Alternative Contexts of Sex Change with Social Control in the Bucktooth Parrotfish, Sparisoma radians

Abstract

Social control of sex change occurs in a variety of hermaphroditic fishes; upon removal of the dominant individual, the largest individual of the opposite sex typically changes sex and acquires mating priority with the remaining members of the social group. Social control may allow a phenotypically plastic response to social situations that convey cues about the relative advantages of functioning as one sex or the other, and should be advantageous in highly heterogeneous habitats such as coral reefs. Parrotfishes (family Scaridae) are dominant members of herbivorous coral reef fish assemblages, and numerous histological examinations of gonads have demonstrated the hermaphroditic life history of many species in the family. However, social control of sex change has never been conclusively demonstrated in the parrotfishes. To test a new version of the size-advantage model for sex change, we conducted removal experiments of dominant male bucktooth parrotfish, Sparisoma radians, in St. Croix, U.S. Virgin Islands. A total of seven females from five different reefs changed sex following removals, clearly demonstrating social control of sex change. In addition, all but one of those individuals changing sex were smaller than the largest females remaining in the harems, and this contrasts with nearly all previous studies of sex change in fishes. Sex change proceeds via a novel sequence of events when compared with previous studies. Rather than behavioral sex change preceding morphological sex change, the appearance of male coloration is followed by the development of male behavior that is fully expressed approximately 20 days after removal. We show how differing arrival rates of bachelor males at our study sites may facilitate alternative contexts of sex change, with sex change occurring within social groups in some locations and with bachelor males filling harem vacancies in other locations. Alternative contexts of sex change further illustrate the astonishing phenotypic plasticity in the social and mating behavior of parrotfishes.