Keita KODAMA, Takamichi SHIMIZU, Takashi YAMAKAWA and Ichiro AOKI reporter: 黃薇蓉 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

 Arthroppda 節肢動物門  Crustacea 甲殼亞門  Malacostraca 軟甲綱  Hoplocarida 掠蝦亞鋼  Stomatopda 口足目  Squilloidea 蝦蛄總科  Squillidae 蝦蛄科  Oratosquilla 口蝦蛄屬  Oratosquilla oratoria  口蝦蛄

 The Japanese mantis shrimp Oratosquilla oratoria (de Haan) (Crustacea: Stomatopoda) is found on muddy bottoms in coastal waters around Japan.  The catch of O. oratoria from Tokyo Bay : the mid to the late 1980s ＞ the early 1990s.

year larval abundance 1980s1990s highJune to JulyAugust to September lowAugustbefore July The hypothesis was examined: because the spawning season of large female shrimps around May has come to be delayed since the early 1990s, there is low larval abundancebefore July. gonadosomatic index ( GSI ) histological development of the ovary for each size class size at first maturity fecundity

 Samples  Histological procedure  Reproductive cycle and size at first maturity  Fecundity  Larval data

( ● ) seven sampling stations for female Oratosquilla oratoria in Tokyo Bay, Japan. January to December in female shrimps 688 individuals

 Body length  Body weight  Gonadal weight  GSI = GW/BW ×100%

paraffin 10% formalin hematoxylin–eosin

 Ovarian developmental stage and the mean GSI were investigated for the size classes that could attain vitellogenic oocytes.  7 ≤ BL <8 cm, 8 ≤ BL <9 cm, 9 ≤ BL <10 cm, 10 ≤ BL <11 cm, 11 ≤ BL <12 cm, BL ≥ 12 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.

 Samples of a fixed ovary of approximately 3–30 mg were dissected for each individual of the ovary.  The following equation was used to estimate the fecundity of each individual: F=F s × GW/ GWs F ： the estimated fecundity of an individual Fs ： the number of oocytes in a sample GW ： the total weight of the ovary GWs ： the weight of the sample of the ovary

 Fecundity in relation to BL was estimated using the following equation: F(x) =ax x ： BL in cm, F(x) ： the fecundity against BL a and b ： constants b

May to October in 2002 Oratosquilla oratoria have 11 larval stages. The smallest size class of larvae was 0.9 ≤ CL < 1.1 mm, corresponding to the third stage. ( △ ) 15 sampling stations for the larvae in Tokyo Bay, Japan.

 Oogenesis  Developmental stage of ovary  Reproductive cycle and size at first maturity  Fecundity  Spawning and larval abundance

Yamazaki and Fuji (1980) Deecaraman and Subramoniam (1983) present study Oratosquilla oratoria Miyakea holoschista Oratosquilla oratoria Mutsu Bay Madras coast in east India Tokyo Bay 4 stages 10 stages

 Oogenesis was divided into 10 stages: (1) Oogonium stage (2) Early chromatin nucleolus stage (3) Late chromatin nucleolus stage (4) Oil globule stage (5) Yolkless stage (6) Primary yolk granule stage (7) Secondary yolk granule stage (8) Pre-maturation stage (9) Maturation stage (10) Atretic stage Yamazaki and Fuji O. oratoria Yano prawns and crabs Most female O. oratoria under rearing conditions spawn at night (K Kodama, 2002)

Developmental stages ovaryoocytes stage 1 (immature)oogonium stage, early chromatin nucleolus stage, late chromatin nucleolus stage. stage 2 (previtellogenesis)oil globule stage, yolkless stage. stage 3 (primary vitellogenesis)primary yolk granule stage stage 4(secondary vitellogenesis)secondary yolk granule stage stage 5(maturation)prematuration stage, maturation stage. stage 6 (spent)oogonium stage, early chromatin nucleolus stage, late chromatin nucleolus stage, with a few oocytes at the atretic stage

Pachygrapsus crassipes

stage 1 immature stage 2 previtellogenesis stage 3 primary vitellogenesis stage 4 secondary vitellogenesis stage 5 maturation stage 6 spent Tokyo Bay in 2002

( △) 25–49% ( ○) 50–74% ( ●) 75– 100%

 The size at first maturity, 7 ≤ BL < 8 cm, was smaller than that recorded in the mid 1980s.  The CPUE of O. oratoria in the 1990s fell dramatically to approximately one-fifth of that in the mid to late 1980s.  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.  The decrease in size at first maturity seems to be concurrent with that in the stock size.

 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 (r = 0.93, n = 25, P < 0.001)  From this formula, the fecundity of shrimp of 8, 10, 12 and 14 cm BL was estimated to be 19300, 36000, and eggs, respectively. Fecundity

previous studyour study linear relationship F=F s × GW/ GWs allometric function F(x) =ax large female shrimps ≥10 cm BL7.1 and 13.9 cm BL b

 Spawning season of O. oratoria in Tokyo Bay in 2002 lasted from April to September.  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. 1980sour study spawning large female→spring small female→summer some large female→summer large female→May

 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. 1980sour study spawning large female→spring small female→summer large female→May Larve occurred before JulyAugust to September

 First, the mortality of the larvae occurring before July could be extremely high.  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.

 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, the reason for the low hatching rate and whether the hatching rate in the field is as low as in rearing conditions remain uncertain.

 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.

Larval abundancesummerspring Egg productionsmall female shrimpslarge female shrimps Fecunditysmall female shrimpslarge female shrimps 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.