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Twinning in the common snapping turtle Chelydra serpentina.

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Twinning in the Common Snapping Turtle,
CheIydra serpentina '
State University of New York, Upstate Medical Center,
Syracuse, New York, and Marine Biological Laboratory,
Woods Hole. Massachusetts
Twinning or duplication was observed in 28 eggs of the common
snapping turtle, Chelydra serpentina serpentina. The methods o f observation made
detection of separate blastodiscs relatively certain, that of duplication within a blastodisc less so. Only two of the latter were found. Twins occurred in about 1% of the
eggs; 26% of the clutches had one or more abnormal eggs. Twinned blastodiscs developed normally in early stages. When incubated through hatching, 12 out of 12
sets of twins developed unequally so that one was essentially normal, the other was
small and not viable. In one case, in which the carapace of the smaller twin was three
quarters the length of the larger, both died during hatching. The pigmentation patterns of the carapace of these twins differed; this indicates that they are fraternal
rather than identical twins.
In their reviews of twinning in turtles,
Hildebrand ('38) and Crooks and Smith
('58) state that the development of two
or more complete individuals within a
single turtle egg is rarely observed. Anomalies involving partial duplication or
conjoined twins are more frequent. In
this report, the word twinning will be
used to refer to development of two separate individuals within a single egg and
the word duplication for fused embryos or
individuals with incomplete doubling of
parts. The instances of complete twinning
cited by Hildebrand ('38) are three early
embryonic twins of Chelone mydas described by GIaesner ('24), the five sets of
Dermochelys coriacea at later stages of
incubation reported by Deraniyagala ('32)
and the set of twin Malaclemmys centrata
described by Hildebrand himself. Subsequently separate embryonic discs in an egg
of E mys urbicularis were observed during
stages of development by Dehnel ('48).
Crooks and Smith ('58) have described
one set of twins in Terrapene Carolina
triunguis, Horning ('63) in Chelydra serpentina and Hunsaker ( ' 6 8 ) in Gopherus
polyphemus. Fujiwara ('64) reported eight
abnormal eggs of Caretta caretta. Seven
of these contained completely separate
twins; the eighth was duplicated anteriorly.
Rarity of twinning is implied by the
literature. However, twin embryos were
ANAT. REC., 166: 4 9 1 4 9 8 .
occasionally found in eggs of the snapping
turtle, Chelydra serpentina serpentina,
when inspected prior to or in the early
stages of incubation. Some observations
on this material are presented.
The presence of twin embryos in the
turtle egg can be determined by observations on the egg opened early in development or during stages such as 10 to 12
(Yntema, '68) when the embryo and its
extraembryonic vessels can be seen
through the intact shell upon candling.
The first procedure was followed to prepare embryos for operating (Yntema, '64)
and the embryo usually remains viable.
In the second procedure, the eggs were
incubated in moist chambers.
For some years, the twill embryos were
noted rather casually; some of these
records are useful in this study. In 1968,
a record was kept of d l eggs observed and
an attempt was made to incubate all sets
of twins found. The females from which
the eggs were taken were either collected
in the region of Syracuse, New York, or
shipped by the Lemberger Company of
Oshkosh, Wisconsin.
Received June 6, '69. Accepted Sept. 24, '69.
1This work was supported by the research grant 8
R01 HD03484 made by the National Institutes of
49 1
Types and incidence of abnormalities
Twinning or duplication was observed
in 28 eggs. In 26 cases, the twinned
blastodiscs were completely separate. In
two cases, duplication occurred within
single or fused blastodiscs. By far the most
frequent abnormality was the formation
of separate blastodiscs which usually lie
close together each in its own pellucid
area; this occurred in 24 eggs. In two
cases, the rostra1 ends of the pellucid areas
were slightly joined. In one of the two
duplications, three blastopores had formed
around the circumference of a single
blastodisc. In the other, the posterior ends
of the embryos were fused while the anterior portions were separate.
During five operating seasons, records
were kept on the incidence of abnormalities detected and the source of the eggs
checked. Of the 28 abnormal eggs considered above, 25 were found during these
five seasons. Many of the eggs were only
candled; in these, duplication within single
or fused blastodiscs would not be detected.
The observations during these five seasons on incidence of twinning and duplication are summarized in table 1. Out of
2,228 eggs, 23 sets of twins or approximately 1% occurred. One clutch, S 57
from Wisconsin in 1959, contained five
abnormal eggs. The incidence of twinning
was higher in eggs from Wisconsin in the
earlier years than in those from New
York. However, in 1962 and 1968, twins
were more frequent in New York eggs.
There appears to be no real difference in
incidence of twinning in eggs from the
two regions.
Development in twinned eggs
A total of 28 eggs with twins or duplicated structures was available for study.
A pair of embryos obtained in 1966 was
preserved at stage 9. They lie about 3 mm
apart in the blastoderm, their longitudinal
axes converge slightly in front (fig. 1 ) .
The vascular areas are fused between the
embryos. Both appear to be normal and
in the same stage of development.
In one duplication, the trunks were
fused and the anterior portions separate.
In the photograph of the ventral view
(fig. 2) the posterior intestinal portae are
seen to be separate; dorsally, the spinal
cords also are distinct from each other.
The axes diverge anteriorly so that the
rudiments of the head, neck and anterior
trunk are distinct and appear normal. The
blastodisc with three blastopores was incubated for a few days. Two major axes
formed and the third was rudimentary.
Further information is not available since
this case of abnormality died before
Hatching of 15 of the 28 available eggs
was attempted. One of these died at an
early stage; two developed to hatching or
nearly so and then died. In 12 of the eggs,
one normal individual hatched and sur-
Sources and incidence of twinning and duplication
Year and
Clutches with
abnormal eggs
1958 N.Y.
1959 N.Y.
1960 N . Y :
1962 N.Y.
1968 N.Y.
Totals N.Y.
1 (33% )
6 (35%)
7 (21% )
1958 Wisc.
1959 Wisc.
1960 Wisc.
1962 Wisc.
1968 Wisc.
Total Wisc.
Totals N.Y.
2 (40% )
2 (67%)
4 (25% )
9 (31% )
16 (26% )
and Wisc.
1 (0.7% )
8 (1.3% )
9 (0.7%)
3 (2.0%)
4 (3.1%)
2 (2.9%)
5 (0.9%)
14 (1.5%)
1 (0.8%
i i i . 4 %j
2 (0.2%)
23 (1.0%
2 (0.1%
Fig. 1 Photograph of ventral view of twin embryos at stage 9, fixed and unstained. Various
tures are evident such as the stomadaea and intestinal portae. x 12
Fig. 2 Photograph of ventral view of fused embryos at stage 9, fixed and stained with alum
mine. Anteriorly, the parts are separate; posteriorly, they are fused. The two posterior intestinal
tae are distinct from each other and separated by a ridge of tissue. This case was one of the
abnormal eggs in clutch S 57. x 16.
vived; the other was small and not viable.
Four of these non-viable twins are illustrated (figs. 4, 6-8). All are small and
abnormally formed; insofar as can be
estimated, they are comparable to stages
25, 15, 18 and 23 respectively. The three
less mature embryos were dead when
recovered; the oldest of the four had a
functioning heart and was relatively well
formed though not viable. This last embryo was attached to its viable mate by
way of a fusion between the yolk sacs
(fig. 3 ) . After hatching, the two were cut
apart. The larger twin was kept for several
weeks and appeared entirely normal.
As stated above, two eggs developed to
hatching or nearly so and then died. The
twins in one of these eggs died shortly
before hatching. I n this case, the twins
were unequal like those discussed above.
In the other case, the twins died during
hatching and the disparity in sizes was
not great (fig. 5). In one, the carapace
is 20 m m long; in the other it is 15 mm.
Both twins are well formed; they have a
yolk sac in common but otherwise are
separate. The pigmentation patterns in the
marginal lamina of the anterior carapace
are not identical. The arrows point to the
midline in each case. The larger twin
(fig. 9;) has a conspicuous marginal media n dark area which is smaller in the
other twin (fig. l o ) . In the larger twin,
heavily pigmented areas a t the intermarginal junctions are more restricted to the
junctions than in the smaller twin. In the
latter, the darkly pigmented areas persist
on the marginal lamina of the lateral portion of the carapace; these become less
marked in the larger individual. These
examples of differences in pigment pattern are held to indicate that these twins
are fraternal rather than identical.
Sizes of viable twins
Eggs from eight clutches of eggs were
incubated along with the eggs i n which
twins were detected. Diameters of the eggs
were recorded as well as the lengths of
carapaces measured two or three days
after hatching. The latter measurement
is delayed in order to permit the carapace
to flatten after release from the confinement of the shell. The data are summarized in table 2; the ranges of measurements are given when there were
differences. In each of the nine cases of
twins, one twin was viable and apparently
normal; the other twin was small and
dead at the time of hatching. The data
show no real differences between sizes of
normal and twinned eggs. The lengths of
carapace after hatching are also essentially the same. The smaller twin did not
prevent the viable one from reaching a
typical size.
lncubation periods of twins
Times of incubation were recorded for
the twins and their siblings. These are
summarized in table 2. On the average,
the twins hatched about one day later
than the control siblings. Such a difference is well within the normal range of
variation for the incubation period.
The question of the origin of twinning
and partial duplication in turtles has been
reviewed and discussed by Dehnel (’25,
’29, ’48). He concluded that these situations resulted from cytoplasmic fusion of
oocytes rather than division of rudiments
derived from a single ovum. Development
in the latter situation would result in
identical rather than fraternal offspring,
whether separate or fused.
In his 1929 report, Dehnel described
an unusual female E m y s which had 13
eggs, “cinq de ces blastodermes portaient
les foyers formatifs doubles, l’un Ctait
pourvu de trois centres embryonnaires independants, et, enfin, un oeuf contenait
deux vitellus etroitement soudCs l’un a
l’autre.” In sections of the ovaries of this
animal he found “43 oocytes plurinuclBBs
ou en train de le devenir, dont 12 cas de
la fusion indubitable de deux oocytes en
un seul, 28 cas d’oocytes B deux noyaux,
un oocyte trinuclCC, un B quatre noyaux,
et, infin, un cas extr6mement compliqu6
de cinq oocytes se fusionnant en un complexe commun” (pp. 65-66). Such abnormalities were not found in other females
studied. If abnormal blastodenns such as
described by Dehnel developed, one would
expect monsters with varying amounts of
duplication. This is in contrast with the
formation of separate individuals on a
common yolk sac found when the blastodiscs are separate and distinct within the
egg as described by Glaesner (’24), Dehnel
(’48) and in this report.
In this study of Chelydra, separate
blastodiscs were seen occasionally. In two
cases, the anterior ends of two pellucid
areas were fused. A single blastodisc with
three blastopores was observed in one case
and fused blastodiscs with two blastopores
in another. These latter two would be
difficult to detect by candling unopened
In the course of collecting materials,
eggs of turtles other than Chelydra were
observed. Out of approximately 200 eggs
of Chrysemys picta, two eggs from different females had twinned blastodiscs. None
were observed in approximately 100 eggs
by Emys blandingii. The number of observations is too small to serve as a basis
for a statement as to incidence of twinning in these forms but indicates agreement with, rather than difference from,
the situation in Chelydra.
Three instances of eggs at or near the
time of hatching with living twinned embryos have been recorded previously. In
the case of Malaclemmys described by
Hildebrand(’38), two like or “identical”
live individuals were dug from a bed of
eggs. They soon died. In the case of
Terrapene reported by Crooks and Smith
(’58) and of Gopherus reported by Hunsaker (’68), one of the twins survived
hatching, the other was dead at the time.
Fujiwara (’64) related twinning to
mechanical injury at the time of laying.
He suggested that the embryonic discs may
be split by the shock of falling during
deposition of the eggs. When eggs are removed directly from the oviduct, they are
not exposed to this trauma (Yntema, ’64);
there has been no indication that twinning
is based on injury this late in develoDment.
According to the conclusions of Dehnel
(’25, ’29, ’48) one would expect turtle
twins to be fraternal rather than identical. In this study on Chelydra, the twins
which died at hatching had different pigment patterns, a finding which agrees with
Dehnel’s conclusions. However, Hildebrand
(’38) carefully checked the twinned Malaclemmys and found them to be identical
because their size, color and individual
markings on shell and skin were identical.
In the case of Terrapene described by
Crooks and Smith (’58), “the completely
separate twins are not identical. In addition to the disparity in size, the larger
specimen has a better developed egg tooth
and posterior fragmentation of the laminae
so that there are six rather than five
centrals and, on the left side only, five
rather than four laterals.” These somatic
variations might be due to environmental
influences; they do not appear to confirm
genetic variation. The findings of Hildebrand do not agree with the conclusions
of Dehnel nor those of this report. Further
pertinent observations are needed; it is
possible that both fraternal and identical
twins may occur.
Several associates have contributed to
this report. The cooperation of Mrs. G.
Berman and Mrs. L. Blaisdell has been
especially important in preparation of the
photographs and in care of the embryos
and the adult snappers.
Crooks, F. D., and P. W. Smith 1958 An instance of twinning in the box turtle. Herpet.,
14: 170-171.
Dehnel, G. 1925 Origine des polyg6nsses et
oeufs plurinuclkbs. C. R. SOC. Biol., Paris, 93:
1929 Recherches sur le d6veloppement
et la g6nsse des monstres composbs chez l a
tortue Polkise ( E m y s orbicularis I,.). Arch.
Nauk. Biol. Warsz. Tow. Nauk., 2: no. 2, 68 pp.
1948 The case of embryonic double
monstruality of the turtle ( E m y s orbicularis
L.) observed during its development. Ann. Univ.
M. Curie-Sklodowska, C, 3: 43-54.
Deraniyagala, P. E. P. 1932 Notes on the development of the leathery turtle, Dermochelys
coriacea. Spolia Zeylanica, 17: 73-102.
Fujiwara. M. 1964 Some cases of .spontaneous
twinning in the loggerhead turtle. Acta Herpet.
Jap., 2 7-8 ( i n Japanese).
Glaesner, L. 1924 Ueber drei Doppelbildungen
von Ckelone m y d a s . 2001.Anz., 60: 185-194.
Hildebrand, S. F. 1938 Twinning in turtles.
Jour. Hered., 29 243-253.
Horning, E., Jr. 1963 Twin snapping turtlesa rarity? Bull. Phila. Herpet. SOC.,11: 9.
Hunsaker, D., I1 1968 Twinning in tortoises.
Turtle and Tortoise SOC.J., 2, No. 6 , 38.
Yntema, C. L. 1964 Procurement and use of
turtle embryos for experimental procedures.
Anat. Rec., 149: 577-586.
1968 A series of stages in the embryonic development of Chelydra serpentina. J .
Morph., 125: 219-252.
The scale line i n the upper left of each figure represents 1 mm of
length i n the specimen.
Photograph of twins from clutch S 331 taken immediately after
hatching. The larger twin survived; the smaller twin (also shown
in fig. 4 ) is not viable. The yolk sacs of the two are joined. x 1.7.
Photograph of smaller twin also shown in figure 3. The animal is
small but at a late stage of development estimated to be stage 25.
x 3.
Photograph of twins from clutch S 300 which died during hatching.
The carapace of the smaller individual is three-quarters the length
of that of the larger one. The two are joined only by their yolk sacs.
x 3-.
6 Photograph of twin from clutch S 300 whose development stopped at
approximately stage 15. The necrotic embryo is abnormally formed.
x 12+.
Photograph of twin from clutch S 287 whose development stopped at
approximately stage 18. The carapace is delimited. x 10 -.
8 Photograph of twin from clutch S 308 whose development stopped at
approximately stage 23. The carapace of this small individual is well
formed. x 6 - .
9 Photograph of the anterior portion of the carapace of the larger twin
shown in the left part of figure 5. The median marginal pigmented
area indicated by the arrow is large. In the regions of the intermarginal junctions, the heavily pigmented area remains fairly close to the
junctions in most instances. Compare with figure 10. x 5 -.
10 Photograph of the anterior portion of the carapace of the smaller twin
shown i n the right part of figure 5. The median marginal pigmented
area indicated by the arrow is small. I n the regions of the intermarginal junctions the heavily pigmented area is relatively large, especially lateral or posterior to the junction. Compare with figure 9. x 5 -.
C. L. Yntema
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common, twinning, serpentina, turtles, snapping, chelydra
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