Abstract
The population structure and life history of the goneplacid crab Carcinoplax vestita were investigated in Tokyo Bay, Japan, from November, 2002, to October, 2003. Catch per unit effort (CPUE) was generally constant (<100) with two exceptional peaks in June (171) and August (390), and it was always highest in the central part of the inner bay than elsewhere. The overall sex ratio was significantly biased towards males, but the proportion of males gradually decreased from November and the sex ratio was significantly biased towards females in June, August, and September. The overall size-frequency distributions for males and females were not significantly different, and there is no sexual dimorphism in body size in this species. The size-frequency distributions were bi- and/or trimodal from November to April or May, but unimodal in later months. The small- and large-sized modes consisted of newly recruited individuals and senile, post-reproductive individuals, respectively. Life-span was estimated to be about one year with exceptional individuals surviving longer. The occurrence of soft-shelled small males peaked in December, April, and August, suggesting a four-month molt cycle; however, large males and both small and large females showed no clear peaks or cycles of molting. Larger females were inseminated first, starting in April. In contrast, females concurrently began to spawn and carry embryos regardless of their body size in August. The time lag between summer hypoxia and the ovigerous period has probably contributed to the establishment of a large population in Tokyo Bay.
