Reproduction in the Hermaphrodite Aeolidiella glauca - A Tale of Two Sexes

Abstract: This thesis focuses on reproduction in a simultaneous hermaphrodite with internal fertilization; the nudibranch Aeolidiella glauca. Unlike most other nudibranchs, where copulation is the rule, A. glauca was found to transfer sperm via external spermatophores that were attached to the partner's back. Despite elaborate courtship the actual spermatophore transfer, which always involved two animals only, was of short duration. In most matings (88%) spermatophores were reciprocally exchanged. A. glauca was further found to be very promiscuous. During mating and sperm transfer the receiver exerts considerable control over sperm, and manipulative behaviours designed to increase the donor's reproductive success are thus likely to have evolved. An example of such manipulative behaviour may be A. glauca's unique spermatophore avoidance behaviour. I found that slugs carrying a sign of previous mating activity, i.e. a spermatophore, were discriminated against in a situation where mate choice was possible. The presence of spermatophores was further found to reduce slugs' ability to interrupt matings, and displace other slugs. Body size, however, had no direct effect on displacement in A. glauca as small slugs interrupted matings as successfully as large ones. Furthermore, pair formation and mating were found to be random with respect to size. This was true also for pairs formed in the field. In addition to mate choice hermaphrodites may increase their reproductive success by differential sex allocation. I tested whether differing mate encounter rates had any effects on allocation to male and female function in A. glauca. Slugs with more mating opportunities mated more, and had higher proportional spermatophore production that others. As predicted they also laid significantly fewer eggs than slugs presented with partners less often.