Abstract: I investigated the process of growth, migration, and reproduction of the Japanese mitten crab Eriocheir japonica in the Saigo River, Fukuoka Prefecture, Japan. Samples were collected by hand-held nets in the lower freshwater, tidal river, and sea coast areas. Above the upper tidal river area, males and juvenile females appeared nearly all year round, but adult females only from August to February. Exuviae and post molt crabs were collected only in this area, from March to November. Below the middle tidal river area, adult females of >38mm CW and males of >28mm CW appeared from September to June. Duration of occurrence was longer in the lower tidal river and sea coast areas than in the middle tidal river area. In the lower tidal river and sea coast areas, postcopulatory guarding pairs were collected in December, January, and March, and ovigerous females appeared from October to the following June. Carcasses(>38mm CW and >34mm CW for females and males, respectively)were collected mostly in the tidal area. Therefore, growth molts of juvenile crabs appeared continuously except in the winter season, and the puberty molt to the adult stage from August to November, mainly in the freshwater area. Adult crabs migrated downstream from September to February, and reproduction took place mainly in the lower tidal river and sea coast areas.
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