Abstract
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.
