Protogynous Hermaphroditism in the Sand Diver Trichonotus filamentosus

Transcription

1 Œ Ÿ ª ŽYŽŽŒ± ê j. Nippon Suisan Gakkaishi 57(1), (1991) Protogynous Hermaphroditism in the Sand Diver Trichonotus filamentosus Janny Dirk Kusen,*1 Kiyoshi Nakagawa,2 Yutaka Yogo,*3 and Akinobu Nakazono*1 (Received June 20, 1990) Sexual patterns of the sand diver Trichonotus filamentosus were studied using specimens collected at Hakata Bay, Kyushu, Japan, between The species was found to be a protogynous hermaphrodite for the following reasons: (1) The males are not only larger in body size than females, but also have a different body form and color, with intermediate types occurring within a range where the male and female sizes overlap. (2) Differences in annuli in otoliths between the sexes suggest that males are older than females. (3) Bisexual gonads occur in 4.58% of the specimens examined histologically. (4) Bisexual gonads and mature testes exhibit secondary structure. The size frequency distributions of the males and females, and the occurrence of specimens with bisexual gonads indicate that the sand diver of Hakata Bay undertakes protogynous sex change at mm SL, after the spawning season in summer. It has also been found that the present species is monandric, because all males have secondary testes. The sand diver Triehonotus frlamentosus is a member of the family Trichonotidae. It has an elongate, cylindrical, and gradually compressed body, and inhabits sandy bottoms. A number of sand diver were collected from Hakata Bay, Fukuoka. According to our observations, the species is most commonly caught with the Japanese sand lance Ammodytes personatus by small trawling net and constitutes one of the major species of fishes found on sandy bottoms in the shallow waters of Hakata Bay. Although Shimada and Yoshino1) suggested protogyny in trichonotid fishes, on the basis of body size differences between the sexes, apparently little is known about the sexuality of the family. The present study was designed to first examine the sexual status of Trichonotus filamentosus by documenting the size distribution of male and female populations, their age differences, and to conduct a histological examination of the gonads. The second objective of the study was to determine if this species is a functional hermaphrodite, following criteria presented by Sadovy and Shapiro2) and Shapiro.3) Materials and Methods Specimens of the sand diver (n=735) used in this study were collected monthly, using a small trawling net, at Hakata Bay, Fukuoka, beginning in January 1986 to December 1989, with the exception of the months of March and November. Specimens were directly fixed with 10% formalinseawater on the vessel of Prefectural Fisheries Experimental Station. Standard length (SL) and body weight (BW) were measured to the nearest 0.1mm and 0.01g, respectively. A total of 131 specimens was randomly chosen for histological examinations. The gonads were carefully taken out, and were measured to the nearest 0.01g and preserved in a 10% formalin solution for at least 24hours. After that, they were rinsed with tap water prior to subsequent processing.4) For histological preparation, only one lobe of larger gonads was used, but two lobes for smaller ones. In some of the smaller specimens, the whole abdominal part was embedded in paraffin. Paraffin sections were cut transversely at 6-12ƒÊm thickness through the entire gonad or at an arbitrary portion. Sections were stained with *1 Department of Fisheries, Kyushu University, Fukuoka 812, Japan ij. D. Kusen, ¾ M F ã B åšw _Šw ŽYŠw È j *2 Fukuoka Prefectural Fisheries Experimental Station, Imazu, Fukuoka , Japan i ì F Ÿ ª *3 Showa Women's High School, Hita, Oita 877, Japan i ]Œá L F º a Žq Šw Z j.

2 Mayer's hematoxylin and eosin.4) Slides were examined under a microscope to determine sex of the specimens and the status of the gonads, i.e. mature or immature, transforming or pure. Otoliths (sagittae) were also removed randomly from both sexes. These samples were used to determine the difference in number of annuli formed. Several parts of calcareous bone are most often used in age determination. However, the otoliths were chosen to count the annuli of the specimens, because scales and opercles did not show clear annuli. After removing the remnant tissues and cleaning in alcohol, the otoliths were burned and carefully separated transversely by small scissors towards the center. Otoliths of sand diver are rice shaped and did not show growth checks under reflected light, but burning of the otoliths made it possible to read the brown rings.5) The number of rings was compared between male Fig. 1. Size frequency distributions of male, female, and intermediate Trichonotus filamentosus collected in Hakata Bay, Fukuoka. and female specimens. To compare gonadal maturation between both sexes, and to determine the spawning season of this species, the gonado-somatic index (GSI= GW in g ~100/BW in g) was calculated using fish chosen from monthly samples, except for March and November when no specimens were collected. Results Sexual Dimorphism and Dichromatism There are a considerable part of external morphological differences which allow sexual discrimination of Trichonotus filamentosus. It was noted early that the male is larger in size than the female. The male has a black spot on the occiput with blue blotches, whereas these markings are absent in the female. The dorsal fin of the female has a black spot just on the anterior portion. The male has a relatively long ventral fin, whereas in the female it is short. Blue spots margined in black along the lateral line are very clear in the male. Specimens with an intermediate body form and color were also found in the sample population. They usually had ventral fins of intermediate length and the black spot on the occiput was either absent or not fully developed. Size-Sex Relationship A total of 735 individuals was measured and most were sexed by external appearance (Fig. 1). It was found that the frequency distribution of Fig. 2. Monthly changes of GSI in Trichonotus, filamentosus. Number shows the sample size. : Female, œ: Male. standard length of the sand diver was bimodal, the females showing the highest frequency at a smaller size when compared to the males. The females ranged in size from mm SL with the highest frequency between mm SL. The males ranged in size from mm SL with the highest frequency between mm SL. Males under 80mm SL, and females larger than 120mm SL were not caught. Females and males overlapped their size ranges between mm SL. Those specimens with an inter - mediate body form and color occurred within the same range where the male and female sizes overlapped. Gonado-Somatic index Between 4 and 63 gonads from both sexes were

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4 Fig. 5. Transverse sections of gonads in Trichonotus filamentosus. A: Immature ovary, 40mm SL, Date B: Mature ovary, 116mm SL, Date, C: Transitional gonad with testicular tissue and rudimentary oocytes (arrows), 95mm SL, Date, , D: Higher magnification of C. E: Testis in fully mature stage. Arrow mark shows the former ovarian wall, 118mm SL, Date F: Entire profile of E. Arrows show the former ovarian wall. OC=Ovarian cavity. FOC=Former ovarian cavity. Scale bar represents 0.1mm. Table 1. Monthly number of Trichonotus flamentosus specimens examined histologically

5 gynous sex change in fishes.2,3,7-10) In the present species, gonads having both atretic eggs and young testicular tissue were not found. However, several signs of gonadal transformation from ovary to testis, i.e. bisexual gonads, were seen (Fig. 5C). Oocytes in these bisexual gonads were small in number and the nucleus was not evident. These oocytes were considered to be degenerating remnants from the previous spawning season. These bisexual gonads were principally composed of testicular tissue in an early developmental stage and spermatids were not visible yet (Fig. 5D). It was supposed that the testicular tissue in these bisexual gonads began to proliferate and oocytes disappear, with the approach of spawning season. Bisexual gonads were recognizable only in specimens obtained in October and December (Table 1). The body sizes of these specimens fell within the same range where male and female body sizes overlapped. Additional evidence of sex change was a large lumen found in the bisexual gonads (Fig. 5C). It has been shown that the testis of many protogynous species has a lumen derived from the former ovarian cavity and this type of testis is referred to as a secondary testis.7) Testis: Mature testes are readily identifiable from mature ovaries by their sizes and color. They are smaller than ovaries, and milky white in color. Histologically, mature testes were characterized by the occurrence of many spermatids and spermatozoa (Fig. 5E). Evidence of sex change was recognizable even in mature testes. A secondary structure, a thin membrane surrounding the entire lobe, was found in all testes examined histologically (Fig. 5F). Yellow bodies were frequenctly found in the male gonads, suggesting the degeneration and coalescence of oocytes. However, it has been pointed out by Sadovy and Shapiro2) and Shapiro3) that yellow bodies are not always derived from atretic eggs. The process of structural change in gonads shown in the present species is similar to that of the Lethrinidael2) and Labridae.13,14) Discussion Sadovy and Shapiro2) and Shapiro3) presented criteria for determination of hermaphroditism in fishes. Following their criteria, the results of the present study on Trichonotus filamentosus are summarized in Table 2. The male and female of the present species Table 2. Diagnostic features of protogynous hermaphroditism in Trichonotus fzlamentosus show marked differences in body size, morphology and coloration. Although direct observations of transformation from the female to male phases were not intended in the present study, specimens in an intermediate body form and color were found within the same size range where the male and female sizes overlapped. This strongly suggested that these specimens were in the process of transformation of body form and color from the female phase to the male phase. In the present study, several other reliable features to demonstrate sex change are available. One is the occurrence of bisexual gonads with degenerating ovarian elements and proliferating testicular tissue. Clear signs of atretic eggs were not found in these bisexual gonads, but small oocytes were scattered in the tissue. These oocytes were supposed to eventually degenerate and disappear as the testicular tissue proliferates. Bisexual gonads occurred only in specimens collected in October and December. These results suggest that the sex change of T. filamentosus occurs primarily after the spawning season. The origin of the male germinal tissue in the bisexual gonad is not known. Degenerating eggs in the yolk stage were seen in some of the specimens collected after the spawning season, but

6 male elements were not visible in these gonads. It is suggested that the male germinal tissue becomes apparent when most of the oocytes in the yolk stage have disappeared. The secondary testicular structure is also indicative of the sexual transformation process. In bisexual gonads, the lumen derived from the former ovarian cavity is large and the testicular tissue is covered with a thick membrane derived from former ovarian wall. The lumen becomes narrower and the membrane becomes thinner, as the testicular tissue proliferates. The secondary structure is recognizable even in mature testes, as well as many labrid species.18,14) In some of other protogynous species, it is known that the secondary structure of testes disappears as the testicular tissue proliferates. 15) Existence of the secondary structure in all testes examined histologically demonstrates that the males of the present species are inevitably derived from females by the protogynous sex change. Reasons why the bimodal size-frequency distribution can not be adopted as conclusive evidence for the sex change are that differential habitat preference and differential growth between males and females can cause such a bimodal size-frequency distribution.2,3) However, bimodal size-frequency distribution of T. filamentosus is apparently a result of protogynous sex change because males and females were collected in the same haul, suggesting no differential habitat preferences between the sexes. Furthermore, according to our underwater observations, the mating system of the present species is haremic, i n which a male and females occur within a narrow range.* Otolith analysis strongly suggests that the size differences between the male and female are due to their ages, but not to differential growth patterns between the two. The sex ratio of collected specimens of T filamentosus was near unity and was not supportive the protogyny in the present species. But it is not enough to deny the above stated evidences of hermaphroditism in T. f ilamentosus. Because, unity in the sex ratio can be easily ob - tained even in the protogynous species, if the smaller fish escape from the net. In the present study, observations on spawning behavior and serial biopsies of gonads were not * Unpublished data. conducted. However, the evidence presented above supports T. filamentosus being a protogynous hermaphrodite. Furthermore, lack of primary males in the present specimens strongly suggests that the species is monandric.7) Acknowledgments We are grateful to Prof. T. Okuda, Kyushu University, for his encouragement and advice through this study. Special thanks go to the Captain and crew of Fukuoka Prefectural Fisheries Experimental Station vessel for assisting collection of the specimens. Thanks are also due to two anonymous reviewers and Mr. Mark Pawluk of Nomo Fisheries Station, Nagasaki University, and Mrs. Marilynn A. B. Pawluk for their critical reading of the manuscript. References 1) K. Shimada and T. Yoshino: Japan. J. Ichthyol., 31, (1984). 2) Y. Sadovy and Y. Shapiro: Copeia, 1987, (1987). 3) Y. Shapiro: BioScience, 37, (1987). 4) G. L. Humason: Animal Tissue Techniques, Freeman, San Francisco, 1962, p ) J. M. Christensen: J. Cons. Expl. Mer., 29, (1964). 6) D. E. Chilton and M. Stocker: N. Am. J. Fish. Man., 7, (1987). 7) R. Reinboth: Mem. Soc. Endocr., 18, (1970). 8) S. T. H. Chan and W. S. B. Yeung: in "Fish Physiology" (eds. by W. S. Hoar, D. J. Randall, and E. M. Donaldson), Vol. 9B, Academic Press, New York, 1983, pp ) R. M. Ross: Copeia, 1984, (1984). 10) T. F. Hourigan and C. D. Kelley: Mar. Biol., 88, (1985). 11) M. Nakamura, T. F. Hourigan, K. Yamauchi, Y. Nagahama, and E. G. Grau: Env. Biol. Fish., 24, (1989). 12) P. C. Young and R. B. Martin: J. Fish. Biol., 21, (1985). 13) R. Reinboth: Zool. Jb. Abt. Zool. Physiol. Tiere, 69, (1962).14) A. Nakazono: Rep. Fish. Res. Lab., Kyushu Univ., No. 4, 1-64 (1979). 15) K. S. Cole and D. R. Robertson: Bull. Mar. Sci., 42, (1988). Nippon Suisan Gakkaishi: Formerly Bull. Japan. Soc. Sci. Fish.