 88 percent of teleost species are gonochoristic , having individuals that remain either male or female throughout their adult lives . The sex of an individual can be determined genetically as in birds and mammals , or environmentally as in reptiles . In some teleosts , both genetics and the environment play a role in determining sex . For species whose sex is determined by genetics , it can come in three forms . In monofactorial sex determination , a single @-@ locus determines sex inheritance . Both the XY sex @-@ determination system and ZW sex @-@ determination system exist in teleost species . Some species , such as the southern platyfish , have both systems and a male can be determined by XY or ZZ depending on the population . 
