Annual fluctuation of life-history traits in the mud crab Macrophthalmus japonicus

Interannual fluctuation of life-history traits in the mud crab Macrophthalmus japonicus (De Haan) was studied for a 7-yr period (1981 to 1987) in an intertidal mudflat in the estuary of the Tatara-Umi River, Fukuoka, Japan. Density of juveniles fluctuated greatly, and maximum annual density was low in 1981 to 1984 (22.0 to 44.0 m^-2) and very high in 1985 to 1987 (98.5 to 188.6 m^-2). In 1980, recruits may have been very few, though there is no data on or before 1980, because no crab of the 1980 cohort was found during the study period. Density of adult crabs was almost stable (11.0 to 32.6 m^-2), perhaps because of density-dependent mortality. Under the high density conditions prevailing from 1985 to 1987 crabs grew slowly and longevity was shorter. Male chela and female abdomen were dwarfish, and females produced few and small broods. The changes of life-history traits under high density conditions were thought to result from strong intraspecific competition for space.