Abstract
Ecological characteristics of Maeropntnalmus japonwus populations were studied in two different parts of a small intertidal mud-flat in Amakusa, west Kyushu. These two places, the upper shore and the lower shore, are separated each other by artificial bank with a narrow creek, and characterized by different nutritional conditions and physical factors related to the difference of the time of exposure to the air. Difference of size composition in-two subpopulations was explained by differences in the intensity of settlement, mortality in prereproductive age and longevity at two places. The lower shore crabs show larger growth rates, earlier sexual maturity, larger size at first reproduction. The brood size and the mean egg size were also different in two subpopulations, the lower subpopulation produce smaller brooding size in larger egg size, and the upper subpopulation produce larger brood size in smaller egg size. The difference of such life history traits in different local populations often regarded as the result of natural selection. In this case, however, genetic difference may not be expected because of complete mixing during the pelagic larval period. From this viewpoint, non-genetical variance of life history traits and the problems to be solved were discussed.
