Sex and the simultaneous hermaphrodite: testing models of male-female conflict in a sea slug, Navanax intermis (Opisthobranchia)

Abstract

navanax inermis is a simultaneously hermaphroditic sea slug with unilateral copulation. Individuals alternate sexual roles over a series of copulations within a period of hours. Laboratory observations were used to test three predictions derived from the hypothesis that the mating system of this species is based on sperm-trading. These predictions were: (1) the middle animal in a courtship chain should be more likely to become female than male in a subsequent copulation: (2) an individual having an opportunity to be female at the end of a copulation as a male should be less likely to resume male behaviour than one that does not; and (3) more copulations should be terminated when the male gets an opportunity to be female than when the female gets an opportunity to be male. The results of the last two tests provide strong support for the sperm-trading hypothesis and are inconsistent with the alternative hypothesis, eggtrading. The results of the first test were more consistent with sperm-trading than with egg-trading. It is concluded therefore, that the mating system of N. inermis is based on sperm-trading and that the gamete-trading model, based on considerations of control of fertilization, better predicts the form of male-female conflict in simultaneous hermaphrodites than does the egg-trading model based on Bateman's principle.