モクズガニEriocheir japonica(de Haan)の繁殖生態(総説)

抄録

Reproductive ecology of the Japanese mitten crab Eriocheir japonica (de Haan) is reviewed mainly based on information collected over the last ten years. This crab is a catadromous species which grows to maturity in freshwater and reproduces in brackish waters and the sea. Adult crabs migrate downstream from fully freshwater areas to marine tidal areas for the purpose of copulation, oviposition, and hatching of eggs. Megalopa larvae settle and metamorphose to the crab stage in upper brackish areas after planktonic zoea larva stage in the sea. Juvenile crabs migrate upstream to freshwater and disperse widely along the river. Larger crabs are distributed in the upper reaches. In both sexes, in the upstream direction the population density tends to decrease, and size at maturity tends to be larger, and the sex ratio becomes female-biased. Details of the process and timetable of maturation (puberty moult and gonad maturation), downstream migration, mating behaviour, oviposition and hatching, the planktonic life of the zoea larvae, and the settlement of the megalopa larvae are noted and their ecological significance is discussed. Ovarian maturation occurs after the puberty moult (terminal moult), taking approximately 4 months. Adult crabs in a hard-shell condition migrate downstream to the tidal area and participate in reproduction, but females migrate downstream even if they have not completed maturation. There are two groups of females that differ in the timing of reproduction. The early-maturing group, which is small in number, develops ovaries in September or October and oviposits in October. In the late-maturing group, which is larger in number, the puberty moult occurs in August, maturation is generally complete by December, and oviposition occurs after December. Mating is initiated without any precopulatory behaviour. Females do not use a sex pheromone to attract mates and males approach without determining the receptivity of the females. Females can copulate only after their ovaries have matured, and they lay eggs within a day after copulation. However, females are not necessarily in a receptive condition in the tidal area, and approaching males are often rejected by females with not fully matured ovaries. The duration of copulation is from 13 to 43 minutes and is negatively correlated with water temperature. Postcopulatory guarding by males continues for several hours, and females may be released by males when oviposition occurs. Females often reject guarding by small males. Females oviposit and hatch eggs up to a maximum of three times. The number of eggs tends to decrease in later clutches. Most crabs seem to die after reproduction without further moulting. Size at maturity varies from approximately 35mm to 75mm in carapace width, but each crab can participate in reproduction only in one season and cannot thereafter grow larger. Thus, there is a wide variation of reproductive success in E, japonica. In the females, fecundity is higher in larger crabs. As for the males, larger ones with larger chelipeds can most effectively guard females after copulation. The duration of embryonic development varies from 2 weeks to 3 months, and that of the planktonic life of zoea larvae in the sea is estimated to vary with water temperature. Settlement of megalopa larvae in the upper tidal reaches occurs from 2 weeks to 4 months after hatching, depending on the river, and it takes place mainly in mid-autumn (October) and early summer (May to June). The young crabs that metamorphose in autumn and winter neither migrate nor grow during the winter. Upstream migration of the autumn settlers and the summer settlers to freshwater occurs from March to June and from July to August, respectively. This mode of settlement with two peaks reflects the temporal pattern of reproduction. Autumn settlers are derived from the eggs produced by the early-maturing females, and summer settlers from the latematuring ones. The reproductive strategy of this crab