〔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. A small piece of level was assumed to be continued, without conspi- ovary, approximately 0.01 g in wet weight, was dis- cuous peak, until November. Then, GI declined steeply sected out from each three sections in an ovary, i. e., from 60% in November to 25% in December. The just behind the gastric caecum (Section A) and at the maximum GI indicated the values larger than 60% distances of 1/2 (Section B) and 3/4 (Section C) of most of the year, and this suggested that ripe females the length between the posterior end of the gastric existed in the population throughout the year (Fig. 4). caecum whorl and the tip of the gonad (Fig. 3). Those The minimum GI in August and November were above small pieces of ovary were weighed with accuracy of 40%. The rest of the year except January and Octo- 0.1 mg to assess homogeneous distribution of oocytes ber, individuals with GI less than 12% appeared con- within the same ovary, and the number of oocytes sistently. The mean GI of the males did not change sig- were counted under a dissecting microscope. Finally, 8 nificantly, and indicated the values between 15 and 25 Reproduction of Turbo marmoratus in the Ryukyus 301

% from January to June and in December. The males Yaeyama Island Group are shown in Fig. 5. The mean indicated smaller GI ranges than those of the females GI for the females was 37 % in August, 1987, and this except for August. The GI declined steeply from 54% level was maintained until October (36%). The mean in November to 15% in December. GI rapidly decreased from 30% in November to 9%

Seasonal GI changes of T. marmoratus from the in December. Recovery of gonad condition appeared to commence between April and May, and the mean GI reached 35% in July 1988. The mean GI of the males fluctuated between August and November 1987 while that of the females did not. (2) short term changes The mean GI of the females (Fig. 6A) demonstrated fluctuations within the period from March 29 to July 29 in 1989. Abrupt declines of GI occurred between April 5 and 9, and between May 10 and 22. On the other hand, the mean GI of the males shifted without fluctuations from March 29 through May 10 (Fig. 6B). Short term changes in mean GI are also represented together with the lunar cycle (Fig. 6). Evidences which showed that spawning activities related to a certain lu- nar phase were not found.

Fig. 4. Seasonal changes in mean gonad indices com- bined for 5 years (1985-1989) in Turbomarmor- atus from TokunoshimaI. Perpendicular bars in- dicate ranges. A: female;B: male.

Fig. 6. Short term fluctuation of mean gonad indices in Turbo marmoratus in Tokunoshima I. (1989). The lunar phases are indicated together. Perpen- dicular bars indicate ranges. A: female; B: male.

Presumptive fecundity Fig. 5. Seasonal changes in mean gonad indices in (1) oocyte size Turbo marmoratus in the Yaeyama Island Group The oocyte size ranged from 180 to 350 rim, and the (1987-1988). Perpendicular bars indicate ranges. A: female; B: male. average was 255pm. Mean oocyte sizes for three 302 T. Komatsu el al. (1995)

gonad positions (Fig. 3) within the same ovary were existed all year round has been reported for Haliotis

not significantly different for 10 individuals measured r rufescens , H. cracheroidii14) and H. kamtschatkana15). (GI ranging from 51 to 73%, ANOVA, P>0.05). Trochus niloticus in Palau16) and in the Andaman There was no significant correlation between mean Islands17) spawned all year round. T. niloticus in the oocyte size and gonad index (GI>50%, r= 0.0429, Great Barrier Reef appeared to spawn, as a population, P>0.05, n=10). at approximately monthly intervals all year round but (2) number of oocytes in ovaries individual females may spawn once every two to four Number of oocytes per 0.01 g counted for three monthsls18). Giorge and DeMartini10) reported that with- positions of ovaries were significantly different in a spawning season, there were three types of (ANOVA, P<0.05). Mean number of 787 oocytes/ females that have different spawning patterns, such as 0.01 g at Section C (Fig. 3) was 13 to 16% greater complete spawning, incomplete spawning- in this pat- compared to those of 679 and 697 oocytes/0.01 g at tern, GI does not indicate full maturation of ovaries more anterior Sections A and B respectively. but these are packed with ripe oocytes, and nonspawn-

There was a significant positive correlation between ing in the same population in H. rufescens. It was pre- shell width of females and the weight of their ovaries sumed by histological examination that the residual (GI=62-73%, r=0.9789, P<0.05, n:8). Ovarian oocytes retained by incomplete spawner or nonspaw- weight of those eight T. marmoratusranged from 15.8 ner of female H. rufescens became necrotic, and even- g for a 12.5 cm individual to 121.2 g for a 19.2 cm in- tually, resorption of the oocytes would be occurred10) dividual (Table 2). Presumptive fecundities were Such the phenomenon was also suggested for T. mar- ranged from 1, 279, 000 oocytes for a female of 12.5 nioratus by unpublished data of histological examina- cm in shell width with GI of 62 % to 7,523,000 tion of ovaries by the authors, and this may suggest oocytes for a female of 19.3 cm with GI of 73% the existence of females which do not participate in re-

(Table 2) . The presumptive fecundities were signifi- production under certain conditions. cantly correlated to shell width (r=0.9821, P <0.05, The relationship between spawning cycle and the lu- n = 8) with the equation: F=64502.9e (o.2428sw) nar cycle has been reported for Trochus niloticus by where F is presumptive fecundity and SW is shell direct observations of tank-held animals19). However, width in cm. Grange20) did not find such relationships in two troehids as Melagraphia aethiops and Zediloma atro- Discussion virens and in one turbinid as Lunella smaragela. The re- It is necessary to confirm whether the year-round lationship between the timing of spawning and the lu- appearance of ripe ovaries is of retention of un- nar cycle was not clear in T. marmoratus. As a possible spawned oocytes or the results of ordinary process of reason why the relationship was not detected, it is oogenesis in T. marmoratus. The fact that ripe females thought that there probably were individuals that

Table 2. Presumptive fecundity in Turbo marmoratus from Tokunoshima I

* M ean of the three sections A, B and C, (•}S.D.). ** R ounded to the nearest thousands. Reproduction of Turbo marmoratus in the Ryukyus 303

spawn in different patterns or intensities, that were script. represented by GI variation, in the same population of References T. marmoratus as reported for H. rufescens10). Concern- ing with the spawning season, patterns and annual fre- 1) Yamaguchi, M. (1988a) : Marine ranching as a quency of spawning in T. marmoratus should be stu- means of resource management. Aquabiology, (57), died. For the species such as T. marmoratus whose 250-255. (in Japanese, with English abstract) population is seriously declined, however, it should 2) Yamaguchi, M. (1987a) : Fisheries management of also be necessary to consider whether they still exhibit gastropod resources. Aquabiology, (51), 260-265. their genuine reproductive ecology or not. (in Japanese, with English abstract) In the estimation of the number of eggs, there were 3) Devambez, L. C. (1961) : Report on a sup- significant differences in the number of oocytes per plementary survey of trochus and green snail 0.01 g of ovary among the different portions in an ov- reefs in the central and southern groups of the ary, especially between the basal part and the tip of New Hebrides. SPC unpublished report, 12 pp. the gonad. A similar trend was found in the Califor- 4) Yamaguchi, M. (1987b) : Biology of tropical nian red abalone H. rufescens10). Since no significant archaeogastropods as marine resources. Aquabiolo- difference was indicated in mean oocyte sizes in each gy, (53), 402-407. (in Japanese, with English of three positions in the ovary, the difference in the abstract) number of oocytes per 0.01 g may indicate the differ- 5) Yamaguchi, M. (1988b): Biology of the green snail ence in specific gravity of individual oocytes in each (Turbo marmoratus) and its resource management. portion. Non-synchronous maturation of oocytes in an Working Paper 11, SPC Workshop on Inshore ovary are suggested for Trochus niloticus2l), and Mono- Fishery Resources, 9 pp. donta liniata and Gibbula umbilicalis22). Asano21) re- 6) Yamaguchi, M. and K. Kikutani (1989) : Feasibility ported that maturation of gametes did not occur syn- study of green snail transplantation to the Feder- chronously throughout a gonad but that it occurred ated States of Micronesia. Rept. FAO/South Paci- gradually from the periphery of the gonoduct in T. fic Aquaculture Development Project, Suva, Fiji, niloticus in Palau. In T. marmoratus, there is also a pos- 25 pp. sibility that its maturation may not be synchronized 7) Murakoshi, M., T. Komatsu, and R. Nakamura throughout an ovary on account of qualitatively diffe- (1993): Development of mass seed production rent groups of oocytes within the same ovary. A more techniques of green snail (Turbo marmoratus) in precise examination on the ripening process (e. g., Okinawan water. Suisanzoshoku,41(3), 299-309. vitellogenesis) is needed to explain the possible differ- 8) Wells, F. E, and J. K. Keesing (1989) : Reproduc- ences in the specific gravity of the oocytes among sec- tion and feeding in the abalone Haliotis roei Gray. tions in an ovary. Aust. J. Mar. Freshwater Res., (40), 187-197. 9) Hahn, K. O. (1989) : CRC Handbook of Culture of Acknowledgments Abalone and Other Marine Gastropods, CRC The authors are most grateful to Professor M. Yama- Press, Florida, 348 pp. guchi who has supplied specimens of Turbo marmoratus 10) Giorgi, A. E, and J. D. DeMartini (1977) : A study and supervised this work. The offer of facilities by of the reproductive biology of the red abalone, Messrs M. Arakaki and J. Isa, the present and former Haliotis rufescens Swainson, near Mendocino, Cali- Directors of the Okinawa Prefectural Sea Farming fornia. Calif. Fish and Game, 63(2), 80-94. Center, is acknowledged. The authors deeply appreci- 11) Hayashi, I. (1980) : The reproductive biology of ate the offer of valuable specimens and information on the ormer, Haliotis tuberculata. J. Mar. Biol. Ass. U. the green snail by Mr K. Kikutani. Finally, the authors K., (60), 415-430. are thankful to Dr M. Yamaguchi, Dr N. A. Sloan and 12) Kakuda, N., T. Watanabe, N. Yurano, and M. Jin- Dr N. Bromage for the critical reading of this manu- nouchi (1986) : Studies on maturation and spawn- 304 T. Komatsu et al. (1995)

ing in Turbo cornutus. Ann. Rep. Yamaguchi Pref. 18) Nash, W. J. (1985) : Aspects of the biology of Open Sea Fish. Exp. St., (21), 38 pp. (in Japanese) Trochus niloticus and its fishery in the Great Bar- 13) Boolootian, R. A., A. Farmanfarmaian, and A. C. rier Reef region. Report to Fisheries Research Giese (1962) : On the reproductive cycle and Branch, Queensland Department of Primary Indus- breeding habits of two western species of Haliotis. tries, and the Great Barrier Reef Marine Park Au- Biol. Bull., 122(2), 183-193. thority, 210 pp. 14) Webber, H. H. and A. C. Giese (1969) : Reproductive 19) Heslinga, G. A. and A. Hillmann (1981) : Hatchery cycle and gametogenesis in the black abalone culture of the commercial top snail Trochus niloti- Haliotis cracheroidii (Gastropoda: Prosobranchiata). cus in Palau, Caroline Islands. Aquaculture, (22), Mar. Biol., 4(2), 152-159. 35-43. 15) Sloan, N. A. and P. A. Breen (1988) : Northern aba- 20) Grange, K. R. (1976) : Rough water as a spawning lone, Haliotis kamtschatkana, in British Columbia: stimulus in some trochid and turbinid gastropods. fisheries and synopsis of life history information. N. Z. Journal of Marine and Freshwater Research, 10 Canadian Special Publication of Fisheries and (1), 203-216. Aquatic Sciences, (103), 46 pp. 21) Asano, N. (1939) : On the spawning season of top 16) Heslinga,G. A. (1980): Report on Palau's tochus shell. Suisan Kenkyushi, 34 (1), 36-38. (in hatchery project. SPC Fisheries Newsletter, (20), Japanese) 4-8. 22) Underwood, A. J. (1972) : Observations on the re- 17) Rao, H. S. (1937) : On the habitat and habits of productive cycles of Monodonta lineata, Gibbula Trochus niloticus Linn. in the Andaman Seas. Rec. umbilicalis and G. cineraria. Mar. Biol., (17), 333- Indian Museum, (39), 47-82. 340.

琉 球 列 島 にお け るヤ コ ウ ガイ の生 殖 につ い て

小 松 徹1)・ 村 越 正 慶2)・ 中 村 良太1)

(1)琉球 大 学 理 学 部 海 洋 学 科 ・2)沖縄 県 栽 培 漁 業 セ ン ター)

琉 球 列 島 の そ れ ぞ れ 北 部 と南 部 に位 置 す る,徳 之 島 お よび 八 重 山 諸 島 に お け る ヤ コ ウ ガ イ,Turbo marmoratusの 性 比 は,そ れぞ れ1対1を 示 した 。 生 殖 腺 指 数 が50%を 越 え る 雌 個 体 に 関 して は,卵 巣 内 の 異 な る 部位 に お け る成 熟 卵 の平 均 卵 径 値 に有 意 な差 は見 られ ず,ま た,異 な る 生殖 腺 指 数 を示 す個 体 間 にお け る平 均 卵 径 値 に も有 意 な差 は現 れ なか っ た。 一 方,上 述 の 個 体 につ い て卵 巣 の単 位 重 量 当 た りの卵 数 を比 較 した結 果,有 意 差 が 見 ら れ た こ と に よ り,卵 巣 内 の 部 位 の 違 い に よる卵 の比 重 変 化 が示 唆 さ れ た。 成 熟 した 雌1個 体 の 卵 巣 中 に お け る 最 大 孕 卵 数 は,殻 幅12.5cmの 最 小 標 本 個 体 に お い て 1.279×106粒,19.3cmの 最 大 標 本 個 体 に お い て7.523×106粒 と推 定 さ れ,個 体 サ イ ズ の増 加 と と もに 孕卵 数 は指 数 関 数 的 に増 加 した。 ヤ コ ウ ガ イの 主 た る産 卵 期 は,徳 之 島 に お い て は8～11月,八 重 山諸 島 に お い て は7～11月 で あ る と推 定 され たが,い ず れ の 海 域 にお い て も雌 雄 の 成 熟個 体 が周 年 出現 した。 徳 之 島 産 ヤ コウ ガ イ に 関 して,産 卵 行 動 と月 齢 周 期 との 関 係 を調 べ た が,明 確 な相 関 は 見 出 され な か っ た 。