collectors the greatest set occurred on
June 20.
Following this date increasing volumes of water
had to be examined in order to provide a clear distribution
curve.
This indicates that the mussel larval population
was decreasing.
As the brood approaches settlement, the
individuals which metamorphose at a larger size bring the aver­
age nearer the mode, until finally the bulls: of the larval
group have settled from the water leaving only the lagging
members of the tail.
For Instance, on June 19, the older
members which previously formed the advancing tail of the
distribution curve have largely settled from the water.
The
lagging members including the mode of the brood, are still
present in their original abundance, so that the average
determination falls somewhat behind at 2
6
7
On June 20,
the lagging individuals have caught up and equal these indi­
viduals of the advancing tail which settle at a larger size
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58
on the other side of the mode.
The mode and the average
fall thus on the same point, 877 u*
On the following days,
however, the lagging tail decreases by gradual settlement
while the advancing tail is maintained in the original fre­
quency by those members which did not settle at modal size.
On the days following peak settlement, therefore, the aver­
age determination becomes increasingly larger.
The mode
which indicates the majority, remains constant and provides
the most reliable indication of the ultimate prodissoconch
height.
By further scrutiny of the data, one would be led to
believe that those individuals which vary from the modal
settling height tend to go through metamorphosis at a greater
rather than at a lesser size.
This is not so apparent in
the last brood of the season, a fact which is brought out
particularly by the lower averages of height-frequency in
the distribution curves around July 14.
The average tempera­
ture during development of Brood S was much higher.
Further
indications of the influence of temperature on the larval
settlement size will be presented later.
It was not possible to follow Brood 2, 19S9, through to
settlement from the plankton
height had reached 273 yU.
tows,
(hi June 24, the modal
On June 30, when the larvae should
have been ready for settlement, high winds and water disturb­
ances must have driven the larvae down below the surface
area where the tow was'taken, for no brood appeared in the
plankton
samples.
This same phenomena was observed in con-
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52
nection with oysters in 1958 (station records).
The water
disturbance must have also prevented attachment for the peak
set did not occur until July 1.
It could perhaps be ex­
plained by the dislodgment of the settled larva by the storm
on July SO, so that the secondary set on July 1 recorded the
peak.
July 1 as a settlement date extends the length of the
free swimming stage beyond that which may be indicated by
the temperature under which the brood developed.
The next
tow taken on July 5, showed that the brood had completed
metamorphosis.
In 1938 the season’s first brood had finished settle­
ment just before the tows were started for the season.
A
few large larvae remaining from the lagging tail of a frequency-distribution curve were still in the water on June
16.
Brood 2 settled about June 20 for, on June 17, the modal
height fell at 260/ a but on June 21 it was 279jx while the
average had passed the mode to 284j a .
The third brood reached
settlement size on July 4 and the modal frequency again fell
at 279yU.
In 1938 tows were examined under a higher magni­
fication than in 1939* which would eliminate a certain dis­
crepancy in measurement.
It is possible that 279ju. is a
more accurate figure for the modal disappearance of the
larvae from the water than is 277
Method 2.
The second approach to the problem of settle­
ment size was the measurement of newly settled prodissoconchs
as they appeared in the experimental spat collectors.
Both
height and length measurements were made in order to compare
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54
with the data obtained in Method 3.
The possibility that the temperature, under which a
brood completes its development may influence the size at
which metamorphosis takes place was further investigated.
I received through the kindness of Dr. J.C. Medeof, a collec­
tion of Mytilus spat from the colder waters of South Gut,
Cape
Breton Island.
The data from both sources is presented
here in graph and tabular form.
That Mytilus broods are sensitive to varying environ­
mental factors is displayed by the irregular settlement
graphs for Bideford.
These irregularities may be caused by
factors varying from brood to brood or by the ”contamination”
of a single brood from the introduction of larvae previously
developing under different conditions.
The measurements
given for the Stage spat include the collections made during
the whole season, while those for South Gut represent the
simultaneous set of one brood.
This partially accounts for
the distinct single mode observed in the size~frequency set
data for South Gut.
It is further explained by the increased
mixing of the water and water temperatures through the dis­
trict.
The average determination for the settled prodissoconch
height at Bideford is 889j m .
If the irregularities are con­
sidered separately, two modes are found, one at 278 ju. and the
other at 313ju*
Referring again to the frequency distribu­
tion curves of the plankton tows (Graphs 13 and 14) there
was observed a difference in the averages of Broods 1 and 3
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55
of 1959, which was thought to he caused by a difference in
the temperature during the developmental period.
If the
settlement curves are considered as composite data for all
three sets, then the circumstances are somewhat parallel.
The first mode corresponds quite well, but the second seems
to fall at a height considerably greater than anything that
could be explained by the superior settling size of Broods
1 or S.
Barring the fact that a certain amount of growth
may occur between the time of the larval disappearance from
the water and its actual attachment, there is the possibility
that since spat collections were made every other day, larvae
settling on the first day could have developed an almost im­
perceptible ring of dissoconch which increased the height
measurements.
Where this ring was visible, it was ignored
in the determination of the prodissoconch height.
Neverthe­
less, from evidence given later, this growing ridge of dis­
soconch distorts and pulls the larval shell to an increased
height.
Brood 2, subjected to storm conditions as it neared
279 jjLy may have withheld metamorphosis for a short time and
settled at a height increased by a day’s growth.
These com­
bined factors may have influenced the unusually large second
mode of the settlement size-frequency curve.
Since there appears to be a variation in the settling
size, so well illustrated here, the figure quoted should be
the average, 289ju..
In 1959, however, the first mode is the
most prominent, indicating that the majority settle earlier
than the average.
During a colder season it is quite con-
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56
eeivable that the peak could fall at the second mode.
If
temperature affects the settling size then the height at
which metamorphosis takes place would have to be calculated
for each individual brood.
At South Gut, C.B.I., the modal and average height
correspond quite closely at 332/*. and 337//*. respectively.
This is about 48 ju above that determined for Bideford Mytilus
where the average temperature during larval growth is higher.
Method 3.
Measurements of the prodissoconch shells re­
tained on the umbo of adult Mytilus are correlated with the
data supplied in Methods 1 and 2.
The first group to be examined was the "ridge" size,
that is, the prodissoconchs which showed a small ridge of
dissoconch protruding below the larval shell.
These were
found as pelagic forms present in the plankton tows? on ex­
perimental collectors, and among the older spot clinging to
eel grass and stage piles.
This size group was still small
enough to be examined as a wet mount with transmitted light.
The older dissoconchs ranging from 2 to 12 mm. were
supported by plasticine and the larval shall resting on the
upturned umbo was measured.
Ridge Size:
At Bideford only a limited number of ridge
size dissoconchs were collected.
Breton Island
The data supplied for Cape
is more dependable since for distortions of
the larval shell from ridge size dissoconchs it represents
measurements from a larger precentage of the population.
The prodissoconch shell, subsequent to settlement and
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57
the onset of dissoconch growth cannot add to its mass.
The
shell gland has changed its function and cannot revert to
its larval activity.
From Graph 10, however, it may be ob­
served that in comparison with newly settled larvae, there is
a marked increase in the height-length measurements.
This is
believed to be due, not to an increase in the substance of
the prodissoconch, but to its distortion caused by the dissoconch growth as it begins to assume its adult shape.
The larval
umbo is flattened and the whole shell pulled downward with
the posterior development of the dissoconch beneath it.
g TO 1-g MM. DISSOCONCHS:
there-appeared
larval shell.
In the macroscopic Mytilus spat
a distortion of another type in the retained
The growth of the internal organs and the en­
larging of the dissoconch valves to accommodate it, spreads
the larval shell at its free edge until both values come to
lie in the same horizontal plane.
The development of the adult
umbo underneath "humps" up the centre of the frail prodisso­
conch and lessens the height-length distances.
This was shown
by the inability to keep in focus, at the same time, both the
centre of the bivalve and its edges.
Considerable inaccuracy
in measurement probably resulted in the tilting of spat in an
attempt to get the prodissoconch level for examination.
The adults were divided into arbitrary groups for the
comparison of the warping of the larval shell at various stages
of dissoconch growth.
Above 12 mm, the prodissoconch begins
to wear off and disappear.
At Cape Breton Island 2 to 12 mm.
dissoconchs were not procured from South Gut, but measurements
were made on spat collected at Stoney Point, a neighboring
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58
district.
From Graph 10, :it may be noted that following the sudden
increase in h»eight-length proportions at ridge size, the length
remains somewhat greater than the average at settlement, at
the expense of the height measurements which fall below.
The facts presented in Method 3 are of interest only in
following the fate of the prodissoconch.
The distortion of
the larval shell, following the appearance of the dissoconch
renders this method useless in the determination of settling
size of mussels.
It is concluded, therefore, that 8 8 9 as
determined by Method 2, may be considered the average settl­
ing height of Bideford Mytilus.
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CHAPTER 5
GROWTH
RATES
Indications of the influence of temperature on the rate
of development of a larval brood may be observed from the
study of Tables 9 to 14 and Graphs 11 to 14.
Table 9 is a summary of the brood histories for 1938
and 1939, the modal height for settlement size being used
to determine the last day of larval growth.
It may be noted
that as the average temperature during the free swimming
period increases, the rate of larval growth increases in
proportion.
Thus under an average daily temperature of
15.5°C., SI days are required for development from spawning
to metamorphosis, while under an average temperature of
S1.9°G. only 15 days are required.
This indicates that for
every 1°C. rise in average temperature the length of the
larval growth period is lessened by approximately 1 day.
Tables 10 to 13 give the height-frequency measurements
which were made of the larval population at stage and station
2001 in 1938 and 1939.
They are graphically represented in
Graphs 11 to 14.
Table 14 presents the growth increments from 1 to 5
days of larval broods at various temperatures.
It may be
observed that the growth per day increases with the tempera­
ture, and that the percentage growth per day increases in a
like manner, but that the greatest increase in proportion
to size is during the straight hinge stage.
59
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60
Graphs 11 to 14 represent the consecutive size-frequency
changes in the larval distribution at the stage and station
2001 in 1958 and 1939.
The frequency polygons are plotted
to correspond with the time intervals between tow samples
so that the progress and direction of the growth curve may
be followed.
The growth curves were constructed by joining
the points which marked the average height of each sizefrequency polygon.
The first broods of 1959 show in their
early straight hinge stages the retarding effect of low
temperatures on their rate of development.
In the other
curves the slope is fairly uniform until it nears settle­
ment, where the factors confusing an average determination
of a size-frequency polygon are beginning to operate. These
factors were discussed in Chapter 4, so that it may be under­
stood that the continuation of the growth curve beyond the
time of average settlement is not of concern in the study
of larval growth rates.
In the path of the growth curves
there are to be observed at various times sudden deviations
from the general slope of the curve.
These may be correlated
with temperature changes of which July 7 to 14, 1959 at the
Stage may be quoted as an example.
From the spawning of
Brood 3 until July 8 the temperature varied between 20 and
22°C.
The slope of the curve during this period is uniform.
On July 10 the temperature rose to 24°C. and remained there
for three days.
During this period the growth curve presents
a distinct deviation toward an increase in the rate of larval
growth.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
61
The problem of the prediction of Mytilus sets, or the
degree to which temperature effects the length of the free
swimming stage is confused by a rapid and steady temperature
rise during the larval period.
Dr. J.C. Medcof (1939) for
the prediction of oyster sets, compiled composite curves
from growth increment data at various constant temperatures.
The range of temperatures to whieh oyster larvae are sub­
jected is about 7-8°C., while that of mussels is about 15°C.
It is difficult therefore, from two seasons’ observations,
to obtain for any one degree of temperature growth increments
which would cover the whole larval period.
Under these
circumstances it was possible to construct only one composite
growth curve from the data available.
This curve is given
in Graph 15 and represents the progress of larval growth
under a constant temperature of 2S°C.
The only method that can be suggested, therefore, for
the prediction of Mytilus sets is the Use of this growth
as a standard reading from it the number of free swimming
days remaining for the larval brood under investigation and
adding one day to that period for every degree of tempera­
ture variation from 22°C.
If the average developmental tem­
perature is greater than 22°C., then the number of days for
each degree temperature change will be subtracted.
It is to be noted that the average settlement size is
used in this curve, and thus any prediction made will fall
approximately a day later than the time of metamorphosis
for the modal group.
Incomplete growth curves constructed
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
62
from the growth increments at other temperatures suggest
the possibility of an inaccuracy in the islope of the 22°C.
growth curve.
It has been given here only as an example
for a method of the prediction of Mytilus sets and must be
substantiated upon the collection of more complete data.
No dependable answer may be given, therefore, to the
thesis problem, other than to throw some light on the be­
havior of Mytilus larvae at Bideford.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
SUMMARY
(1)
Experiments on the determination of the spawning thres­
hold of adult Mytilus were carried out under laboratory con­
ditions at Bideford.
Male mussels were found to spawn from
11 to 12°C. and female mussels from 12 to 13°C.
A rising
temperature and the duration of exposure to each degree of
temperature change, tended to increase the percentage of
individuals which spawned*
(2)
A histological study of the Mytilus gonad was made
from specimens collected periodically during the spring and
summer of 1939 and the late winter of 1940.
The seasonal
changes in the germ cells, the interfollieular connective
tissue, and the blood cells, were followed.
Germinal pro­
liferation was observed to continue unbroken from March 28
to September 8, but only three successful and early spawn­
ings occurred during each season. (1938 and 1939).
It is
suggested that genital products formed subsequent to normal
spawnings may be discharged into the water but fail to
develop because of some lethal factor, such as unfavorable
temperature, or their removal is effected within the gonads
by their disintegration and reabsorption through the action
of the germinal epithelium and blood phagocytes.
(3)
The period of trochophore development from the time of
sperm impregnation to the first straight hinge stages was
determined from artificial fertilization experiments to be
4 days at a temperature of 20°C.
63
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
64
(4)
Plankton tows were taken at station 2001 and Stage on
alternate days during the summers of 1938 and 1939.
Size-
frequency distribution curves were constructed from the
height measurements made of the Mytilus larvae present in
these tows.
(5)
Data on the time of settlement of Mytilus larvae was
obtained by observations made on alternate days of experi­
mental bivalve collectors hung at station 2001 and Stage.
(6)
The modal height at which a larval brood disappears
from the plankton tows was found to fall at 277j i .
The
modal height at which Mytilus spat were observed to settle
on experimental collectors was found to fall at 278/i.
The
average height of the newly settled prodissoconchs was found
to be 289 /t.
(7)
The changes in the larval shell brought about by the
developing dissoconch
w
& bs followed in Bideford Mytilus and
also in Cape Breton Island Mytilus. the latter having develop­
ed under a lower average temperature.
(8)
The histories of the larval broods for 1938 and 1939
were tabulated and correlated with temperature.
Spawning
dates were determined by the deduction of 4 days from the
appearanee of the brood*s first mode in the plankton tows;
settlement dates by observation of experimental collectors were
substantiated by the disappearance of prodissoconchs from
the tows.
The length of the free swimming period of Bideford
Mytilus varied bdtween 15 and 21 days and the average maxi­
mum temperature between 15.5° and 21.9°C.
Approximately
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
65
1°C. rise in temperature thus lessens the larval growth
period by 1 day, within
(9)
M u
Growth curves for individual larval broods of 1938
and 1939 were constructed from size-frequency distribution
data.
(10)
Larval growth increments were tabulated in respect
to constant temperatures.
The rapid and steady rise in
temperature during the free swimming period of Mytilus at
Bideford prevented the establishment of any accurate method
of set prediction.
A composite larval growth curve, age
plotted against height, was constructed for the constant
temperature of 22°C.
It is suggested that this curve be
used as a standard from which deductions from the larval
growth period may be made for the varying temperatures.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
BIBLIOGRAPHY
Baster, J.
1762.
"Opuscula subseciva."
Harlemi.
Battle, Helen. 1932. "Rhythmic sexual maturity and spawn­
ing in certain bivalve mollusks." Jour, of the Fish.
Research Board of Can., Vol. 4 (4).
Belding, D,l. 1909. "A report upon the mollusk fisheries
of Massachusetts." Special Report of the Mass. Commis­
sion on Fish, and Game, 243, p. 50.
Bigelow, H.B. 1926. "Plankton of the Gulf of Maine.”
Fisheries, 40, p. 2.
Bur.
Blegvad, H. 1914. "Food and conditions of nourishment
among the communities of invertebrate animals found
on or in the sea bottom in Danish waters." Report,
Danish Biol. St., Vol. XXII, p. 41.
Coe, W.R. 1914. "Sexual rhythm in the California oyster.”
Science, 74, p. 247.
Coe, W.R. 1938. "Development of the gonads and gametes
in the soft shelled clam.” Jour, morph., 62 (l), p. 91
Cole, H.A. 1938. "The fate of the larval organs in the
metamorphosis of Ostrea edulis.” Jour. Marine Biol.
Ass. U.K., 22 (2).
Cole, H.A. and E.W.K. Jones. 1939. "Some observations
and experiments on the settling behaviour of the larvae
of Ostrea edulis." Jour, du Council Intern, pour
1*Exploration de la Mer., Vol. 14 (l), p. 86.
Coulthard, H.S. 1929. "Growth of the sea mussel."
to Can. Biol. N.S. 4, p. 121.
Contrlb
Daniel, R.J. 1921, 1922. "Seasonal changes in the chemical
composition of the mussel." Rep. Lancs. Sea-fish.
Labs., 1921, p. 205; 1922, p. 27.
Daniel, R.J. 1925. "The effects of starvation on the
common mussel." Proc. Trans. Liverpool Biol. Soc.,
40, p. 52.
Dawson, 1875.
From White 1937.
Dodgson, R.W. 1928. "Report on mussel purification".
Ministr. Agr. and Fish. Invest., Ser. II, Vol. X.
66
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67
Field, Irving A.
1921.
nBiology and economic value of the
sea mussel, Mytilus edulis."
58, p. 127.
Bull. U.S. Bur. Fish.,
Galtsoff, Paul S. 1938. "Spawning reactions of female
and male Ostrea virginica." Biol. Bull., 74 (5),
p. 461.
Ganong, W.F.
1889.
"The economic mollusca of Acadia.”
Bull. Nat. Hist. Soc. of N.B., No. VIII.
Goutsell, J.S. 1931. Natural history of the bay scallop."
Bull. U.S. Bur. Fish., XLVI, p. 569.
Gray, J. 1925. "The mechanism of ciliary movement”.
Roy. Soc. (London), 95, p. 6.
Proe.
Henderson, J.T. 1929. "Lethal temperatures of Lamellibranchs.” Contrib. Can. Biol. N.S., 4, p. 399.
Huntsman, A.G. 1921. "The effect of light on growth In
the mussel." Trans. Roy. Soc. London, 15, p. 23.
Hopkins, A.E.
1931.
Bull. U.S. Bur. Fish., 47, p. 57.
Johnstone, James. 1908. "Conditions of life in the sea."
Cambridge Biol. Series XII.
Kollman, M. 1908. "Recherches sur les leucocytes et le
tissu Lymphoide des Invertebres." Ann. Sci. Nat.
Zool., Ser. 9, Vol. VII, p. 1.
Larsen;, Ert. J.
1937.
Inv. Cult, methods p. 551.
Mathews, Annie. 1913. "Notes on the development of Mytilus
edulis and Alcyonium digitatum in the Plymouth Laboratoiy." Jour. Marine Biol. Ass. of U.K., Vol. 9, (4).
Medcof, J.C. 1939. "Larval life of the oyster." Jour.
Fish. Research Bd. of Can. Vol. 4 (4), p. 287.
Mossop, B.K.E. 1921. "A study of the sea mussel."
trib. to Can. Biol., 15, p. 48.
Nelson, 1921, T.C.
Con­
Bull. N.J. Agrie. Exp. St., 351, p. 1.
Nelson, T.C. 1928. "Pelagic dissoconchs of Mytilus edulis,
with observations on the behavior of the larvae of
allied genera." Bio. Lab., 55 (3), p. 180.
Nelson, T.C. 1928. "On the distribution of critical tem­
peratures for spawning and ciliary activity in bivalve
molluscs." Science, 67, p. 220.
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Nelson, T.C. 1938. "Feeding mechanisms of the oyster."
Jour. Morph., 63 (l), p. 1.
Orton, J.H. 1926-27. "On the rate of growth of Cardinm
edule. Growth rings of Mytilus." Jour. Marine Biol.
Ass. N.S., 14, p. 239.
Poli, J.K, 1791. "Testacea utriusque Siciliae eorumque
Historia et Anatome." Parma.
Prytherch, H.F. 1928. "Oyster farming."
Fish., 44, p. 429.
Bull. U.S. Bur.
Rice, E.L. 1908. "Gill development in Mytilus."
Bull. Woods Hole, Vol. XIV, No. 2, p. 61.
Biol.
Richards, Oscar. 1928. "Growth of the mussel Mytilus edu­
lis." Nautilus, 41, p. 99.
Stafford, J. 1912. "On the recognition of bivalve larvae
in plankton collections." Contrib. to Can. Biol.,
1906-10, p. 221.
Stohler, R. 1930. "Sex cycle in Mytilus californianus."
Zool. Anz, 90, p. 263.
Warren, A.E. 1936. An ecological study of the sea mussel.
Jour, Biol. Bd. of Can., II, p. 89.
White, K.
1937.
"Mytilus."
L.M.B.C. Memoirs, March, 1937
Wilson, 1886. "On the development of the common mussel."
5th Rep. Fish. Bd. Scot. App. F, No. VI, p. 247.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
ACKNOWLEDGMENTS
The author wishes to express her indebtedness to
•everal persons who, through their hearty cooperation,
have made this work possible.
Special thanks are due
to Professor A.D. Robertson under whose supervision
this thesis was written, for his wise council and help­
ful criticism; and to Dr. A.W.H. Needier, Director of
the Marine Biological Station, Prince Edward Island,
who suggested the problem and superintended the collec­
tion of data.
Appreciation is likewise extended to
Dr. J.C. Medcof, whose experience in a similar line
of work has been at the writer's disposal; to Dr. Helen
Battle for her kind interest and ready suggestions; and
to C. Robert Turnbull for the photographing of plates.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Reproduced with permission of the copyright owner. Further reproduction
TABLE 1
MYTILUS SET DISTRIBUTION ON EXPERIMENTAL COLLECTORS AT STAGE 1939
June
prohibited without perm ission.
Bottom
15
16
17
18
19
0
0
0
0
0
—
0
0
0
0
0
0
0
0
6
0
1
2
0
1
0
1
5
2
4
0
0
0
0
0
0
5
4
2
1
2
0
0
0
0
0
1
3
1
0
2
14
0
0
1
0
1
17
9
8
3
0
0
0
0
0
0
0
0
0
0
0
*
0
0
1
0
0
0
1
0
0
3
0
2
0
0
5
0
0
0
0
0
5
0
0
1
1
0
0
6
0
0
0
1
0
75
1
2
1
2
0
0
23
0
0
0
2
0
4
3
1
0
0
0
3
8
1
0
0
0
0
2
12
0
1
5
0
3
43
1
0
0
3
1
86
16
6
2
0 '
5
57
1
0
1
3
2
103
25
14
6
B
B
B
T
B
T
36"
T
30
1
0
0
0
2
2
0
0
0
0
0
2
0
0
0
0
0
3
0
0
0
0
0
4
0
54
6
0
0
3
72
0
0
2
8
0
Total
Too Surface
0)
Td
•H
•IH*1 CO
13
14
5
6
0)
•O
18
•H
-P
(out)
tn
0
«h <X>
Top
H.C
X! C
18
Surface .p‘H (covered)
«S C
36
20
18
(out)
0
7
8
9
10
11
12
3
4
DATE
X
■i
p
ft
<D
a
July
21
29
m
^ jS
Ctjo
18
Surface ^.h (covered)
-p
36
a
•H
X
4I
*
54
ft
<D
total Botto: a
Surface
Total Top
and Bottom
Modal
Depth
72
13
T
54"
72"
54"
72"
.
36"
54"
.
B
36"
£
72"
18"
Reproduced with permission of the copyright owner. Further reproduction
TABLE 2
MYTILUS SET DISTRIBUTION ON EXPERIMENTAL COLLECTORS AT STATION 2001, 1939
DATE
+> ©
18®
(out)
0
cdTt
•H
XS -P 18
Surface -P
© H 56
G cd
XI
54
Total Top
Surface
Top
prohibited without perm ission.
Bottom
Surface
18®
■p © (out)
Cfl*d 0
Xi 'P
■P
18
© r—1
a at
xi 36
54
Total
Bottom Surf.
Total Top
and Bottom
Modal
Depth
June 21
20 22
23
24 26
25 27
27
28
29
50
July
1
2
3
4
5
6
7 9,10 12
8 11 15
14
15
16
17
18
19
6
1
0
0
0
0
0
1
0
0
0
0
-
0
0
-
0
5
0
1
0
0
0
0
2
0
0
0
0
0
0
4
0
0
.0
2
1
1
1
1
0
6
0
0
0
0
1
3
3
3
0
1
2
1
3
0
0
0
2
1
0
0
-
0
0
0
1
2
0
1
0
0
0
0
1
0
0
0
0
0
—
1
1
2
1
1
10
0
0
10
4
8
4
0
0
0
0
0
0
1
0
0
0
0
1
1
0
0
0
0
0
0
4
0
0
0
1
1
0
0
1
0
1
4
0
0
0
1
2
1
0
0
0
2
1
2
0
0
1
1
0
0
0
1
0
0
7
5
2
0
1
1
4
6
17
11
5
4
2
0
5
-
T
36
-
1
B
36
T TB
18® 36®
(out)
0
2
2
0
4
1
2
0
1
0
2
0
1
0
-
0
2
0
2
2
0
0
0
-
0
1
1
0
6
0
Q
0
3
2
9
4
5
16
7
0
0
0
4
5
17
10
8
T
18
B
0
-
-
-
B
0
T
18
TB
0
T
18
0
TABLE 3
PRODISSOCONCH MEASUREMENTS OF NEWLY
SETTLED MYTILUS AT BIDEFORD
Height in microns
05
«J*
CO
Qi
C$
264
CO
h
02
05
02
^
O
CQ
co
H
CO
in
10
co
CQ
1
1
276
1
3
Length
in microns
291
304
2
6
4
1
7
3
2
1
4
5
21
1
2
1 13
2
1
6
1
1
5
3
4
1
2
2
1
2
1
318
535
346
360
1
7
2
2
4
6
3
2
1
1 2
1
1
1 1 3
1
374
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
TABLE 4
PRODISSOCONCH MEASUREMENTS FROM
RIDGE SIZE ADULTS AT BIDEFORD
Height in microns
CD
W
CO
02
<D
fcQi
<T>
02
O
to
00
I—I
to
to
to
K>
304
Length
in microns
318
335
346
360
374
387
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
to
TABLE 5
PRODISSOCONCH MEASUREMENTS FROM
2-11 mm. ADULTS AT BIDEFOBD
Height in microns
w
?P
2
3
H
02
O)
c*W
xsH
oo
Oi
co
on
W
10
o
to
296
305
Length
in microns
313
322
330
339
34?
1
1
1
4
2
1
3
3
4
3
1
2
3
5
1
2
3
4
2
3
3
1
3
4
3
5
2
1
5
2
2
2
1
1
4
3
4
10
1
2
1
2
1
1
3
2
6
1
2
1
354
1
1
1
1
1
1
364
1
1
1
1
4
1
1
1
2
2
1
1
373
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
212
^
in
CVi
TABLE 6
PRODISSOCONCH MEASUREMENTS OF NEWLY
NEWLY SETTLED 1YTILUS AT SOUTH GUT, C.B.I
Height in microns
CO
05
CM
^
O
tO
H
H
t
C
O
O
H
t
l
O
O
W
O
C
0}
tO
tO
t O W t O
O
t
^
t
O
O
O
lO
t O
C
'
t
f
C O < D l >
t O W t Q l
518
3S5
559
Length
in microns
546
553
11
560
IS
10
367
10
33
374
19
11
388
10
394
401
408
415
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
H
CO
Q
982
H
05
03
TABLE 7
PRODISSOCONCH MEASUREMENTS PROM
RIDGE SIZE ADULTS AT SOUTH GUT, C.B.I.
Height in microns
Of
H
Length
in microns
to
CO : to ■
H
to
to
H
349
1
357
2
to
: H
to
365
2
378
3
380
1
388
05
r~
tfi
to
to
to
LO
O
W
t-
t o
O
00
t o
CO
00
t o
to
05
t o
3
<-t
05
£-
- s
i 1
^
H
H
3
3
4
404
2
3
2
1
1
7
5
1
419
2
7
5
3
427
3
3
7
5
435
5
5
442
2
450
^
3
396
411
O
^
3
1
6
4
2
2
458
3
466
1
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
W i
TABLE 8
PRODISSOCONCH MEASUREMENTS FROM
-
2-11 mm. ADULTS AT STONE5f POINT,
C.B.I.
1
288
280
2
to
to
in
to
CO
4
5
4
1
4
1
2
1
3
2
1
3
3
4
582
3
1
4
590
2
2
4
4
2
1
1
3
4
3
373
in microns
to
t
02
to
i—
399
3
1
1
6
407
416
1
2
424
1
1
2
432
1
449
02
oo
to
t
1
1
1
2
1
441
to
to
E-
1
365
Length
r-(
992
in
o
to
622
356
en
02
5
839
348
E'­
122
293
271
Height in microns
1
1
1
1
1
1
458
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
1
1
Reproduced with permission of the copyright owner. Further reproduction
TABLE 9
MXTILUS BROOD HISTORIES IFOR 1938 and 1939
Location Brood
Spawning date
Spawning
temperature
(arerage
bottom °C.
Set
Length
of free
Swimming
period
(davs)
Average surface
and bottom temperatures during
free swimming
Deriod
Average
Maximtm
dally (oc) daily (°
prohibited without perm ission.
•Stage
2
June 1, (approx.) 1938
15.6
June 20, 1938
19
19.4
Stage
3
June 17, 1938
22.2
July 4, 1938
17
21.0
2001
2
June 1, 1938
15.6
June 20, 1938
19
19.4
2001
3
June 17, 1938
22.2
July 4, 1938
17
21.0
Stage
1
lay 30, 1939
12.8
June 20, 1939
21
14.8
15.5
Stage
2
June 11, 1939
July 1, 1939
20
17.3
17.8
Stage
3
June 29, 1939
16.0
(approx.)
18.2
July 14, 1939
15
21.3
21.9
2001
1
lay 30, 1939
June 20, 1939
21
14.8
15.5
2001
2
June 11, 1939
July 1, 1939
20
17.3
17.8
2001
3
June 29, 1939
July 14, 1939
15
21.3
21.9
12.8
16.0
(approx.)
18.2
Reproduced with permission of the copyright owner. Further reproduction
TABLE 10
SIZE-FEEQUEICY DISTRIBUTION AT STAGE, 19S9
Height in microns
Date
« C r > ( D t O O t ~ ^ l
«> is 00 0> 0> O
H
June 2
5
7
9
12
15
17
20
22
24
27
prohibited without perm ission.
29
J u ly S
5
7
5
8 24 14
8 42
1
14
17
19
10 H CO 10 03
<0
10
( J f i O ' I I O D f f l l > H H H H H H H H
O t S t j t H C O l O H O O
O O C O O T O O H W W
H H H W N N W W
ID M
10
02 N
®
®
^ W
CD 03
O
«0 I S
03 W
H
co cn
03 02
CO rff
o> O
02 10
H
H
CO 10
0
02 tf
to Hto 1
0 tc CO to
1
1
1
105.0
115.3
154.2
206
2
5
236
4
25 13 17 10 14
3 5 1
9
4
25 23 24 15
4
3
1
1
1
1
2
1
1
8 5 2 3
1
1
4 14 22 45 34 46 21 10 5 9 2 4 1 1 2 2 2 1 1 1 1
1 2 1 2 1 2 3 1 2
5 2 1 3 5 9 11 20 32 32 27 19 19 11 5 2 3 1
1 1
1 4 3 3 5 7 12 8 33 18 27 22 23 22 4 3 1
1
1 2 1 5 2 4 4 6 13 11 9 15 18 25 18 20 22 29 26 17 5 3
1 2
2 1 4 5 4 3 2 2 2 4
1 6 8 7 10 20 17 18 30 16 25 18 12 10
2
2
3 1 2 8 5 6 7 3 6 18 22 24 16 20 .4
1
1 2
12 57 65 38 13
14 26 51 34 27
£—
8 8 .8
24 23 19 12 19
28 16 26 12 16
1
£L_
77.4
1
5 1 2
1
2 1 4
48 46 46 9 2
5 10 25 16 22 3
1 2 1
3 4 2 4 13 20 45 28 30 10 6
2
1 1
1 3 1 7 3 4 5 14 10 28 22 25 22 18 9 15 7 7 2 1 1
20 24 19 6
1 1 2 1 3 4 2 4 4 11 12 14 15 17 19 15 19 10 3
7 29 84 50 8 2 1
2
2 9 21 33 42 28 12 10 2 1
8 6 5 7 4 10 17 15 18 21
4 3 5 5 8 10 18 15
1 1 7 9 35 23 49 37 49 4 4 5 3 1 7
1 2 2 3 4 17 22 31 30 47 29 20 5 9 7 4 2 5 4 12 14
1
1
1
1
2 2 5 7 4 12 17 25 31 42 25 27 23 18 3
1 2
2 3 7 19 16 28 44
1
2
1 2
is
is
02
29
10
12
H 00
H H
H H
Brood 1 Brood 2 3rood 3
average average iverage
01 tO
in
in
in
to ^
2
1
1
1
4
1
2
277.7
281.7
5. ]
285.5
83.5
97.9
130.5
153
191
234
273
89
112.5
134
1
3
170
2
223
256
1
7
3
3
1
1
271.3
283.5
Reproduced with permission of the copyright owner. Further reproduction
TABLE U
SIZE-FREQUENCY DISTRIBUTION AT STATION 2001, 1939
Brood
D ate
June
o> <0
<0
2
6
8
10
19
21
23
26
prohibited without perm ission.
28
30
J u ly
3
4
6
8
11
13
15
18
a o
H
1
2 12 31 27 5
4 14 66 57 23
1 1 8 25 43
2 2 5 1
13
16
co o
05
co
1
HI co co H co in cm o> to » o
•cF H CO to H CO to CM 0) (0
t>
CO
H
H H CM to co ^ to to co t- co co 01 O O
CM CM CO
to CO g t> co Oi 05
H H H H H H H H H r*i H H H CM CM CM CM CM CM CM CM CM CM CM CM CM CM CM
2
2
rH CM
05 CO
CO
CO
6
3
CO
78
35 15 1
2 1 26 34 43 37 4
4 1 5 5 10 12 19 21 43 17 22
1
1
1 3
6 3 1
8 15 24 20 11
1 5 18 52 64 43 14
2
average h eig h t in
l
1
96.2
3
1
1
108.5
1
l
1
10 10
1
6
2
6 18 15 56 37 43 6 5 4
1 1 5 8 14 20 40 36 37 21 14
1 3 1 4 2 7 9 19
2
1 2 5 4 3
1
co 10
to to co
H
Hi
1
2
51
10
t
CO
1
1
3
9 12 14 21 13 13 13 6 19 7 10 1 5
4 5 2 8 4 12 6 21 15 24 20 27 18 13 5 6
5 2 8 3 8 7 9 11 14 12 31 28 18 11 15
4 2 11 6 13 14 36 24 32 14 16
4 5 1 2
1
1
1
12 35 21 23 21 14 5 6
133.6
162.4
2
2
2
5
4 10 13 18 17 42 21 24 15 17
2 1 1 1 6 3 4 2 4 A
3
3
7
1
1
230.3
267
280
7
2
283.2
*
5
6
«
1
2
4 19 68 59 29 2
2
1
1 1 2
1
2 16 41 66 45 16 3 2
1 1
2 1 6 8 23 59 56 21 17 6 3
2
1
2 9 23 16 41 50 44 35 16 5 9 1 2 1
1
1
1 1
3 1 11 5 18 21 36 25 39 35 41 16 23 12 4 3 3 1
3
1
1
1 2 1 2 7 1 3 7 8 8 20 5 25 16 26 30 35 27 18 10 22 4 6 1
1 2 2 1 6 9 18 11 15 14 18 21 21 16 5 6
1
1
4 9 17 26 19 15 12 18
1
1
1
1
92.3
117.2
140.8
172.9
212.2
250.8
86.5
97.5
122.5
146.3
199.6
4
4
240.4
266.2
1 ;
2
1
1
283.4
Reproduced with permission of the copyright owner. Further reproduction
TABLE 12
SIZE-FREQUENCY DISTRIBUTION AT STAGE, 1938
H eight in-m icrons
lO 60
CO 03
Date
o
o
H
co <d
o H
rH H
cm
H
CM
60
H
o
t-
ta
10
H
H
H
to H
to
H
June 16
2
June 17
1
June 21
June 23
3 13
3
June 27
2
prohibited without perm ission.
June 29
June 30
J u ly
2
J u ly
4
J u ly
8
3
.O t- lO
r-t H CM
CM CM CM CM
W
O
3
5
l
2
12
4
6
1 2 .
2
2
o
H
60
CO
H
CO
<0
CM
60
60
60
height
m p.
9
3
1
2
4
8
5
3
1
238
284
2
1
220
1
1
6 13 23 60 57 27 18
8
4
3
2
1
1
2
9
5
3
2
2
5
2
1
1
4
5 17 13 11
6
2
3
1
8 13 14 19 23 25 13 19 21 12 13 7
2
1
4
6 10
93.2
118.0
155.8
182.0
3
3
Brood 2
average
m p
7 38 51 42 37 23 13
5
1
e
heig
1
3
2
60
2
2
2
o
5
1
3
60
7
3
5
LO
03
CM
9
1 3 1
6
03 IS
00
CM N
8
6 11 15 19 21 27 32 41 34 24 19 11 4
5
CM CM CM CM
5
1
2
CM
CM
UJ (O t- N
7
1
9 10 11 20 30 41 24 15 12 4
2
oo co
H
^
5
1
8
60
60
CM
4 14 24 22 38 52 34 23 11
2
3 13 21 32 19 17
June 25
h- .10
t- CO 03
H H H
Broo
aver
213.2
3
9 20 29 24 35 34 20 14 14 16 8
2
1
221.4
2
7
8
9 10 18 16 14 30 40 24 13 19 9
4
1
260.5
2
5
3
9 11 15 24 23 31 18 27 34 34 17 5
3
272.8
3
3
6
9
6
6
16 8
12 6
6 11
8
2
Reproduced with permission of the copyright owner. Further reproduction
TABLE 13
SIZE-FREQUENCY DISTRIBUTION AT STATION 2001, 1938
Height in microns
Date
io n
o o o ' « ) ^ w O N i Q i < 5 H c « i O ' ! H w o i > - i o n H
CO cn OH OH HH Hwn'tf'tflOiOfr-C'-COCJiOHrlWW'ji
H H H H H H H H H W C 3 M N W 0 3
June 21
June 23
June 28
June 30
12 21
5
9
4
2
6 18 33 27 14
1
1
4
4
3
1
2
1
1
1
2
7
1
3
4
5
3
«
5
< o < # w c r > £ - i n n o a ) ! 0
^
I
O < O E - f - C D a i O HW HO«Bn
W N W « « M ( l l O .
7 16 22 44 45 41 18 14
93.0
118.3
8 16 18 19 10
6
6
6
3
2
2
1
204.0
7 16 16 17 29 20 21 18 20
14
9
3
3
1
3
218.0
7 2
7 13 12 11 18
24 20 31 23 34 27 13
J u ly
5
1 4
1 6
8 12 12 14 17
32 30 29 25 40 21 14 11
268.0
8 3
7 6
2
84.0
8
1 1
4
in/a .
2
4
5
2 1
ffii
3
J u ly
7
9
Brood 3
average
height
4
6 14 12 18 19 21 37 34 46 25 13
1
co
ood 2
srage
Lght
1
267.2
prohibited without perm ission.
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DISTRICT
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
FIGURE
2.
»
BIDEFORD
RIVER
D I S T R I C T .
BIOLOGICAL
S TA T I ON
I
STAT I ON
2
STATION
2001
4
STATION
2002
5
MUD DI GGER
P. E l-
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LANDING
STAGE
POINT
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t* wirtrti i
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HEIGHT
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4
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GRAPH
HEIGHT
5.
IN MICRONS
30
O AVERAG E
HE I G H T
2 9 0 *4
FREQUENCY
20
/\
L EN GT H
IN
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AVERAGE
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RIDGE
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PRO-
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BIDEFORD
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6.
G RAPH
HEI GHT
IN MI CRONS
30
O AVERAGE
HEIGHT =
O AVERAGE
LENGTH
279H
20
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=
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SIZE-FREQUENCY
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DI STRI BUTI ON
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BIDEFORD-
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GRAPH
HEIGHT
7*
IN M I C R O N S
<\l
oo
co
5 O
c
40
average
3371^
height
FREQUENCY
30
20
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50
O AVERAGE
40
L E N G T H = 37 2
FREQUENCY
30
20
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SIZE-FREQUENCY
DISTRIBUTION
NEWLY
PROD I S S O C Q N C H S.
SET TL ED
SOUTH
OF
G U T ; C. B
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GRAPH
HEI GHT
IN
8.
MICRONS
30
AVERAGE
HEI GHT = 3 f l i q
20
>
u
z
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FREQUENCY
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GRAPH
HEIGHT
0
IN M I C R O N S
to
to
O
AVERAGE
HEIGHT
=
3 2 3 JL^
20
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FROM
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OF
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POINT ; C.B.I.
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PRO —
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H E I GH T
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LENGTH
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tA B V A L
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GRAPH 12
L A RV AL
IC tS A
I S MlCHC'Nt-
ALSO
F >CP Rt
SC NTS
:s
Hf i CMT
MCASLtftCMCNTS
i nDIV i OU A L
SIZE-FREQUENCY
DISTRIBUTION
OF
MYTILUS
L A R VAE.
S T A T I O N
2 0 0 1
1938
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
C H A P H
SI
7 t
F R F O U f c N C Y
13
D I ST M IB U T I O N
L A « V A E
F
Y T I L U S
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(, H A I ’H
SI 7 E
I H M l U t N C y
Dl
I
i4
* »T H I I UJ
T I O N
M Y T I LU
A H V A L
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1 9 ^ 9
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GRAPH
IS
Average settling
si z e .
Z70
Height
“in >i
/9o
I BO
170
Approximate
gr o
160
//o
loo —
&0
/6
/&
So
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
PLATE I
First stages in the transition of hyaline blood
cells to eoslnophiles and phagocytes.
Camera lucida
drawing made with a Bausch and Lomb compound micro­
scope.
Number 10 ocular, and oil immersion.
Taken
from the mantle connective tissue of a female mussel
on May 6, 1939.
and 6ji wide.
The young hyaline cell is 15ju. long
The eosinophiles are 1 6 ^ in diameter
and the phagocytes 20jul .
These blood cells are aggregated
in spaces between the connective tissue cells arranged
to resemble capillary vessels.
Similar aggregations
were found within the lumen of follicles.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
PLATE
I.
Y O UNG
HYALIN (
C ELL .
PHAGOCYTE
MANTLE
COSINOPHILC
TRANSITIONAL
HYALINE
C ELL .
MANTLE
CONNECT IVE
T ISSUC
TRANSITION
OF
CYTES
HYALINE
AND
BLOOD
CELLS
TO
PHAGO­
E O S I N O P H I LE S .
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
PLATE 2
Phagocytic aggregations in the mantle connective
tissue.
Camera lucida drawing made with Bausch and
Lomb compound microscope, number 10 ocular and oil
immersion.
Taken from the mantle tissue of a male
mussel on May 6, 19559.
The eosinophiles are 15 ju
in diameter, the hyaline cell 17ju. long and 9Ji wide.
The phagocytes vary between 18 and 25yu in diameter.
Phagocytes are distended with ingested debris, of a
yellow color with brown inclusions.
They represent
the final stage of phagocyte formation.
Their out­
lines are indistinct and somewhat indistinguishable
from the debris present in the interstitial space.
Similar aggregations were observed within the lumen
of follicles and canals.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
PLATE 2.
MANTLE CONNECTIVE
EOSINOPHILE
PHAGOCYTIC
HYALINE
CELL
HYALINE
AGGREGATION
OF
PHAGOCYTES.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
BLOOD
CELL
PLATE 3
First stages in the development of oocytes.
Samera Tueida drawing made with Bausch and Lorab
compound microscope, number 10 ocular and oil
immersion.
Taken from a female mussel on May 6,
1939.
Largest oocyte is 5 5 ^ in height and 27ju
wide.
Germinal vesicle is about 1 8 in diameter
and the nucleolus 11/f.
Oogonia are about 7
in diameter.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
PLATE
3.
GERMIN A L E P I TH EL ­
IUM .
OOCYE
ATTA CHED
TO
EPITHELIUM
i-vs'-sy,
NUCLEOLUS
GERMINAL
VESICLE
OOGONIUM
STAGES
OF
tion
DEVELOPING
OOCYTES.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
IN d e p o s i ­
YOLK.
PLATE 4
Stages in the deyelopment of sperms.
Camera
lueida drawing made with Bauseh and Lomb compound
microscope number 15 ocular and oil immersion.
Taken from a male mussel on May 6, 1939.
Youngest
spermatids are about 14 ju in diameter and decrease
to 7 jjl> in the mature sperm.
Axial filaments are
about 1 8 long and the tails of mature sperm about
45 jUs.
Note the hyaline phagocytes present in the
canal follicle.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
PLATE 4.
MANTLE
CONNECTIVE
SPERMATOGONIUM
GERMINAL
EPITHELIUM
S P E R M AT I0 5
AXIAL F I L A M E N T
MATURE
SPERM
i
HYALINE
PHAGOCYTE
m
CANAL
DEVELOPING
EPITHELIUM
SPERM.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
TISSUE
PLATE 5
Final stages in the development of oocytes.
Camera lueida drawing made with Leitz compound
microscope, number 10 ocular and 16 mm. objective.
Taken from female mussel on July 21, 1959. Oocytes
are about 6 5 in diameter and the germinal vesicles
about 5
2
Three stages are represented here; a
typical mature oocyte similar to those found pre­
vious to the normal spring spawnings; oocytes
with broken down germinal vesicles showing mitotic
figures typical of the final ripening stage in
specimens collected subsequent to the normal spring
spawnings; and third, what has been considered the
first process of ova disintegration, or reabsorption
of the yolk contents by the epithelium.
Note the
chromatin granules scattered through the germinal
vesicle of type three.
The mantle connective tissue
is somewhat thin and invaded by blood cells.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
PLATE
5.
FIRST STAGE IN OOCYTE
DIS I N T E G R A T I O N ?
OOCONIA
YOUNG
OOCYTES
EPITHELIAL
6ROKEN
NUCLEUS
DO W N
GERMINAL
VE S IC L E
Ml T O TIC
GERMINAL
FIGURE
EPITHELIUM
M A NTLE
T I SSUE
LAST
STAGES
IN
DEVELOPMENT
OF
OOCYTES.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
PLATE 6
Follicles containing mature sperm and
typical of the pre-spawning state.
Camera
lucida drawing made with Leitz compound micro­
scope, number 10 ocular and 16 mm. objective.
Taken from male mussel on July 21, 1939. Mature
sperms 52 p, over all.
Note the congestion of
blood cells near the follicles*
Preparatory
to the breaking down of the germ cells.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
PLATE
6.
SPCRMATID
G ERMINAL
EPITHELIUM
DEVELOPING
MATURE
SPERMATOCYTE
SPERMS
MANTLE
EOSINOPHILE
MOTILE
EOSINOPHILE
HYALINE
MANTLE
BLOOD
CELL
C O N N ECTIVE
TISSUE.
MATURE
SPERM
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
PLATE 7
FIGURE 1
Action of phagocytes on oocytes.
Camera lueida
drawing made with Leitz compound microscope number 10
ocular and 16 mm. objective.
Taken from a follicle of
a female specimen collected Fuly 87, 1939.
Numerous
blood cells of all types present, particularly hyaline
phagocytes, congregated around ova in what appear to
be stages of disintegration.
Cell bodies have parti­
ally disappeared leaving the germinal vesicle to the
action of the phagocytes.
Germinal vesicles are about
35 y- in diameter, the hyaline phagocytes 20 ju, and the
eosinophiles 25^.
Note the oogonia forming in the
germinal epithelium.
FIGURE 2
Action of phagocytes on sperms.
Camera lucida
drawing made with Leitz compound microscope, number 10
ocular and oil immersion.
Taken from a genital canal
of a male specimen collected September 5, 1939. Sperms
are observed in what may be considered stages of dis­
integration. Sperm tails have disappeared leaving the
axial filaments or the sperm body alone.
Some sperm
bodies are swollen to about 12 ju. within which the chro­
matin granules are loosely scattered.
Hyaline phago­
cytes were more prevalent on September 5 than on July
27, but fewer spermatogonia were observed within the
germinal epithelium.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
PLATE
7
fig.
I
HYALINE CELL.
OOCYTES
IN STAGES
OF
DISINTEGRATION?
HYALI NE P H A G O C Y T E
E O S I N O P H IL E
GERMINAL
ACTION
OF P H A G O C Y T E S
ON
EPITHELIUM.
OOCYTES.
F I G.
CANAL
HYALIN E
SPERMS
EPITHELIUM
CELL .
/n STAGES OF
PHAGOCYTIC
DISINTEGRATION?
HYALINE
CELL.
ft
EOS INOPHIL E S.
Cl LI A .
ACTI ON
OF
PHAGOCYTES
ON
2
SPERMS.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
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