The blue swimming crab, Portunus pelagicus (Linnaeus, 1758), is one of the most economically important crustaceans in Thailand. In order to gather more in-country data on its biological aspects, a comprehensive observation of the annual reproductive cycle of female P. pelagicus, which were gathered each month from Kung Krabaen Bay, Thailand, from October 2017 to September 2018 was undertaken. The ovarian structure, gonadosomatic index (GSI) and histological description of P. pelagicus were

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... ved during this time. It was revealed that six distinct stages-I (immature stage), II (early maturing stage), III (late maturing stage), IV (early matured stage), V (middle matured stage) and VI (late matured stage)-occur during the annual reproductive cycle of the females. Moreover, the percentage of the oogenic proportion of P. pelagicus appeared continuously throughout the year. The ovarian development was exclusively found in stage VIstage I was also observed, especially during October 2017 (49.5%), implying its important involvement in supporting oogenic production. It would be suggested that the successive spawning season of the P. pelagicus population clearly occurred during October-December each year. Therefore, this would be the best time to support hatchery seed production, during which P. pelagicus collection should be avoided.