Body size and mating strategies in the simultaneous hermaphrodite Oxynoe olivacea (Mollusca, Opisthobranchia, Sacoglossa)

1 To better understand the role and importance of body size in hermaphroditic mating system theory, the mating behaviour of the shelled sacoglossan Oxynoe olivacea was studied. This simultaneous hermaphrodite exhibits bilateral and unilateral sperm transfer and thus it is particularly suitable for studies on hermaphrodite sexual conflict. 2 In this study three hypotheses on body size were tested: that O. olivacea partner size has an effect on (i) mating mode choice, (ii) duration of mating modes, and (iii) choice of sexual role. Furthermore, we tested Charnov's hypothesis that in O. olivacea, like many simultaneous hermaphrodites, a sexual conflict exists and the male role is preferred. 3 A laboratory experiment was done to test the aforementioned hypothesis and to observe the general mating behaviour of O. olivacea. Pairs of similar (S) and different (D) sizes were considered for a total of five different treatments (S1, S2, S3, D1, D2). 4 During the five copulatory treatments O. olivacea mated up to 93 times. Specifically, pairs of the same size mated with bilateral sperm transfer while unilateral copulation was common in pairs of slightly differing size. Contrary to predictions, animals of small body size started mating in the female role. O. olivacea showed an active alternation of sexual roles and thus did not specialize in one particular sexual role. 5 This study produced evidence of a relationship between mating mode and body size in O. olivacea and highlighted the need for new studies on this simultaneously hermaphroditic species.