〔水産増殖43巻3号297-304Suisanzoshoku(1995-H7)〕
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〔水産増殖43巻3号297-304SUISANZOSHOKU(1995-H7)〕 A Study on the Reproduction of the Green Snail, Turbo marmoratus in the Ryukyu Islands, Southern Japan Toru KOMATSU1),*1, Masayoshi MURAKOSHI2), *2, and Ryota NAKAMURA1), *3 1)Department of Marine Sciences , University of the Ryukyus, 1 Senbaru, Nishihara, Okinawa 903-01, Japan 2) Okinawa Prefectural Sea Farming Center , 853-1 Ohama, Motobu, Okinawa 905-02, Japan Abstract The green snail, Turbo marmoratus had sex ratios of 1 : 1 both in Tokunoshima Island and the Yaeyama Island Group, the northern and southern Ryukyu Islands, Japan. No significant differences were found in mean oocyte sizes either between different sections in an ovary or between ovaries with different gonad bulk indices (GI: percentage gonad area to the total cross- sectional area of the gonad-digestive gland) when GI exceeded 50%. The number of oocytes per unit weight was significantly different among different portions in an ovary, and this could be attributed to the difference in the specific gravity of oocytes. The maximum number of oocytes in an ovary was estimated between 1.279 •~ 106 and 7.523 •~ 106 for females of 12.5-19.3 cm in shell width. The main spawning season was August-November in Tokunoshima I, and July-November in the Yaeyama Island Group. Mature individuals were found during all seasons in both localities. The relationship between the spawning activity and the lunar cycle was examined for T. marmoratus from Tokunoshima I., but the relationship was not found. The largest species of the family Turbinidae, the the Ryukyu Islands. Over-exploitation occurrs else- green snail Turbo marmoratus is dioecious, and broad- where in the western Pacific and Southeast Asia1,2) cast its gametes into the water. It can exceed 20 cm in where T. marmoratus is economically important. Today, shell width and 3kg in total weight. It has been ex- T. marmoratus does not constitute an important fishing ploited from ancient times as one of the most important target as the animal is just an occasional by-product fishery resources in the Ryukyu Islands, Okinawa, among others caught by diving fishery in Okinawa. Japan1). Not only has the meat been eaten, but its nac- In order to help facilitate recovery of this resource, reous shell has been valued for mother-of-pearl inlay a positive approach such as restocking with mass- works and various ornaments. produced seed may be necessary in addition to closure Yamaguchi1) noted that the over-exploitation of T. of T. marmoratus harvesting in the Ryukyu Islands and marmoratus was already in progress in the 1880's in other areas. Compared with other commercially Received : April 4, 1994 Key words: Green snail, Turbo marmoratus, Reproduction *1 Present address: Overseas Fishery Cooperation Foundation , Akasaka Twin Tower, 17-22, Akasaka 2, Minato-ku, Tokyo 107, Japan *2 Present address: Okinawa Prefectural Fisheries Experimental Station , 1 Nishizaki, Itoman, Okinawa 901-03, Japan *3 Fisheries & Aquaculture International Co ., Ltd., 7th Kojimachi-bldg., 4-5 Kojimachi, Chiyoda, Tokyo 102, Japan 298 T. Komatsu et al. (1995) important gastropods like Haliotis spp., Turbo cornutus Guts and gonad-digestive glands of 501 T. marmor- and Trochus niloticus, information on the biology and atus were excised, and then fixed and stored in 10 % ecology of T. marmoratus is limited1,2,3,4,5,6), *1, *2 formalin solution for the examinations of sex ratio, Mass seed production of T. marmoratus has been fecundity and gonad bulk index mentioned later. undertaken at the Okinawa Prefectural Sea Farming Center since 19897. In order to establish restocking Measurement of shell size methods of T. marmoratus with mass produced seed, in- In order to estimate the shell sizes, which were formation on fisheries biology of this species is needed. necessary for estimation of fecundities, of the speci- To make seed production of T. marmoratus predict- mens without shells, shell widths (SW, after able, reproductive cycle, sex ratio, fecundity and re- Devambez3)) and the operculum greater diameters lationships between spawning activities and the lunar (OGD) of specimens from Tokunoshima I., the cycle were examined in the present study. YaeyamaIsland Group and Okinawa I.: 25, 26 and 29 individuals respectively, ranging 11.3-21.5 cm in SW, Materials and Methods were measured (Fig. 2). The correlation between SW A total of 530 T. marmoratus, of which 80 were and OGD was shown by the equation: SW=-3.8422 whole animal and 450 were soft body with operculum +2.5945 OGD (r=0.8913, P<0.05, n=80). without shell, were collected from different areas in different months from 1985 to 1990 (Table 1). Speci- Gonad bulk index mens were collected at Tokunoshima Island belongs to Gonads are located along the entire length of the the Amami Island Group, the Yaeyama Island Group digestive gland, and overlap it laterally (Fig. 3). In and Okinawa Island (Fig. 1) . In order to examine a re- gonads of mature individuals, the sexes were disting- lationship between a lunar phase and a spawning bout, uishable macroscopically, i. e., ovaries exhibited a short term changes in gonad conditions were studied. granular structure in olive green or dark green just For this purpose, specimens were collected at intervals beneath a translucent epithelium, while testes appeared of 4 to 47 days spanning a lunar cycle during the much smoother with a creamy white color. period from March 29 to July 29, 1989 from Toku- The gonad bulk index (GI) of Newman (1967, cited noshima I. in Wells and Keesing8) and Hahn9) ) was used to Table 1. Numbers of specimens of Turbo marmoratus in each collecting area *1 Honma, K. (1987) : Growth of coral-reef gastropods Trochus niloticus and Turbo marmoratus. Undergraduate thesis, Department of Marine Sciences, Univ. of the Rvukyus (unpubl.), 29 pp. *2 Shimoike, K. (1990):A study on the stock-enhancement of Turbo marmoratus. Report of Aka-jima Marine Sci- ences Laboratory, Establishment of Tropical Marine Ecological Research (unpubl.), 14 pp. (in Japanese) Reproduction of Turbo marmoratus in the Ryukyus 299 Fig. 1. Map of the Ryukyu Islands. Areas enclosed by broken lines are where Turbo marmoratus sampled. describe gonad conditions. Gonad-digestive glands were cut at the three positions described by Newman (1967, cited in Giorgi and DeMartini10)), and GI were monitored at Section A, where is just behind the gas- tric caecum11,12). *3(Fig. 3). The diameters of the tot- al gonad-digestive gland and that of the digestive gland at Section A were measured to the nearest 0.1 mm with vernier calipers. Gonad bluk index obtained by these measurements (GI') were calculated from GI' =100{(R2-r2)/R2} , where R and r are the radii of the total gonad-diges- tive gland and of the digestive gland respectively. The GI' values were correlated with GI values which were obtained preliminarily based on direct measurements of the cross-sectional areas of gonad-digestive gland and that of gonadal portion. Therefore, the GI' values converted into GI values by the following regression equations: GI=-1.7115+0.9349GI' (r=0.9693, P< 0.05, n=78) for female and GI = 0.5367 + 0.842GI' Fig. 2. Measurements of shell width (SW, after (r = 0.9478, P<0.05, n=77) for male. Devambez3)) and operculum greater diameter (OGD) of Turbo marmoratus. *3 T anaka, H. (1988): Reproductive biology and relative growth of Turbo argyrostomus. Undergraduate thesis, De- partment of Marine Sciences, Univ. of the Ryukyus (unpubl.), 24 pp. (in Japanese) 300 T. Komatsu et al. (1995) ovaries with GI higher than 60 % were examined the number of oocytes contained, using the mean number of oocytes per 0.01 g of those at the three sections in an ovary and the total wet weight of the ovary. For 10 females selected from every GI level of 50, 60 and 70%, sizes of 50 oocytes were determined with a profile projector and vernier calipers under magnifica- tion of •~50. Most of the oocytes measured, however, appeared square or rectangular in profile, so that two neighboring sides of a quadrilateral oocyte profile were measured. The area of the oocyte profile was calculated, and then it was calculated into a diameter of a circle, which was considered to be oocyte size in the present study. Results Sex ratio Twenty-six out of 372 individuals from Tokunoshi- ma I. were spent. Of the remaining 346 snails, 175 (50.6%) were female and 171 (49.4%) were male. For the specimens from the Yaeyama Island Group, 67 Fig.3. Cut-portions (above) and linear measurement specimens (52.3%) were female and 61(47.7%) (below) of the total 'gonad-digestive gland' dia- were male but sex could not be determined in one meter (R) and that of digestive gland (r) . Sec- tion A: just behind the gastric caecum; Sections specimen. In both localities, sex ratios were not signifi- B and C: at the distances of 1/2 and 3/4, respec- cantly different (x2 test, P>0.05) compared to the tively, of the length between the posterior end of expected 1:1 ratio. the gastric caecum whorl and the tip of the gonad. g: gonad; dg: digestive gland. Gonad bulk index Presumptive fecundity (1) seasonal changes Twenty female T. marmoratus (GI>50%) from the Fig. 4 is the monthly GI changing patterns of T. mar- specimens collected at Tokunoshima I. in 1989 were moratus from Tokunoshima I. for the five years com- assessed for fecundity by means of the number of eggs bined (1985-1989). The mean GI for females was present in ovaries, which was called as presumptive maintained above 30% -level most of the year. The fecundity. Ovaries were excised, and their total wet mean GI of 60 % was marked in August, and such the weights were measured individually.