MATURATION AND OVIPOSITION IN THE JAPANESE MITTEN CRAB ERIOCHEIR JAPONICUS (DE HAAN) RELATION TO THEIR DOWNSTREAM MIGRATION

The process of downstream migration and maturation of the Japanese mitten crab Eriocheir japonicus was determined by investigating the relationship between gonad maturity and the distribution off. japonicus in the Kaminokawa River, Kagoshima, Japan. In females, migration occurs following the puberty moult. Gonad maturation occurs after the puberty moult, taking approximately 4 months. The gonadosomatic index (gonad weight x 100/cheliped-free body weight) increased from below 1.0 to above 10.0. With maturation, oocyte diameter increased from less than 60 mu m to more than 300 mu m. However, crabs migrated downstream even if they had not necessarily completed maturation. The migration appears to end earlier further upstream and there are two seasonal periods of maturation. The early maturing group, which was small in number, developed ovaries in September or October and oviposited in October. In the late maturing group which was larger in number, puberty moulting occurred in August, and maturation ended nearly in December and oviposition occurred after December. The early group tended to be larger. In males, the GSI was always below 1.0, and tended to increase from March to Au,oust. Most crabs seemed to migrate downstream in the mature condition with their vas deferens full with spermatophores. The maturity size tended to be larger for individuals from further upstream.