SEXUAL MATURITY, SPAWNING AND SEX-RATIO O F THE 2 FORMS OF FLATFISH, GENUS PLEURONICHTHYS, IN KIISUIDO CHANNEL WATERS OFF THE PACIFIC COASTOF CENTRAL JAPAN

Sexual maturity, spawning season and sex ratio in the two sibling spec ies of flatfish, genus Pleuronichthys, Honmeita and Bakemeita were sur veyed. Spawning season in the Honmeita was estimated to be from the mi ddle of November to the beginning of January, while the Bakemeita was estimated to be from the middle of January to the beginning of May. Th e Existence of seasonal isolation in the spawning season between the t wo forms was distinctive. However, there remained the possibility of r eciprocal crossing between the two forms in January, because the perio d in which mature males with fully matured testis were observed was mu ch longer than the spawning season. In this report, the existence of a nother reproductive isolation mechanism, such as geological isolation of spawning ground, is suggested, considering the absence of heterozyg ous variants of these two forms in isozyme analysis and the lack of co mmon mtDNA haplotypes between the two forms. Differences in the sex ra tio were also demonstrated between the two species. The sex ratio of H onmeita and Bakemeita were 1.5 and 1.0 respectively.