Keita KODAMA, 1 * a Takamichi SHIMIZU, 2 Takashi YAMAKAWA 1 AND Ichiro AOKI 1

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1 Blackwell Science, LtdOxford, UK FISFisheries Science Blackwell Science Asia Pty Ltd 705October Reproductive biology of Oratosquilla oratoria K Kodama et al /j x Original Article734745BEES SGML FISHERIES SCIENCE 2004; 70: Reproductive biology of the female Japanese mantis shrimp Oratosquilla oratoria (Stomatopoda) in relation to changes in the seasonal pattern of larval occurrence in Tokyo Bay, Japan Keita KODAMA, 1 * a Takamichi SHIMIZU, 2 Takashi YAMAKAWA 1 AND Ichiro AOKI 1 1 Department of Aquatic Bioscience, Graduate School of Agricultural and Life Sciences, The University of Tokyo, Yayoi, Bunkyo, Tokyo and 2 Resources and Environment Division, Kanagawa Prefectural Fisheries Research Institute, Jyogashima, Misaki, Miura, Kanagawa , Japan ABSTRACT: The reproductive traits and the monthly larval abundance of the mantis shrimp Oratosquilla oratoria were investigated in Tokyo Bay, Japan, in The goal of the study was to elucidate the cause of changes in the monthly pattern of larval abundance from the 1980s to the 1990s as these changes relate to variation in the stock size of the adult shrimp. Oogenesis was divided into 10 stages by histological observation. The developmental stage of oocytes in an individual s ovary was synchronous, suggesting that almost all the oocytes in an ovary are spawned at the same time. The size at first maturity was estimated to be 7 body length (BL) < 8 cm. Fecundity was expressed as a function of BL, ranging from eggs for 8 cm BL to eggs for 14 cm BL. Small female shrimps (<10 cm BL) spawned around August. Most large female shrimps ( 10 cm BL) spawned around May, and some large female shrimps also spawned until September. Although most large female shrimps spawned in spring, the larval abundance was low before July and high from August onwards. The results suggest that a substantial decrease in the stock size of large individuals causes the low larval abundance before July. KEY WORDS: fecundity, larvae, mantis shrimp, maturation, Oratosquilla oratoria, reproductive structure, spawning season, Tokyo Bay. INTRODUCTION The Japanese mantis shrimp Oratosquilla oratoria (de Haan) (Crustacea: Stomatopoda) is found on muddy bottoms in coastal waters around Japan, and is exploited commercially in many regions, mainly by small bottom trawlers. Oratosquilla oratoria is the most dominant species in the demersal fauna of Tokyo Bay, central Japan, 1 and it is the most commercially valuable species for the bottom-trawl fishery in Tokyo Bay. 2,3 The catch of O. oratoria from Tokyo Bay was high in the mid to the late 1980s but dropped dramatically in the early 1990s, 3 which has resulted in *Corresponding author: Tel: Fax: kjr8455@a6.mnx.ne.jp a Present address: Endocrine Disruptors and Dioxin Research Project, National Institute for Environmental Studies, Onogawa, Tsukuba, Ibaraki , Japan. Received 12 November Accepted 15 April a setback for the bottom-trawl fishery. Concurrent with the variation in the catch, changes in the monthly pattern of larval abundance have been reported: larval abundance was high from June to July and decreased from August onward in the mid 1980s, 4 whereas it was high from August to September and low before July in the 1990s. 5 The changes in the monthly pattern of larval abundance suggest that the shrimp s reproductive patterns have changed. Size-related differences in spawning season were reported in the mid 1980s: most large female shrimps ( 10 cm in body length [BL]) spawned around May, whereas most small female shrimps (<10 cm BL) spawned around August. 6 The larvae occurring before July seem to derive from broods spawned around May by large female shrimps. 2 Although several postulations concerning the change in the monthly pattern of larval abundance are possible, the following hypothesis was examined: because the spawning season of large female

2 Reproductive biology of Oratosquilla oratoria FISHERIES SCIENCE 735 shrimps around May has come to be delayed since the early 1990s, there is low larval abundance before July. To test the hypothesis, the reproductive traits of O. oratoria with regard to monthly pattern in the gonadosomatic index (GSI) and in the histological development of the ovary for each size class, size at first maturity and fecundity were investigated. MATERIALS AND METHODS Samples Samples of female O. oratoria were collected on a monthly basis from January to December in 2002 at seven sampling stations during bottom-trawl surveys in the southern part of Tokyo Bay, Japan (Fig. 1). Sampling surveys were conducted during the daytime by a small 5-t trawler chartered for the surveys. A beam trawl with a cod end mesh size of 1.8 cm was used for the sampling. The mouth of the trawl was 5.5 m wide and 0.6 m high. A total of female shrimps of various sizes were caught in the sampling. Sub-samples of more than 50 individuals covering a wide size range of female shrimps were taken randomly every month for the present study, amounting to a total of 688 individuals. These subsamples were fixed on board in 10% neutral buffered formalin, except for several individuals each month that were fixed in Davidson s fixative. 7 Before fixation, the exoskeleton of each individual was slit with dissecting scissors from the dorsal side of the sixth thoracic somite to the telson to expose the ovary directly to fixative without damaging the ovary. In addition, to reveal the final maturation stage of the oocytes, female shrimps with vitellogenic ovaries were collected separately from the subsamples during the sampling in July 2002 and kept in 1-t aquariums until September An individual in the middle of spawning under the rearing condition was collected in August 2002 and fixed in Davidson s fixative in the same manner described above. Detailed descriptions of the rearing procedure have been given in previous studies. 8,9 Body length (from the base of the rostrum to the anterior edge of the median notch of the telson) was measured to the nearest 0.1 cm in the laboratory. In addition, body weight and gonadal weight Fig. 1 Location of ( ) seven sampling stations for female Oratosquilla oratoria and ( ) 15 sampling stations for the larvae in Tokyo Bay, Japan. Dotted lines show depth contours in meters.

3 736 FISHERIES SCIENCE K Kodama et al. were measured to the nearest 0.10 g and 0.01 g, respectively. The GSI was calculated by dividing gonadal weight by body weight and multiplying by 100. Histological procedure Tissue from the middle of one ovary from each female O. oratoria was dissected for histological examination. The tissues were dehydrated in ascending ethanol concentrations from 70 to 100%, transferred to Lemosol (Wako Pure Chemical Industries, Osaka, Japan) and embedded in paraffin. The tissues were sectioned to 6 mm and stained with Mayer s hematoxylin eosin (HE). Sections from the fore, middle and rear areas of the ovaries from five individuals per month were preliminarily investigated for any histological differences among the areas. To examine the presence of oil globules in the cytoplasm of the oocytes, selected tissues were fixed in 10% neutral buffered formalin, immersed in 30% sucrose solution overnight at 4 C, embedded in optimal cutting temperature compound (Sakura Finetek, Tokyo, Japan), made into 6-mm sections using cryostat at -20 C and stained with Sudan Black B. The histological stage of development was determined by making reference to those of other crustaceans The longest diameters of 20 oocytes and the thicknesses of 30 follicle cells were measured for each developmental stage. The data are expressed as mean ± standard deviation. Reproductive cycle and size at first maturity The monthly patterns of ovarian developmental stage and the mean GSI ± standard error were investigated for the size classes that could attain vitellogenic oocytes at 1-cm BL intervals as follows: 7 BL < 8 cm, 8 BL < 9 cm, and so forth. Individuals of BL 12 cm were defined as a single size class, because of the small sample size for the individuals of BL >13 cm. Size at first maturity was defined as the smallest size class at which more than 50% of individuals attained vitellogenic oocytes in the ovary. Fecundity Female shrimps with vitellogenic oocytes in a late developmental stage were used for the estimation of fecundity. Three samples of a fixed ovary of approximately 3 30 mg were dissected for each individual from three positions (fore, middle and rear) of the ovary. Each of the samples was weighed to the nearest 0.1 mg, immersed in 20% glycerol solution and counted under a dissecting microscope by manipulating the ovary with forceps and a needle. The following equation was used to estimate the fecundity of each individual: GW F = F (1) s GWs where F is the estimated fecundity of an individual, F s is the number of oocytes in a sample, GW is the total weight of the ovary and GW s is the weight of the sample of the ovary. The mean of the estimated fecundity of the three samples within each individual was regarded as the individual s fecundity. Using this method, the coefficient of variation (CV) of the estimated fecundity for each individual was less than 10%. Fecundity in relation to BL was estimated using the following equation: F(x) = ax b (2) where x is BL in cm, F(x) is the fecundity against BL, and a and b are constants. Larval data Larval samples were collected monthly during the daytime from May to October in 2002 by the research vessel (R/V) Ushio, Kanagawa Prefectural Fisheries Research Institute. The sampling was carried out using a remodeled NORPAC net (net mouth area m 2, net length 1.3 m, mesh size 0.33 mm; Rigosha, Saitama, Japan) with a flow meter, by a vertical tow through the entire water column at 15 stations in Tokyo Bay (Fig. 1). The distance traveled by the net during a tow was calculated as (stop revolution number - start revolution number) blade constant (3) where stop and start revolution numbers were the number of revolutions recorded on the flow meter at the end and beginning of each tow, respectively, and the blade constant was The sample volume was calculated as the product of the distance traveled by the net and the net mouth area. Carapace length (CL) of the larvae (the distance from the base of the anterolateral spine to the base of the posterolateral spine on the carapace) was measured to the nearest 0.1 mm. Oratosquilla oratoria have 11 larval stages, and the duration of the larval development is approximately 5 7 weeks. 13 The first and second stage larvae remain on the bottom, and the pelagic phase starts after the second stage. 13 The smallest size class of larvae collected in the water column by the NORPAC net

4 Reproductive biology of Oratosquilla oratoria FISHERIES SCIENCE 737 was 0.9 CL < 1.1 mm, corresponding to the third stage that is, 3 7 days after the hatch date. 13 In the present study, the larval abundance was calculated for the third stage larvae <1.1 mm CL to minimize the lag between the date of egg-laying and larval sampling, thereby clarifying the relationship between the peak of spawning and the larval occurrence. The larval abundance at each station was calculated as the number of larvae at the third stage divided by the sample volume, and the mean larval abundance of all the sampling stations for each month was regarded as the monthly larval abundance, measured in individuals/m 3 per tow. RESULTS Oogenesis Oogenesis was divided into 10 stages: (i) oogonium stage; (ii) early chromatin nucleolus stage; (iii) late chromatin nucleolus stage; (iv) oil globule stage; (v) yolkless stage; (vi) primary yolk granule stage; (vii) secondary yolk granule stage; (viii) prematuration stage; (ix) maturation stage; and (x) atretic stage. Oogonium stage The oocyte diameter is 10.0 ± 2.4 mm (Fig. 2a). Strongly basophilic chromatin materials show scattering distribution in the nucleus. The cytoplasm is hardly visible. Early chromatin nucleolus stage The nucleus and oocyte increase in diameter to 18.9 ± 3.8 mm and 26.3 ± 8.5 mm, respectively (Fig. 2a). Strongly basophilic chromatin materials are visible in the nucleus. The cytoplasm is thin and basophilic. Follicle cells around the oocyte are not conspicuous. Late chromatin nucleolus stage The nucleus and oocyte increase in diameter to 30.6 ± 5.8 mm and 55.5 ± 18.8 mm, respectively (Fig. 2b). A large, round, strongly basophilic nucleolus appears in the nucleus. The cytoplasm remains basophilic. Follicle cells of 4.4 ± 1.3 mm in thickness are visible around the oocytes. Oil globule stage The nucleus and oocyte increase in diameter to 45.1 ± 6.7 mm and ± 22.6 mm, respectively (Fig. 2c). The cytoplasm is weakly basophilic. Vacant globules are found in the cytoplasm that is not stained with HE. These globules are stained black with Sudan Black B, indicating the presence of oil globules. Follicle cell thickness is 4.6 ± 1.0 mm. Yolkless stage The nucleus and oocyte increase in diameter to 52.6 ± 6.7 mm and ± 21.6 mm, respectively (Fig. 2d). The oil globules are not visible at this stage. The cytoplasm is weakly basophilic. Follicle cell thickness is 4.2 ± 0.8 mm. Primary yolk granule stage The nucleus and oocyte increase in diameter to 54.3 ± 10.5 mm and ± 46.2 mm, respectively (Fig. 2e). Acidophilic yolk granules appear from the circumference of the cytoplasm. The yolk granules do not occupy the inside of the cytoplasm yet. Although the nucleus does not usually have an affinity for dyes, it becomes basophilic during the latter part of this stage. Follicle cell thickness is 4.1 ± 1.0 mm. Secondary yolk granule stage The nucleus and oocyte increase in diameter to 59.5 ± 13.4 mm and ± mm, respectively (Fig. 2f). Acidophilic yolk granules increase in number and occupy the cytoplasm completely. The shape of the oocyte becomes more irregular and polygonal. The nucleus becomes basophilic and is warped out of the outline. Follicle cells become thinner, reaching 2.4 ± 0.7 mm. Pre-maturation stage The oocyte diameter reaches maximum (608.4 ± 66.8 mm) by the increase in acidophilic yolk granules (Fig. 2g). The nucleus shrinks to 48.1 ± 17.8 mm in diameter and moves to the periphery of the oocyte. Thereafter, germinal vesicle breakdown (GVBD) occurs. Follicle cell thickness is 2.3 ± 0.5 mm. Maturation stage Figure 2h shows the oocyte at the maturation stage collected from a female shrimp in the middle of

5 738 FISHERIES SCIENCE K Kodama et al. Fig. 2 Oocyte development of Oratosquilla oratoria in Tokyo Bay. (a) Oogonium and early chromatin nucleolus stage; (b) late chromatin nucleolus stage; (c) oil globule stage; (d) yolkless stage; (e) primary yolk granule stage; (f) secondary yolk granule stage; (g) pre-maturation stage; (h) maturation stage; (i) atretic stage. All of the oocytes were stained with Mayer s hematoxylin eosin, except (c), which were stained with Sudan Black B. (a c) Bar = 10 mm; (d, e, i) bar = 50 mm; (f h) bar = 100 mm. ec, oocyte at early chromatin nucleolus stage; f, follicle cell; fy, fused yolk; n, nucleus; nu, nucleolus; oi, oil globule; oo, oogonium; pf, postovulatory follicles; ve, vitelline envelope; y, yolk granule. spawning under rearing conditions at night. The oocyte diameter is ± 65.9 mm at this stage marginally smaller than oocytes at the prematuration stage, although a significant difference in diameter between the two stages was not detected (Student s t-test, P = 0.37). It is difficult to find the nucleus in the cytoplasm filled with acidophilic yolk granules at this stage. Some yolk granules are partly fused to one another. Follicle cells are not found around the oocyte; instead, a vitelline envelope with a thickness of approximately 1.6 mm coats the periphery of the oocyte. Postovulatory follicles, indicative of ovulation, are found near the oocytes at this stage.

6 Reproductive biology of Oratosquilla oratoria FISHERIES SCIENCE 739 Table 1 Frequency distribution of GSI, mean and SD of GSI and constituent oocytes for each ovarian developmental stage of female Oratosquilla oratoria + Frequency at each ovarian developmental stage (%) GSI (%) Stage 1 Stage 2 Stage 3 Stage 4 Stage 5 Stage No. samples Mean GSI (%) SD Constituent oocyte Oo Og Pyg Syg Pm Oo Ecn Yl Mt Ecn Lcn Lcn At + Females < 7 cm body length were omitted from the measurement. At, Atretic stage; Ecn, early chromatin nucleolus stage; GSI, gonadsomatic index; Lcn, late chromatin nucleolus stage; Mt, maturation stage; Og, oil globule stage; Oo, oogonium stage; Pm, pre-maturation stage; Pyg, primary yolk granule stage; SD, standard deviation; Syg, secondary yolk granule stage; Yl, yolkless stage. Atretic stage The matured oocytes, remaining in the ovary after spawning, begin to be atretic (Fig. 2i). The vitelline envelope is not found, and yolk granules in the cytoplasm begin to fuse from the peripheral region. Developmental stage of ovary The developmental stage of almost all oocytes in the fore, middle and rear areas of ovaries was isochronal, which suggests synchronous development of oocytes in the whole ovary. Because of the limited sample size, the 10 developmental stage of oocytes were combined into six stages for the following analyses (Table 1), which indicate the developmental stages of the ovary: stage 1 (immature) was made up of oocytes at the oogonium and early and late chromatin nucleolus stages; stage 2 (previtellogenesis) was made up of oocytes at the oil globule and yolkless stages; stage 3 (primary vitellogenesis) was made up of oocytes at the primary yolk granule stage; stage 4 (secondary vitellogenesis) was made up of oocytes at the secondary yolk granule stage; stage 5 (maturation) was made up of oocytes at the prematuration and maturation stages; and stage 6 (spent) was made up of oocytes at the oogonium and early and late chromatin nucleolus stages, with a few oocytes at the atretic stage. No individuals <7 cm BL attained vitellogenesis. The minimum size with a vitellogenic ovary in the field sample was 7.1 cm BL at stage 4. The relationship between GSI and each ovarian developmental stage for female shrimps 7 cm BL, the stage at which sexual maturity was potentially attainable, was investigated (Table 1). The GSI remained low before yolk accumulation at stages 1 and 2 and began to increase after yolk accumulation started at stage 3. The GSI showed considerable scatter around the mean value of 11.6% at stage 4, probably because of the rapid increase in the size of oocytes during the secondary yolk granule stage. Matured ovaries at stage 5 showed high GSI values of more than 18%. The GSI of all female shrimps with ovaries at stage 6 was less than 2%. Reproductive cycle and size at first maturity The size at first maturity, at which 50% of individuals attained vitellogenic ovaries (stage 3 6), was found in the size class of 7 BL < 8 cm in July (Fig. 3). Frequencies of female shrimps with vitellogenic ovaries increased as the body size increased (Fig. 3).

7 740 FISHERIES SCIENCE K Kodama et al. Fig. 3 Monthly changes in percent occurrence of ovaries at the six developmental stages of female Oratosquilla oratoria in Tokyo Bay in 2002 for each size class. The classification of the developmental stages of the ovary was based on histological observation: stage 1 immature; stage 2 previtellogenesis; stage 3 primary vitellogenesis; stage 4 secondary vitellogenesis; stage 5 maturation; and stage 6 spent. Numbers on each column indicate sample size. In the 7 10 cm BL classes, a peak of both the occurrence of individuals with vitellogenic ovaries and GSI was found from July to August (Figs 3,4). Few individuals attained vitellogenic ovaries during winter to early spring in these size classes (Fig. 3). In the body size classes of 10 cm BL, vitellogenesis began in winter and early spring; the occurrence of individuals with vitellogenic ovaries and GSI reached a peak in April to May and decreased gradually until October (Figs 3,4). Individuals with vitellogenic ovaries were not found from October onwards (Fig. 3), during which time the GSI fell to approximately 1% in all size classes (Fig. 4). To examine the relationship between growth and maturity, three levels (25 49%, 50 74% and %) of frequency of female shrimps with vitellogenic ovaries (Fig. 3) were incorporated into the monthly BL histograms (Fig. 5). There were two modes from January to April (Fig. 5). The right mode ( 10 cm BL) includes the population born in 2000 and the preceding years, the majority of which spawn in spring. The left mode (4 cm BL) is likely to include the population born in 2001, some of which attain the minimum size at maturity and spawn in summer. Young-of-the-year individuals began to appear in October approximately 4 cm BL in the histogram (Fig. 5).

8 Reproductive biology of Oratosquilla oratoria FISHERIES SCIENCE 741 Fig. 4 Monthly changes in the mean gonadosomatic index of female Oratosquilla oratoria in Tokyo Bay in 2002 for each size class. Vertical bars indicate standard errors. Individuals with ovaries at stage 5 or 6 were scarcely found in field samples (8 of 660 individuals of 7 cm BL; Table 1). Only two individuals at stage 5 were collected in the field survey in May 2002: one had a 9.9-cm BL with oocytes at the prematuration stage, and the other had a 12.9-cm BL with oocytes at the maturation stage. Six individuals classified into stage 6 had ovaries with a few oocytes in the atretic stage in a section in which the late chromatin nucleolus stage was the most progressed stage of oocytes, suggesting the occurrence of spawning a few days prior to the sampling date. Fecundity A total of 25 individuals, ranging from 7.1 cm to 13.9 cm BL, with ovaries at stage 4 were investigated for the estimation of fecundity. The relationship between body length x in cm and fecundity F(x) is given by F(x) = 58.4 x 2.79 (r 2 = 0.93, n = 25, P < 0.001) (4). From this formula, the fecundity of shrimp of 8, 10, 12 and 14 cm BL was estimated to be , , and eggs, respectively (Fig. 6). Monthly pattern in larval abundance Figure 7 shows the monthly pattern of larval abundance from May to October Larvae occurred from June to September. The larval abundance was considerably higher in August and September than in June and July. DISCUSSION Oogenesis Studies on the oogenesis of stomatopods are scarce. Yamazaki and Fuji classified the oogenesis

9 742 FISHERIES SCIENCE K Kodama et al. Fig. 6 Relationship between fecundity F(x) and body length x for Oratosquilla oratoria in Tokyo Bay. Fig. 5 Histograms of the body length (BL) at size classes of 1-cm BL intervals (x BL < x + 1 cm) of female Oratosquilla oratoria in Tokyo Bay between January and December Three levels of frequency of female shrimps with vitellogenic ovaries (stage 3 primary vitellogenesis, stage 4 secondary vitellogenesis, stage 5 maturation and stage 6 spent) were incorporated into the histograms: ( ) 25 49%; ( ) 50 74%; and ( ) %. of Squilla (= Oratosquilla) oratoria in Mutsu Bay, northern Japan, roughly into four stages, but the description of some oocyte developmental stages was apparently merged or absent in their study. 14 In a study of other stomatopods, Deecaraman and Subramoniam classified the ovarian development of Squilla holoschista on the Madras coast in east India into four stages, but descriptions for each stage were incomplete. 15 Fig. 7 Monthly changes in the mean larval abundance of Oratosquilla oratoria in Tokyo Bay between May and October Vertical bars indicate standard errors. In the present study, the oogenesis of O. oratoria was investigated in detail and divided into 10 stages based on morphological differences, size and affinity to stains. The oogenesis of O. oratoria generally resembled that of brachyuran crabs. 10,12 Oil globules were found in the cytoplasm of oocytes before yolk accumulation, which was not described in a previous study by Yamazaki and Fuji. 14 Yano suggested that oil globules are functionally related to the accumulation of yolk protein in prawns and crabs. 11 In brachyuran crabs, cytoplasm changes from basophilic to acidophilic after oil globules move to the periphery of the oocyte and diffuse. 10,12 Yolk granules of O. oratoria begin to accumulate from the periphery of the oocyte at the primary yolk granule stage, as observed in

10 Reproductive biology of Oratosquilla oratoria FISHERIES SCIENCE 743 brachyuran crabs, crayfish and lobsters, 12,16,17 which is different from those of penaeid shrimps. In Penaeus japonicus, yolk accumulation begins from the center of the oocyte. 11 Moreover, a rapid increase in the size of follicle cells at the oil globule stage and cortical crypts in the cytoplasm of the matured oocytes, which have been observed in penaeid shrimps, 11,18 have not found in O. oratoria. A significant scattering of GSI values was found in the ovaries during yolk accumulation (stage 4). This is likely to be a result of the rapid increase in the weight of the ovary during yolk accumulation. Such a scattering and rapid increases in GSI values have also been reported for the lined shore crab Pachygrapsus crassipes. 10 Oratosquilla oratoria with oocytes at the prematuration or maturation stage were rarely collected from field surveys during the daytime and were not described in the previous study by Yamazaki and Fuji. 14 It is likely that the spawning of O. oratoria occurs mostly at night. Most female O. oratoria under rearing conditions spawn at night (K Kodama, unpubl. data, 2002). In P. japonicus, GVBD begins in the evening and is completed in 5 11 h; after GVBD, the time from ovulation to spawning is expected to be very short. 11 This provides an explanation for the small number of O. oratoria with oocytes at the pre-maturation or maturation stage during the daytime survey, given that the final maturation process of O. oratoria is similar to that of P. japonicus. The developmental stages of O. oratoria oocytes were isochronal among different areas of the ovary. Moreover, oocytes after the oil globule stage, except those in the atretic stage, were not found in the ovary after spawning (stage 6). It is likely that almost all oocytes in the ovary develop synchronously and are extruded simultaneously. Reproductive cycle and size at first maturity It is estimated that the spawning season of O. oratoria in Tokyo Bay in 2002 lasted from April to September, based on the monthly pattern of ovarian development and GSI. Size-related differences in the spawning season were found in the present study, as reported in a previous study in the mid-1980s: 6 large female shrimps spawn in spring, whereas small female shrimps, in addition to some large female shrimps, spawn in summer. The majority of large female shrimps spawning in the spring appear to have been born 2 years previously. Small female shrimps born in the previous year begin to grow from May onwards, and fast-growing individuals are likely to spawn in the summer, whereas slow-growing individuals do not spawn within 1 year of birth. Some large female shrimps still spawn in summer. Two explanations can be considered for this. One possibility is that female shrimps that did not attain the minimum size at maturity in spring have increased their size to 10 cm BL by summer and experience their first spawning in summer. Another possibility is that large female shrimps have two broods in a spawning season. The female O. oratoria probably lays almost all of its oocytes at one time and broods the egg mass for 3 4 weeks until hatching, 13 during which time the brooding female O. oratoria ceases feeding. 19 Although multiple spawning of O. oratoria under rearing conditions has been reported, these female shrimps failed to brood the egg mass until hatching and resumed feeding for the next spawning. 9,19 In these rearing situations, the interval between the first and second spawning was days. 9,19 Therefore, at least 2 months is needed for the second spawning if a female shrimp succeeded in the first brooding. The interval between the two spawning peaks observed in the present study was approximately 2 3 months, which suggests that multiple spawning could occur in the field. The size at first maturity observed in the present study, 7 BL < 8 cm, was smaller than that recorded in the mid 1980s. Ohtomi et al. investigated the GSI of O. oratoria for a wide range of BL in Tokyo Bay in the mid 1980s and they found that the GSI of the female shrimps <8 cm BL was less than 1% throughout a year. 6 The decrease in size at first maturity might be explained by the positive relationship between stock size and size at first maturity as seen in other crustaceans. 20,21 The trawl survey in Tokyo Bay revealed that the catchper-unit-effort of O. oratoria in the 1990s fell dramatically to approximately one-fifth of that in the mid to late 1980s. 1 In addition, the annual catch of O. oratoria in Tokyo Bay was high in the mid to the late 1980s, followed by an abrupt decrease in the early 1990s, and the catch has since remained low. 3 The decrease in size at first maturity seems to be concurrent with that in the stock size. Fecundity The fecundity of O. oratoria in Tokyo Bay has been estimated in a previous study, 22 in which a linear relationship between fecundity and BL was suggested by examining the fecundity of large female shrimps 10 cm BL. However, extrapolative estimation for the fecundity of small female shrimps <10 cm BL using linear regression apparently results in considerable underestimation, because

11 744 FISHERIES SCIENCE K Kodama et al. fecundity does not correlate linearly to BL. In the present study, fecundity was estimated from various sizes of female shrimps ranging between 7.1 and 13.9 cm BL and was expressed as a positive allometric function of BL. Spawning and larval abundance In the present study, most large female shrimps spawned around May, much as they did in the mid 1980s, 6 which suggests that the larvae could occur before July, as the expected duration between egglaying and hatching is 3 4 weeks. 13 In the present study, however, the larvae occurred predominantly in August and September and the larval abundance was low before July, which is a characteristic pattern of the monthly changes in larval abundance since the early 1990s. 5 These results do not support the hypothesis that the decrease in larval abundance before July beginning in the early 1990s was caused by a delay in the spawning season of large female shrimps around May Three other possibilities could explain the low larval abundance before July in recent years. First, the mortality of the larvae occurring before July could be extremely high. This explanation does not seem plausible, however. The larval abundance was investigated at the third stage, immediately after starting the pelagic phase, and, under rearing conditions, the mortality of the larvae was low until the fifth stage. 23 If the mortality of the larvae at the early developmental stage born in spring had been low in the field, as observed in rearing conditions, the larval abundance before July would have been high. However, predation and/or environmental factors could affect the survival of the early-stage larvae in the field, which should be assessed in a future study. Second, the hatching rate of the eggs laid by large female shrimps in spring could be low. Although the hatching rate of fertilized eggs of O. oratoria under rearing conditions was low, 24 the reason for the low hatching rate and whether the hatching rate in the field is as low as in rearing conditions remain uncertain. In other crustaceans, it has been reported that various factors such as the nutritional condition of the spawner, 25 environmental factors, 26 and chemical substances 27,28 affect the hatching rate, which should be investigated for O. oratoria. Third, a substantial decrease in the stock size of large female shrimps could result in decreased egg production in spring and low larval abundance before July. The decrease in the stock size of large female shrimps has been apparent since the fishing industry catch, in which large individuals 11 cm BL were exploited; 29 numbers dropped abruptly from the late 1980s to the early 1990s and have remained low. 3 In fact, the larval abundance before July decreased one order of magnitude from the mid 1980s (mean abundance between June and July: approximately 1.8 individuals/m 3 per tow) 4 to 2002 (0.13 individuals/m 3 per tow in the present study), which seems to be concurrent with the decrease in the stock size of large female shrimps. In the present study, the larval abundance was high in summer, which suggests that the egg production of small female shrimps in summer is higher than that of large female shrimps in spring, in spite of the fact that the fecundity of large female shrimps is several-fold higher than that of small female shrimps. To reveal the monthly pattern in larval abundance in relation to egg production, the monthly egg production for each size class should be investigated based on abundance, fecundity and proportion of mature individuals in each month for each size class. Nevertheless, the third possibility seems to be a major determinant of the low larval abundance before July, although the other two possibilities can not be excluded completely. Undoubtedly, fishery regulation during the spawning season is required to enhance the resilience of the stock size of O. oratoria. In particular, regulation around the spawning peak in spring, during which most large female shrimps with high fecundity spawn, might be effective for recovering larval abundance before July. ACKNOWLEDGMENTS We thank the fishers of the Shiba Branch, Yokohama City Fisheries Cooperative Association, as well as the captain and crew of the R/V Ushio of Kanagawa Prefectural Fisheries Research Institute for their assistance in field sampling. Thanks are also due to R Yamada, T Masu, S Nagai, R Nakagawa and H Nemoto of the University of Tokyo, W Doi of Tokyo University of Marine Science and Technology and Dr T Funamoto of Hokkaido National Fisheries Research Institute for their help with field sampling. We also thank Dr T Kaneko of the University of Tokyo for guidance on histological procedures with the cryostat. REFERENCES 1. Kodama K, Aoki I, Shimizu M, Taniuchi T. Long-term changes in the assemblage of demersal fishes and invertebrates in relation to environmental variations in Tokyo Bay, Japan. Fish. Manage. Ecol. 2001; 9:

12 Reproductive biology of Oratosquilla oratoria FISHERIES SCIENCE Shimizu T. On the resource of Japanese mantis shrimp Oratosquilla oratoria (de Haan) in Tokyo Bay I: Summarization on the resource utilization and life history. Bull. Kanagawa Pref. Fish. Res. Inst. 2002; 7: Kodama K, Shimizu T, Aoki I. Possible factors causing the fluctuation of the recruitment of Japanese mantis shrimp Oratosquilla oratoria in Tokyo Bay. Bull. Kanagawa Pref. Fish. Res. Inst. 2003; 8: Nakata N. Larval distribution of Oratosquilla oratoria (de Haan) in Tokyo Bay. Bull. Kanagawa Pref. Fish. Exp. Stn. 1986; 7: Shimizu T. On the survival rate at larval stage of Japanese mantis shrimp, Oratosquilla oratoria, in Tokyo Bay. Bull. Kanagawa Pref. Fish. Res. Inst. 2000; 5: Ohtomi J, Shimizu M, Vergara JAM. Spawning season of the Japanese mantis shrimp Oratosquilla oratoria in Tokyo Bay. Nippon Suisan Gakkaishi 1988; 54: Moore KL, Barr ML. Nuclear morphology, according to sex, in human tissues. Acta Anat. 1954; 21: Kodama K, Yamakawa T, Aoki I, Fukuda M, Shimizu T. Salinity tolerance of pelagic larvae of the Japanese mantis shrimp Oratosquilla oratoria in Tokyo Bay. Bull. Jpn. Soc. Fish. Oceanogr. 2003; 67: Kodama K, Yamakawa T, Aoki I, Fukuda M, Shimizu T. Multi-spawning under rearing condition, and reduction in size at maturity of the Japanese mantis shrimp Oratosquilla oratoria in Tokyo Bay. Bull. Kanagawa Pref. Fish. Res. Inst. 2003; 8: Chiba A, Honma Y. Studies on gonad maturity in some marine invertebrates VII. Seasonal changes in the ovary of the lined shore crab. Nippon Suisan Gakkaishi 1972; 38: Yano I. Oocyte development in the kuruma prawn Penaeus japonicus. Mar. Biol. 1988; 99: Minagawa M, Chiu JR, Kudo M, Ito F, Takashima F. Female reproductive biology and oocyte development of the red frog crab, Ranina ranina, off Hachijojima, Izu Islands, Japan. Mar. Biol. 1993; 115: Hamano T, Matsuura S. Egg size, duration of incubation, and larval development of the Japanese mantis shrimp in the laboratory. Nippon Suisan Gakkaishi 1987; 53: Yamazaki M, Fuji A. Reproductive cycle of the mantis shrimp, Squilla oratoria de Haan, in Mutsu Bay. Bull. Fac. Fish. Hokkaido Univ. 1980; 31: Deecaraman M, Subramoniam T. Synchronous development of the ovary and the female accessory sex glands of a crustacean, Squilla holoschista. Proc. Indian Acad. Sci., Anim. Sci. 1983; 92: Beams HW, Kessel RG. Electron microscope studies on developing crayfish oocytes with special reference to the origin of yolk. J. Cell Biol. 1963; 18: Nakamura K. Maturation of the spiny lobster Panulirus japonicus. Mem. Fac. Fish. Kagoshima Univ. 1990; 39: Courtney AJ, Montgomery SS, Die DJ, Andrew NL, Cosgrove MG, Blount C. Maturation in the female eastern king prawn Penaeus plebejus from coastal waters of eastern Australia, and considerations for quantifying egg production in penaeid prawns. Mar. Biol. 1995; 122: Hamano T, Matsuura S. Egg laying and egg mass nursing behaviour in the Japanese mantis shrimp. Nippon Suisan Gakkaishi 1984; 50: Polovina JJ. Density dependence in spiny lobster, Panulirus marginatus, in the Northwestern Hawaiian Islands. Can. J. Fish. Aquat. Sci. 1989; 46: Lipcius RN, Stockhausen WT. Concurrent decline of the spawning stock, recruitment, larval abundance, and size of the blue crab Callinectes sapidus in Chesapeake Bay. Mar. Ecol. Prog. Ser. 2002; 226: Nakata N. The assessment of stock and stock condition on the Japanese mantis shrimp Oratosquilla oratoria (de Haan) in Tokyo Bay. Bull. Kanagawa Pref. Fish. Exp. Stn. 1990; 11: Kudo S, Hasegawa T. The interaction between Sagami Bay and Tokyo Bay based on the distribution of the larvae of Squilla oratoria. Bull. Jpn. Soc. Fish. Oceanogr. 1966; 9: Hamano T. Mating behavior of Oratosquilla oratoria (de Haan, 1844) (Crustacea: Stomatopoda). J. Crust. Biol. 1988; 8: Djunaidah IS, Wille M, Kontara EK, Sorgeloos P. Reproductive performance and offspring quality in mud crab (Scylla paramamosain) broodstock fed different diets. Aquacult. Int. 2003; 11: Ehlinger GS, Tankersley RA. Larval hatching in the horseshoe crab, Limulus polyphemus: facilitation by environmental cues. J. Exp. Mar. Biol. Ecol. 2003; 292: Amim OA, Rodriguez EM, Hernando M, Comoglio LI, Lopez LS, Medesani DA. Effects of lead and cadmium on hatching of the southern king crab Lithodes santolla (Decapoda, Anomura). Invert. Reprod. Dev. 1998; 33: Lee R, Kim GB, Maruya KA, Steinert SA, Oshima Y. DNA strand breaks (comet assay) and embryo development effects in grass shrimp (Palaemonetes pugio) embryos after exposure to genotoxicants. Mar. Environ. Res. 2000; 50: Ohtomi J, Nakata N, Shimizu M. Discarding of the Japanese mantis shrimp Oratosquilla oratoria by smallscale trawlers in Tokyo Bay. Nippon Suisan Gakkaishi 1992; 58:

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