Mating Systems, Sex Change and Sexual Demography in the Rainbow Wrasse, Thalassoma lucasanum

Abstract

Previous work indicates that sex change becomes less advantageous and less common in fish species which live at relatively high population densities. This study extends these observations to the abundant rainbow wrasse (Thalassoma lucasanum) of the eastern tropical Pacific. In this species, large territorial males in terminal phase coloration are uncommon, and the overwhelming majority of spawnings occur in groups of smaller initial phase males. For T. lucasanum, sex change itself is a relatively rare event; fewer than 30% of the terminal phase individuals are derived from females. The initial phase population consists of nearly 50% primary males, their theoretical maximum abundance. Laboratory experiments indicate that sex and color change is socially mediated: the largest individual changed color (and sex if she was a female) in ten groups initiated without a terminal phase male, while no transformations occurred in 15 control groups. The retention of sex-changing ability should carry little or no cost even in densely distributed populations, and can be of great advantage in any small groups that might exist. In addition to ensuring proper timing of transformation in small groups, social mediation of sex change may be of value in preventing sex change from occurring at all in situations where the costs of transformation are too great.