PROTOGYNOUS HERMAPHRODITISM IN THE BRACKISH AND FRESH-WATER ISOPOD, GNORIMOSPHAEROMA NAKTONGENSE (CRUSTACEA, ISOPODA, SPHAEROMATIDAE)

Protogynous sex change in Gnorimosphaeroma naktongense, inhabiting bra ckish water and freshwater bodies, was examined by culturing precopula tory guarding pairs. The transitional stages of masculinizing of a fem ale were observed clearly after releasing juveniles from a female mars upium. About forty percent of the females cultured showed direct evide nce of sex change to males in the outer morphology at the end of labor atory experiments. As histological observations were not done to make clear the processes of gonad changes, the possibility of protogynous s ex change was strongly suggested. Precopulatory mate guarding may be a primary selective pressure favoring protogyny in G. naktongense.