Abstract
The occurrence of sex reversal was investigated in two species of the Platycephalidae, Kumococius detrusus and Inegocia japonica. In the former, all the individuals, and in the latter, at least 50 percent of the fish (perhaps all individuals) reverse their sex protandrously in function in their life span.In K.detrusus, as the sex reversal from the functional male to the functional female occures, the gonad of the fish is transformed to the ovary from an ovotestis in which only the testicular part is functional, whereas in I.japonica, the development goes to ovary from testis through the hermaphroditic stage.Accordingly in the former case the ovotestis may be recognized easily even by the naked eye, whereas in the latter it is difficult to discern the hermaphroditic gonad from the testis except by histological observation.Considering the differences among such patterns as Kumococius-type, Onigocia-type and Inegocia-type, the evolutional process of the sex reversal phenomena was conjectured.If it is assumed that protandrous sex reversal or sex change from functional male to functional female is advantageous to reproduction, the Inegocia-type could be more adaptive where the gonadal form corresponds suitably to the change in sexual function.Therefore, the pattern of sex reversal would have evolved from Kumococius-type to Inegocia-type in which simultaneous hermaphroditic tendency disappears.
