JOURNAL OF EXPERIMENTAL ZOOLOGY 284:299–308 (1999) Hyperplastic Development and Hypertrophic Growth of Muscle Fibers in the White Seabass (Atractoscion nobilis) ANASTASIA MARIA ZIMMERMAN AND MARY SUE LOWERY* University of San Diego, San Diego, California 92110 ABSTRACT Because skeletal muscle is the main contributor to body weight in most fish, it is probable that the size of a fish is limited by the growth of this tissue. Several aspects of both somatic size and skeletal muscle growth were investigated in white seabass (Atractoscion nobilis). One hundred and four subjects ranging in size from 1.3–91.8 cm standard length were examined to discern growth patterns throughout the entire life span of this species. Relationships of somatic growth were evaluated by weight, length, and age comparisons while muscle growth was assessed from cross-sectional area measurements of white muscle fibers. The average cross-sectional fiber area increased from approximately 150 µm2 to 4300 µm2 as the fish increased in size, indicating that hypertrophy plays an important role in growth. Hyperplastic fiber recruitment accounted for approximately 93% of muscle growth in recently hatched fish and gradually decreased with fish length. Although the contribution of hyperplasia declined to less than 1% in the largest subject, the persistence of hyperplasia at standard lengths equal to or greater than 91.8 cm as observed in the white seabass has yet to be documented in other fish species. This sustained fiber recruitment may be responsible for the impressive ultimate standard length of 133 cm (41 kg) that white seabass are capable of attaining. The white seabass currently represents the largest marine fish species for which the dynamics of muscle fiber growth have been described. J. Exp. Zool. 284:299– 308, 1999. © 1999 Wiley-Liss, Inc. Somatic growth is generally viewed as an increase in body size resulting from growth of several if not all of the tissues comprising the organism. Skeletal muscle tissue represents a significant portion of the mass of most fish, and may compose between 30–80% of their live weight (Weatherley and Gill, ’87). Because skeletal muscle may be the main contributor to body mass it is probable that the size of a fish is limited by the growth of this tissue. Investigating muscle growth is an important focus of study from a biological standpoint as the growth of this tissue may underlie the overall somatic growth in fish. Muscle growth is a dynamic process in fish that begins early in their development and continues throughout much if not all of their life span. During initial myogenesis several mitotically active stem cells, referred to as myosatellite cells, fuse together to form individual multinucleated muscle fiber cells (Nag and Nursall, ’72; Campion, ’84; Schultz, ’89; Matschak and Stickland, ’95). Resulting muscle fibers are the fundamental unit of skeletal muscle tissue, as hundreds to several thousand fibers collectively form a single muscle mass. Further recruitment of new fibers (hyperplasia) and enlargement of existing fibers (hypertrophy) within © 1999 WILEY-LISS, INC. a muscle mass often continues well past the age of sexual maturity in fish. Individual fibers are often classified as being red, pink, or white based on metabolic and structural properties. In fish musculature different fiber types are not intermingled but are separated into either superficial muscle masses comprised exclusively of red fibers or deep muscle masses composed entirely of white fibers. Because deep muscle masses may comprise more than 90% of the total volume of muscle tissue in fish (Weatherley and Gill, ’89) it is probable that increasing body size is due in large part to growth of white muscle fibers. Several studies have demonstrated that recently recruited fibers in fish are relatively small in size and that these fibers increase in cross-sectional area through hypertrophic growth(Moss and LeBlond, ’71; Stickland, ’83; Ennion et al., ’95). Because hyperplasia is associated with small fibers and hypertrophy is correlated with fibers of greater dimensions, the size of individual fibers can therefore be used to assess muscle growth at *Correspondence to: Mary Sue Lowery, USD Biology Department, 5998 Alcala Park, San Diego, CA 92110. E-mail: firstname.lastname@example.org Received 25 July 1998; Accepted 25 November 1998. 300 A.M. ZIMMERMAN AND M.S. LOWERY different life stages. Hyperplastic and hypertrophic growth patterns have been described for several species of fish representing different taxonomic families which display a broad range of maximum reported sizes (Weatherley and Gill, ’81, ’84, ’85; Stickland, ’83; Weatherley et al., ’88; Higgins and Thorpe, ’90; Vegetti et al., ’90; Rowlerson et al., ’95). The contributions of hyperplasia and hypertrophy appear dependent on both the species and ultimate somatic size of the fish. In species of small maximum reported body sizes, it is generally found that hyperplasia ceases early in development and most growth after hatching is attributed to hypertrophy. Conversely, in species capable of attaining large sizes hyperplasia continues to be an important contributor to muscle growth into the adult phase. These findings provide strong indications that the overall contribution of hyperplastic growth is responsible for determining the ultimate attainable size of a fish. The purpose of this study was to investigate several aspects of somatic and muscle fiber growth in the white seabass. Somatic growth was evaluated through weight, length, and age relationships and muscle growth was investigated by measuring the cross-sectional area of fibers representative of recently hatched to sexually mature specimens. These measurements enabled us to assess the contributions of hyperplasia and hypertrophy throughout the entire life span of this species of fish. We believe our findings provide considerable insight into the processes that enable white seabass to grow to their impressive ultimate total length of 1.5 m and maximum reported weight of 41 kg (Miller and Lea, ’76). This understanding, in addition to its biological interest, has potential value for enhancing applied aquaculture techniques and furthers the knowledge of hyperplastic and hypertrophic growth patterns in fish. The white seabass currently represents the largest marine fish species for which the dynamics of muscle fiber growth have been discerned. were established and four fish representative of each size class were obtained. These fish were progeny of hatchery-spawns and were raised under similar (if not identical) conditions. Several larger hatchery-reared subjects of 21.5–50.5 standard length were also obtained for comparison. White seabass exceeding these sizes are quite rare in the wild and are prized as hatchery broodstock fish when caught in southern California. Therefore, in order to investigate extremely large white seabass, specimens were obtained as hatchery broodstock mortality occurred, resulting in acquisition of subjects ranging from 56.5 to 91.8 cm standard length. MATERIALS AND METHODS Measurement of fiber size Subjects Specimens of white seabass were generously provided by the Hubbs Sea World Research Institute’s fish hatchery located in Carlsbad, CA. Subject selection was based on standard length (tip of lower jaw to the base of the caudal fin) since this parameter is a reliable indicator of body size. Twenty size classes separated by 1-cm divisions ranging from 1.0 to 20.9 cm standard length Muscle samples Fish were transferred to the University of San Diego and upon arrival were sacrificed by an overdose of methanesulfonate salt. Samples of epaxial white skeletal muscle were extracted from the right side of each fish at a region directly beneath and posterior to the first ray of their dorsal fin. This site was easily located on any size white seabass and provided the greatest abundance of skeletal muscle tissue comprised entirely of white muscle fibers in these fish. The size of each muscle sample was approximately 0.5 cm in width and 2.0 cm in length although for subjects smaller than 3.0 cm, muscle samples were inevitably smaller in accordance with fish length. Histological techniques Samples were fixed by immersion in 10% buffered formalin, dehydrated through a graded series of alcohol, and embedded in paraffin. Tissue blocks were orientated so fibers were cut at a right angle to their main axis during sectioning. Muscle sections (5 µm) were stained with haemotoxylin and eosin and mounted on microscope slides. Resulting sections exhibited a mosaic appearance and individual fibers were polyhedral in shape as shown in Fig. 1. The cross-sectional areas of individual fibers within each section were determined using an image analysis system consisting of a Leica Q500MC computer with corresponding image analysis software and a video camera attached to a light microscope. This setup enabled a field of muscle fibers viewed under the microscope to be projected onto the computer monitor screen. The area of a slide representing a single pixel on the monitor screen was determined through calibration with MUSCLE FIBER GROWTH IN WHITE SEABASS 301 number of fibers contained in fish measuring 1.7, 7.0, 18.2, 31.4, 73.0, and 88.0 standard length were determined. The entire body region located between the first and second dorsal fins of each fish was sectioned. Depending on the size of the body cross-section, it was viewed under either a dissecting microscope or a non-magnified video camera that was attached to the previously described image analysis system. Using this system the total surface area of white muscle tissue from each body section was measured. A small sample of muscle tissue was subsequently removed from each section and its corresponding area was also measured. These samples were processed in the same manner described for all other muscle samples and the fiber cross-sections present in each sample were counted. Because this sum represented a known proportion of the total white muscle it was possible to extrapolate a realistic estimate of the total number of fibers present in each fish. Assessment of hyperplasia and hypertrophy Fig. 1. White fiber cross-sections from a white seabass of 19.2 cm standard length. Mosaic appearance of muscle section and polyhedral shape of individual fibers are characteristic of all subjects. Bar = 20 µm, the diameter of the largest fibers considered to be hyperplastic. a stage micrometer. Using the system software the sum of pixels contained within an individual fiber viewed on the monitor screen was determined and converted into the actual cross-sectional area of each fiber. In order to ensure a significant sample size, the cross-sectional areas of 500 different muscle fibers were measured for each fish. Extensive editing of images was often done to ensure that fibers were viewed individually and free from any debris. Fibers exhibiting an extremely elongated cross-sectional shape were not measured as it was presumed that these fibers were not true cross-sections and might exhibit a significantly larger area than if cut at a right angle. Estimation of total fiber number To estimate the total number of fibers present throughout the life span of the white seabass, the A new fiber arising through hyperplasia is relatively small in size and through hypertrophic growth it increases in size. Several researchers have considered fibers less than 20 µm in diameter to represent fibers recruited by hyperplasia and those exceeding this diameter to represent fibers which had subsequently grown by hypertrophy (Stickland, ’83; Weatherley and Gill, ’84; Weatherley et al., ’88; Rowlerson et al., ’95). Hyperplastic and hypertrophic growth in the white seabass was similarly assessed using this established 20 µm diameter criterion in order to make comparisons to other fish species. The conversion of cross-sectional areas to equivalent diameter values was made according to Weatherley et al. (’84) using the assumption that individual fiber crosssections are circular. Under this assumption the cross-sectional area of a fiber is considered to be equal to that of a circle (πr2). Therefore, fibers exhibiting areas under 314 µm2 can be attributed to hyperplasia whereas those exceeding 314 µm2 are considered a result of hypertrophic growth. RESULTS A total of 52,000 fibers from 104 white seabass ranging in size from 1.3 to 91.8 cm standard length were measured. The smallest fish in this range was a recently hatched specimen whereas the 91.8 cm fish was relatively close to the 133 cm maximum recorded standard length of this species. Eighty of these fish consisted of members 302 A.M. ZIMMERMAN AND M.S. LOWERY from the established 20 size classes (1.0–20.9 cm SL) that were spawned and reared under similar conditions at the hatchery facility. The larger fish had experienced more varied conditions when compared to those of the established size classes. In addition to their acquisition following mortality, the largest fish (56.5–91.8 cm SL) were originally members of a wild population prior to their capture and transfer to the hatchery facility. The biases attributed to these variances could not be assessed and therefore statistical analysis was performed only on data collected from subjects comprising the established size classes. The inclusion of the largest subjects in this study is essential, however, as their results prove to have important comparative value. It was not possible to determine the sex of immature specimens and it was presumed that the ratio of male to female subjects was likely to be equal. Thomas (’68) demonstrated that the weight and length relationship of late juveniles and mature white seabass is unaffected by gender. Recently hatched and early juvenile white seabass used in our investigation probably also exhibit an unbiased size relationship; therefore, gender was not considered a factor biasing the results obtained in this investigation. Standard length was compared to weight, age, total fiber number, average fiber area, and fiber distribution, and the relative contributions of hyperplasia and hypertrophy were determined. Body weight and length relationship The measurements of standard length (SL) to corresponding body weight are represented in Fig. 2. Their statistical relationship was determined by calculation of the following regression curve: (Weight) = 0.038(SL2.707) (r2 = 0.995). The shape of this curve indicates that the amount of weight gain associated with increasing length becomes progressively greater as the fish increase in size. Fig. 2. Relationship between standard length (SL) and corresponding weight (W) for white seabass comprising the 20 established size classes. The relationship was fitted using an exponential curve: (W) = 0.038(SL2.707) (n = 80, r2 = 0.995). Total fiber number The total number of fibers contained within fish of 1.7, 7.0, 18.2, 31.4, 73.0, and 88.0 cm standard length are shown in Fig. 4. These results clearly indicate that the white seabass develop new fibers during much of their lives. However, because of the low sample number, it is not possible to determine whether recruitment continues throughout their entire size range or if a plateau of relatively constant fiber numbers occurs. Average fiber area Age and length relationship The age of each fish was calculated as the time elapsed from the date at which it was spawned until the date at which it was sacrificed. Standard length plotted against age is shown in Fig. 3. The statistical relationship existing between these variables was calculated utilizing a linear regression analysis and was determined to be: (SL) = 0.075(Age) – 0.055 with an r2 value of 0.947. The increase of standard length with age indicates that these subjects were in a state of growth. The average fiber area was found to increase linearly with the length of the fish (Fig. 5). Regression analysis determined r2 = 0.828 for the following relationship: (Average fiber area) = 98.617 + 28.714 (SL). This increasing fiber area indicates that hypertrophy is an important contributor to overall muscle growth. Although only the established size classes (1.0–20.9 cm SL) were included in our statistical analysis, the data obtained from the larger fish appear to follow similar trends as shown in Table 1. MUSCLE FIBER GROWTH IN WHITE SEABASS Fig. 3. Relationship between age and standard length (SL) for white seabass comprising the 20 established size classes. The solid line was fitted using a linear regression: (SL) = 0.075(Age) – 0.055 (n = 80, r2 = 0.947). 303 Fig. 4. Relationship between total fiber number and standard length for white seabass. The solid line represents the following linear regression: (Total fiber number) = 8283(SL) – 25764 (n = 6, r2 = 0.941). Range of fiber area The dynamics of fiber growth are perhaps best illustrated by investigating the distribution of fiber area in relation to standard length. The series of histograms shown in Fig. 6 depicts the relative percentage of fiber area in relation to increasing standard length. The 20 established size classes are consolidated into the first four histograms and the remaining four histograms reflect groupings of the larger subjects into classes each spanning a range of 20 cm in standard length. Hyperplasia and hypertrophy The relative contribution of hyperplasia in relation to standard length of the established size classes is illustrated in Fig. 7. The percentage of fibers exhibiting hyperplastic growth for the entire range of standard lengths is shown in Table 2. Hyperplasia appears to be the dominant growth process in recently hatched white seabass and although this type of growth decreases as these fish increase in size it remains an important contributor well into their adult phase. The decrease of fibers associated with hyperplasia coincides with a proportional increase of fibers attributed to hypertrophic growth. Hypertrophy of existing fibers may make a larger contribution to overall muscle growth as hypertrophic fibers account for a progressively greater percentage of overall total fiber volume as the white seabass increase in size. DISCUSSION Origin of recruited fibers The total number of fibers comprising individual white seabass was found to increase with increasing body size (Fig. 4) indicating that white seabass develop new fibers during much if not all of their lives. Considerable evidence indicates that small recruited fibers originate from the differentiation of myosatellite cells (Nag and Nursall, ’72; Campion, ’84; Schultz, ’89; White and Esser, ’89; Veggetti, ’90). It has been suggested, however, that increasing fiber number may result from the splitting of existing fibers. Although fiber splitting has been demonstrated in the eel, Anguilla anguilla 304 A.M. ZIMMERMAN AND M.S. LOWERY Fig. 5. Relationship between average fiber area and standard length of white seabass. Each point is an average calculated from 500 individual fibers from each of 80 subjects. The illustrated line represents the following linear regression: (Average fiber area) = 28.714(SL) + 99.617 (r2 = 0.828). (Willemse and Van den Berg, ’78), and suggested in the gray mullet, Mugil capito (Scapolo et al., ’84), the preponderance of evidence discounts fiber splitting as the probable mechanism for increasing fiber number in most fish (Greer-Walker, ’70; Stickland, ’83; Weatherley and Gill, ’84; Powell et al., ’89; Koumans et al., ’90, ’93a,b, ’94; Koumans and Akster, ’95; Matschak and Stickland, ’95). Furthermore, in some species small fibers have been shown to contain unique embryonic myosin isoforms, supporting a new origin of the small fibers rather than formation by splitting of existing fiFig. 6. Distribution of fiber area in relation to increasing standard length. Each histogram represents a consolidation of size classes as indicated in the upper right corner. The first four histograms (1.0–20.9 cm) each reflect measurements from 20 different fish and 10,000 individual fibers. The remaining histograms are representations of the following: 5 fish and 2500 fibers (21.0–39.9 cm); 7 fish and 3500 fibers (40.0–59.9 cm); 10 fish and 5000 fibers (60.0–79.9 cm) and 2 fish and 1000 fibers (80.0–91.8 cm). MUSCLE FIBER GROWTH IN WHITE SEABASS 305 TABLE 1. Weight, age and average fiber area of larger white seabass subjects1 SL (cm) No. of fish Weight (g) Age2 (days) Fiber area (µm2) 21.0–29.9 30.0–39.9 40.0–49.9 50.0–59.9 60.0–69.9 70.0–79.9 80.0–89.9 90.0–99.9 3 2 5 2 3 7 1 1 199 ± 83 607 1,741 ± 206 2,459 3,406 ± 503 4,653 ± 1,182 7,140 8,270 254 ± 7 410 464 ± 0 Unknown3 Unknown3 Unknown3 Unknown3 Unknown3 813 ± 74 1,393 1,152 ± 137 1,027 1,601 ± 114 1,561 ± 446 2,518 4,315 Values stated as means ± standard deviation where possible. Age calculated as time elapsed from spawn until sacrifice. 3 Fish that were once members of wild populations. 1 2 bers (Ennion et al., ’95; Rowlerson et al., ’97). Small fibers recruited in the white seabass are considered to have originated from myosatellite cells and not by fiber splitting. This conclusion is in agreement with results from other investigated fish species and is further supported by our discovery of myosatellite cells in the white seabass (unpublished data). Fiber distribution The average fiber area was found to increase with standard length (Fig. 5) with individual fish often comprised of fibers exhibiting a wide range of areas. This range of fiber area was responsible for the mosaic appearance of muscle sections (Fig. 1). Histograms, shown in Fig. 6, clearly indicate that the distribution of fiber area varies considerably with length. In the smallest fish (1.0–5.0 cm SL), fibers are initially confined to a small range between 10 and 1,130 µm2 with a high percentage exhibiting areas under 200 µm 2. As the subjects increase in length this range becomes progressively broader and in the largest fish (91.8 cm SL) the range of fiber sizes was found to extend between 168 and 10,730 µm2. This wide range of fiber sizes in each class indicates that at every length investigated, both hyperplastic development and hypertrophic growth occur. Hyperplastic and hypertrophic growth Hyperplasia is the dominant growth process in recently hatched white seabass and gradually decreases as these fish increase in size (Fig. 7 and Table 2). In the smallest size class (1.0–1.9 cm SL) approximately 93% of their fibers are considered to be in a state of hyperplastic growth whereas the contribution of hyperplasia in the Fig. 7. Relative percentage of hyperplastic fibers in relation to standard length (SL) of white seabass comprising the 20 established size classes. The illustrated curve represents the best fit between these variables: (% of hyperplastic fibers) = 98.67e–0.0603(SL) (n = 80, r2 = 0.84). largest specimen (91.8 cm SL) was found to have decreased to 0.8%. This finding that the relative contribution of hyperplasia is greatest early in the life of the white seabass and decreases with standard length is in agreement with skeletal muscle growth patterns reported in other species (Stickland, ’83; Weatherley and Gill, ’84, ’85; Weatherley TABLE 2. Hyperplastic fibers from white seabass of all size classes1 SL (cm) 1.0–5.9 6.0–10.9 11.0–15.9 16.0–20.9 21.0–39.9 40.0–59.9 60.0–79.9 80.0–91.3 1 2 No. of fish No. of fibers measured Relative % hyperplastic fibers2 20 20 20 20 5 7 10 2 10,000 10,000 10,000 10,000 2,500 3,500 5,000 1,000 82 ± 9 59 ± 9 44 ± 6 33 ± 8 22 ± 10 20 ± 7 9±5 1 Hyperplastic fibers are those <314 µm2. % values stated as means ± standard deviation where possible. 306 A.M. ZIMMERMAN AND M.S. LOWERY et al., ’88; Higgins and Thorpe, ’90; Veggetti et al., ’90; Rowlerson et al., ’95). However, several previous investigations found not only that hyperplasia decreased with standard length but also that once the fish reached a certain size hyperplastic growth ceased completely (Weatherley et al., ’80b, ’88; Stickland, ’83; Weatherley and Gill, ’84; Kundu and Mansuri, ’90, ’92). The persistence of hyperplasia at standard lengths equal to or greater than 91.8 cm as observed in the white seabass has yet to be documented in other species. In perhaps the most extensive study of this sort of growth, Weatherley and his colleagues (Weatherley et al., ’79, ’80a, ’80b, ’88; Weatherley and Gill, ’81, ’84, ’85, ’89) investigated hyperplasia in 10 distantly related fish species from three orders and five families. The maximum reported sizes exhibited by these ten species ranged from 10 to 165 cm total length although the largest subjects they examined ranged between 65.0 and 69.9 cm. In these investigations hyperplasia was found to cease in these fish at lengths which were approximately 44% of the ultimate total length of each species. According to their relationship hyperplasia in the white seabass is predicted to cease at 66.0 cm total length (57.7 cm SL). This was not the case in our investigation as we found white seabass to clearly exhibit hyperplasia at sizes greater than 57.7 cm standard length. In fact, hyperplasia persisted well past this predicted value within the next largest size class (60.0–79.9 cm SL) where 9% of these fibers were scored as hyperplastic (Table 2). The occurrence of hyperplasia was not concentrated in the lower range of this size class, rather hyperplastic fibers were evenly distributed between 60.0 and 79.9 cm standard length. Four hyperplastic fibers ranging in size from 168 to 305 µm2 were also documented within the 500 fibers measured from the largest white seabass (91.3 cm SL) indicating that hyperplasia occurs at every length examined in this investigation. Our data clearly indicate that white seabass retain their hyperplastic ability far beyond that which has been documented in other species. In fact, the occurrence of hyperplastic fibers in our largest subject indicates that white seabass exhibit hyperplasia at lengths which are 74% of their ultimate size. Even with a conservative approach that considers 79.9 cm as the largest size with substantial hyperplasia, white seabass exhibit hyperplasia at 61% of their ultimate size. Either percentage is substantially greater than the 44% which would be predicted using Weatherley’s relationship. There are several possible reasons for this discrepancy, perhaps the most intriguing of which is that white seabass may have uniquely retained the ability to recruit muscle fibers throughout their entire lives. It is also plausible that the white seabass may lose their hyperplastic ability at lengths greater than those observed in our study. When making comparisons between these types of investigations one should consider that the histological methods utilized, although similar, are not completely uniform. Variation in the fixatives and subsequent processing of muscle samples is likely to produce size differences in the resulting fiber cross-sections. These differences are often a result of variation in shrinkage imparted to both fibers and their surrounding connective tissue during fixation (Bourne, ’72). Fiber shrinkage was observed in the white seabass tissue; however, since each sample was processed in the same manner, the extent of this shrinkage was considered to be consistent. This artifact of fiber shrinkage was similarly addressed in several of the previously cited studies although it was never quantified. Therefore, it is not possible to determine if the white seabass fibers endured a greater or lesser degree of shrinkage that might lead to either an over- or underestimation of hyperplasia when compared to previous studies. It is for this reason that direct numerical comparisons between investigations may be problematic. Nevertheless, although the accuracy of numerical comparisons may be uncertain, it is reasonable to draw reliable conclusions by comparing overall trends between different investigations. Hyperplasia facilitates ultimate size Previous research has shown that species of fish that experience long periods of hyperplasia reach larger body sizes than those that grow primarily through hypertrophy (Weatherley et al., ’80b, ’88; Stickland, ’83; Weatherley and Gill, ’84, ’85, ’89; Calvo, ’89; Kundu and Mansuri, ’90, ’92; Veggetti et al., ’90; Battram and Johnson, ’91; Koumans et al., ’93b; Rowlerson et al., ’95). The findings of the current investigation are in agreement with this trend, as hyperplasia was found to persist well into the adult phases of the white seabass, a species that attains a relatively large ultimate size. Considerable evidence therefore suggests that fiber recruitment holds the main role in determining the ultimate size of a fish. This phenomenon of hyperplasia facilitating ultimate size can be explained by considering the metabolic exchange processes of individual fibers. MUSCLE FIBER GROWTH IN WHITE SEABASS During hyperplasia the individual muscle fiber is small in size, thereby creating a high surface-tovolume ratio that facilitates processes such as amino acid assimilation and metabolic removal of cellular waste products. Hypertrophic growth begins as existing fibers are augmented in size, leading to a decrease in their surface-to-volume ratio. As hypertrophy continues, an increasingly inefficient and potentially unfavorable situation develops as a result of this decreasing surface area which causes a limiting of metabolic exchange. Weatherley et al. (’88) proposed that beyond a critical fiber size the continuation of hypertrophic growth would result in impairing metabolic exchange. According to their hypothesis the ultimate size to which a fish could grow would correlate with the attainment of this final critical size by all comprising fibers. Weatherley (’90) suggests that the critical fiber diameter at which surface area becomes limiting ranges from 120 to 270 µm in fish. The largest white seabass fiber measured in our investigation corresponds to a 117 µm diameter and was found in the largest subject (91.8 cm SL). It is probable that white seabass exceeding 91.8 cm standard length would have fibers that had grown to the metabolically restrictive sizes indicated by Weatherley (’90). The prolonged hyperplasia and lack of a restrictive maximum fiber size throughout most of the life span observed in the white seabass is likely to delay the onset of impaired metabolic exchange in these fish. It is therefore reasonable to conclude that this prolonged hyperplastic growth is responsible for enabling this species of fish to grow to their impressive ultimate size. LITERATURE CITED Battram J, Johnston I. 1991. Muscle growth in the Antarctic teleost, Notothenia neglecta (Nybelin). Antarct Sci 3:29–33. Bourne G. 1972. 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