Temporal dynamics of a lentic microphagotroph community were studied during leaf litter decomposition from December to May. Small plastic vessels containing leaf litter were placed on a pond bottom. They were sampled periodically to collect microphagotrophs. Three abiotic factors and abundance of two food items were also measured to analyze the autogenic and allogenic phenomena during a microphagotroph succession. Three behavior types were recognized in dominant taxa: a free-swimming type, a vagile (creeps on substratum, sometimes swims) type, and a voluntarily fixed type. Dominant taxa changed from the free-swimming to the vagile type up to mid-March, and the reverse change occurred from mid-April. Principal component analysis (PCA) indicated four factors affecting the dynamics of the community: water temperature as a seasonal factor, detritus volume on the litter surface as a habitat factor, and densities of bacteria and small flagellates as food factors. Taxa replacement appeared to occur through two mechanisms. (1) Dominance of small holotrichs, a free-swimming type, was brought about by a high bacterial density caused by seasonal events, i.e., leaf fall in December and detritus formation by litter feeders in mid-April. This is an allogenic aspect of community dynamics. (2) The free-swimming type was replaced by the vagile one during the period with high taxa diversity. This replacement occurred through intertaxa competition for scarce food and/or selective predation by larger microphagotrophs. It is an autogenic process within the community.

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Jesogammarus spinopulps M0RIN0 (Anisogammaridae) is one of the freshwater amphipods endemic to Japan. The life history and reproduction of this species were studied in a small lowland pond in Tokyo. J. spinopulps bred in winter and the life cycle was annual. The growth rate of juveniles was retarded from May to August in the field, and the laboratory experiment indicated that it was partly due to high water temperature (>25°C). The high temperature (25°C) accelerated only the growth of newborn juveniles, but repressed the growth rate of larger juveniles, maturity and survival. Juveniles began to develop sexual characteristics from October, and males became significantly larger than females prior to maturity. Reproduction occurred in a cold season from late December to early May, and a female yielded three or four clutches. The reproduction of J. spinopulps is the typical "many small egg" type as compared to other lotic and marine species of similar size. Such a reproductive type may be advantageous in environments where anisogammarids are exposed to harmful high temperature in summer and constrained to breed within a limited period.

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