Hermit crabs are anomuran decapod crustaceans which have evolved the behabiour of utilizing empty gastropod shells as portable shelters. This paper briefly reviewed various aspects of population ecology of the hermit crabs. Hermit crabs in the field are generally unable to procure empty shells from living snails. Rather they are dependent ultimately upon snail mortality for shells. Snail size affects some parameters of the population through the individual-level processes of the crabs. Some studies indicate that crab growth is slowed by occupation of too small or too large shells. Fecundity is also affected directly by shell occupancy. Reduced clutch size is correlated with reduced shell size, even after the effects of crab size are taken in account. The flexibility in life history traits which shells regulated trade-off between growth and reproduction was reported. Crabs with large shells put effort to growth, while crabs in shells too small to permit growth allocate more time and energy into reproduction. There are considerable inferential and some direct evidences that hermit crab population are limited by supplies of empty shells. Adding many empty shells to the crab population in the field resulted in increasing the crab density. Finally, some problems of life history tactics of hermit crabs and other marine invertebrates are discussed.

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Biomass of meiobenthos were studied during 1918-1983, in the eastern area of Hiuchi-nada and in Suo-nada, Seto Inland Sea. Biomass of meiobenthos were composed mainly nematodes and polychaetes. In the eastern area of Hiuchi-nada, large biomass, mean 610 mg/ m² in wet weight, were obtained at coastal area less than 20m depth, small biomass, mean 170 mg/m², were obtained at offshore area more than 20m depth. On the other hand, in Suo-nada, large biomass ranged from 100 to 500 mg wet weight/m² were recorded at eastern area more than 40m depth, small biomass renged from 0 to 100 mgwet weight/m² were recorded at western area less than 20m depth. The distributional pattern of the four feeding types of nematodes were connected with sediment condition in both areas, Hiuchi-nada and Suo-nada.

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Recent studies on marine microturbellarians are surveyed and summarized. The microturbellarians consist of eight orders such as Nemertodermatida, Acoela, Catenulida, Macrostomida, Lecithoepitheriata, Prolecithophora, Proseriata, and Neorhabdocoela. The systematics of the worms becomes a major field in research work especially since the last few decades. This is considered to be mainly ascribed to discovery of rich interstitial fauna including the turbellarians. In this consequence the systematics has brought a remarkable advancement of both electron-microscopial and ecological study (life-cycle, distribution pattern, population structure, abundance dynamics, etc.). These studies have enables us to adopt a more detailed comparative method for clarifying the phylogeny of the Turbellaria.

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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.

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