Abstract
Population density and structure of the intertidal mud crab Macrophthalrnus japonicus were compared among five stations in June, August and November 1984. Population density was higher where sediment particles were finer and nitrogen content was higher than at other stations. However, large crabs were more abundant in stations with coarser sediments and lower nitrogen, indicating higher survival rate in these stations. Stations with fine sediments and higher nitrogen had higher percentage of ovigerous females in both June (early breeding season) and August (late breeding season). But in other stations, it was higher only in August. These differences indicate that high food availability may enable large reproductive effort throughout the breeding season, but also may cause high mortality under high population density. In habitats where food availability was high but crab survivorship low, early and large reproductive effort may be advantageous because probability of further reproduction may be very low.
