Sperm trading and sex roles in the hermaphroditic opisthobranch sea slug Navanax inermis : eager females or opportunistic males?

Abstract

The sea slug Navanax inermis (Gastropoda, Opisthobranchia) is a well-known example of a simultaneous hermaphrodite in which mating partners trade sperm. According to previous work by others, sperm trading follows from a general preference for the female role when the expected variance in reproductive success in the male role is higher and the risk of failure therefore larger. However, this view contradicts theoretical and empirical studies of other systems, which predict a general preference for the male role; sperm trading is assumed to follow from the fact that individuals benefit from sperm receipt as a nutritional compensation for sperm investment. In this study, we investigate the behaviour of N. inermis in more detail. In addition to observations of regular pairs, we also paired individuals with a partner that had been isolated for 33 days in order to induce changes in sex-role preference in the non-isolated partner. We also collected all clutches produced throughout the study to check for signs of infertility as a consequence of allosperm depletion. Fertility of field-collected and isolated individuals suggested that sperm depletion occurs under natural conditions and may be caused by a lack of partners. Although this argues in favour of female preference, low mating rates both reduce variance in male reproductive success and remove the intent to use sperm for nutritional purposes, thus eliminating the conditions under which both hypotheses are intended to operate. The observational data indicate that animals are eager to mate as males, particularly at the beginning of a mating session. Intromissions lasted longer when a simultaneous intromission was received from the partner. Increases in intromission were recorded in non-isolated individuals with partners which were previously isolated and therefore more attractive as females. This response would not have been expected were the female role the preferred one. A summary of the arguments concerning preference for either gender suggests some preference for the male role, but indicates that sexual preferences may actually change or become neutral within an individual, even in the course of a mating session. Overall, our results clearly confirm previously published observations of sperm trading in N. inermis . © 2003 The Linnean Society of London . Biological Journal of the Linnean Society, 2003, 78 , 105 - 116. ADDITIONAL KEYWORDS: sexual conflict ‐ sexual selection ‐ Gastropoda ‐ reciprocity ‐ gamete trading ‐ sperm trading ‐ egg trading.