SEX-CHANGE IN BOTH DIRECTIONS BY ALTERATION OF SOCIAL-DOMINANCE IN TRIMMA-OKINAWAE (PISCES, GOBIIDAE)

The mating system of the gobiid fish Trimma okinawae is one of polygyn ous hermaphroditism, in which the largest female of a social unit chan ges sex following the removal of the dominant male. Histological obser vations of the gonads however, revealed that males have an ovarian tis sue within a functional testis. The occurrence of ovarian tissue in th e functional male suggests that T. okinawae males should be able to re vert back into being functional females. To test this prediction, we p laced females in an aquarium and allowed them to change sex. After con firming sex change from female to male, we individually placed new mal es into another aquarium and added a larger male to each. Our experime nts revealed that females change sex and become males upon the removal of dominant males, and that those males changed sex again and became females in the presence of larger males. Sex change in both directions may be advantageous when a male is forced to become subordinate follo wing the take over of the social unit by a larger male.