CRAYFISH(CRUSTACEA, DECAPODA) IN LAKE MASHU, HOKKAIDO

Abstract

Crayfish in Lake Mashu, Hokkaido, Japan, were captured with tangle nets in the summer of 1992. All specimens, 171 males and 517 females, had intermediate diagnostic character between two subspecies, Pacifastacus leniusculus leniusculus and P, l. trowbridgii. Although three very large individuals of unidentified crayfish were captured in this lake by poachers of trout in 1975 and 1985, there is no certain confirmation because of illegal samplings. One of the present authors took a chance to measure one of them, a fresh male crayfish with very large chelae, in 1975 when he was 11 years old and recorded only its giant size, i.e. 47 cm carapace length. However, the largest P. leniusculus collected in this study was 5.7cm in c. l. Neoteny may have occurred because there is no secondary sexual character on chelae of adult P. leniusculus males. This lake has no river. Age of the lake is estimated as about 2000 years old. Further, all species of fish and crustaceans in the lake were artificially introduced from other waters. Of crayfish, only P. leniusculus occurs and was introduced from Portland, Oregon, U.S.A., in 1930. From these, the unidentified gigantic crayfish seems to have been P. leniusculus grown to a giant size.