Population structure of Ilyoplax pusilla (Crustacea: Ocypodidae) in the Kurae River Estuary, Amakusa, Kyushu, Western Japan.

Abstract

The population structure and reproduction of Ilyoplax pusilla (DE HAAN) (Brachyura:Ocypodidae) were studied from August, 1991, to September, 1992, in the Kurae River Estuary (32°30' N, 130°24' E), Amakusa Kamishima Island. The density of mature crabs (adults) was ca. 200/m² throughout the study period, with no strong seasonal change. Juveniles occurred from July to October, with maximum numbers in August. The ovigerous females occurred from April to September, with a peak in July. Cohort analysis revealed the following: maturity was attained at an age of 1 year; females grew faster than males before maturity, whereas males grew faster after attaining maturity; males were estimated to survive for at most 3 years and to experience 3 reproductive seasons during their lives, whereas females were estimated to survive for 2 years plus a few months and to experience 2 reproductive seasons. Compared with other local populations in temperate and subtropical Japan, the population of the Kurae River Estuary was characterized by smaller maximum sizes of both sexes, smaller minimum sizes of mature females, and the earlier appearance of ovigerous females. Settlement of juveniles occurred over a longer period than in the other populations.