Relatedness of mates influences mating behaviour and reproductive success of the hermaphroditic freshwater snail Physa gyrina

Questions: Does the relatedness of potential mates influence search effort, mating behaviours or reproductive success in simultaneous hermaphrodites? Do search effort and discriminatory mating behaviours adaptively reflect fitness patterns? Does discrimination depend on current gender role or body size? Organism: Laboratory-reared snails, Physa gyrina (Gastropoda: Pulmonata), from family lines that originated from two populations near Lexington, Kentucky, USA. Physid snails are hermaphrodites with easily observable mating behaviours. Eggs are laid in transparent, gelatinous masses. Methods: We examined the effects of inbreeding and outbreeding on behaviour and components of fitness in a hermaphroditic mating system. We created experimental treatments by pairing individuals of known relatedness, or by forcing individuals to self-fertilize. Results: Snails exhibited behavioural discrimination between partners of varying genetic relatedness, with behavioural strategies dependent on current gender roles. Male behaviours did not vary significantly with relatedness. Female-role resistance behaviours increased significantly with male escalation in both highly inbred and highly outbred pairings, but not in pairings of intermediate relatedness. Pairings of intermediate relatedness also had the highest reproductive success, fitting the concept of optimal outbreeding. Crawling rates were not affected by the relatedness of potential mates, but offspring of self-fertilizing parents crawled less than those of outcrossed parents. Snails with longer crawl distances exhibited greater escalations of mating interactions. Body size was positively related to crawling rates and reproduction, and influenced gender roles and mating behaviour.