Japan Sea National Fisheries Research Institute, Fisheries Research Agency, Niigata, Niigata 951-8121, Japan
A total of 545 males of Chionoecetes japonicus collected in the area from Mogami Basin to the west of Oki Island in the Sea of Japan, were examined and their carapace width, chela size, molting stage and weight of vas deferens were measured. The differentiation in relative size of chela as secondary sexual characters was recognizable by principal component analysis of allometric relationships between chela size (length, height and width) and carapace width. Bivariate discriminant analyses (carapace width and any one of the chela measurements) could assign individuals to the correct groups in 99% of cases. All pre-molting stage males had small chela. This indicates that males with large chela might not molt any more or less frequently than ones with small chela. Relationships between weight of vas deferens and carapace width showed that their physiological maturation starts after the attainment of an allometrically enlarged chela as a secondary sexual character.