Japan Sea National Fisheries Research Institute, Fisheries Research Agency, Niigata 951-8121, Japan
A total of 6,489 Benizuwai crabs, Chionoecetes japonicus, were collected by otter trawl from 19 stations in the depth range of 500-2000 m deep in the central part of the Sea of Japan for the purpose of clarifying their bathymetric distribution pattern. Adolescent males (i.e. chelae undifferentiated) and morphologically immature females with a narrow abdomen were highly concentrated in areas deeper than 1700 m. Adult males (i.e. chelae differentiated) distributed widely in the depth range of 1200-2000 m deep, although larger ones tended to occur in shallower bottom areas. It was suggested that adolescent males migrate upward with molting and partly developing secondary sex characters. Bathymetrically the distribution pattern of terminally molted females was different whether they were non-oviparous or oviparous. The density of non-oviparous females, most of which were before first oviposition, was greater in bottom areas deeper than 1700 m, however, oviparous females tended to distribute in rather shallower areas. It was suggested that immature females that typically concentrated in areas over 1700 m deep, start upward migration after they molt and attain morphological maturity. The clutch fullness of oviparous females was significantly different by depth. What leads to the difference in clutch fullness is unknown but the sex ratio and/or density and mean size of mature males are possible reasons.