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Gonadosomatic index (OSI), condition index, the reproductive cycle with gonadal development of the bladder moon, Glossaulax didyma (Roding), were investigated histologically, based on the samples which have been collected from the intertidal and subtidal zones of Piungdo, Kunsan Coast of Korea, from March 1997 to February 1998. G. didyma is dioecious and oviparous. Monthly changes in the gonadosomatic index studied for determination of the spawning period were closely associated with the gonadal phases. The GSI began to increase from March and reached the maximum in May. And then, their values sharply decreased from July to August due to spawning. The condition index calculated for determination of the spawning period was coincided with changes in the OSI and gonadal phases. Reproductive cycle with the gonad developmental phases of this species can be divided into five successive stages in females and four stages in males: in females, early developing stage (December to February), late developing stage (February to March), lipe stage (April to July), spawning stage (late May to August). recove교 stage (September to November); in males, developing stage (December to March), ripe stage (March to July), copulation stage (early May to August), recovery stage (August to January). Fully mature oocytes were approximately 250-270 μm.
Spawning occurred between late May to August in females and early May to August in males, and spawning peak was observed between July and August when the seawater temperature rose to above 19°C. The sex ratio of female: male was significantly different from a 1: 1 (X² = 5.76, p<0.05).

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