Population Differences in Fecundity Components in the Hermaphroditic Freshwater Snail Planorbella trivolvis

Abstract: Fecundity in outcrossing species can be influenced by both maternal and paternal parents. To tease out these influences, we observed egg production in two populations of the hermaphroditic freshwater snail, Planorbella trivolvis (Say 1817). We carried out both intra- and inter-population matings between individuals from an inbred albino laboratory strain and individuals one generation removed from a natural population, and measured egg production for two 3-week periods - immediately after mating and 11 weeks later. In the female role, individuals from the albino laboratory population produced significantly more egg masses than the wild derived snails, regardless of whether they were mated to a partner of wild or laboratory origin, indicating that egg mass production is controlled by the maternal parent. Conversely, regardless of their own origin (laboratory or wild), snails mated to individuals from the wild derived population laid more eggs in each egg mass than those fertilized by sperm from the lab population. These results indicate that the number of egg masses a snail lays is determined at least in part by a maternal contribution, and that the number of eggs deposited in each mass may be influenced by the origin of the sperm donor. In addition, we confirmed a short-term trade-off between growth and reproduction as differences in egg production among mating types were mirrored by differences in body size. Further investigation into the nature of these differences will lead to better understanding of the reproductive biology of these hermaphrodite mollusks.