Social decision-making in a freshwater gastropod

Denis Meuthen , Dustin Raeke, Jannis Bednarzik, Mauricio Bravo Soldevilla, Niclas Kuper, Melanie Dammhahn

Animals are hypothesized to plastically adjust their social behaviour expression dependent on their environment and their phenotype to ensure that its benefits outweigh its costs. There is a strong research bias towards socially complex vertebrates and eusocial insects. Thus, here we study mollusks, a taxon whose social behaviour is less well studied. As predation is one of the strongest drivers of sociality, we study antipredator plasticity in the freshwater gastropod Physella acuta. Using a split-clutch design, we raised individuals from birth onwards until adulthood under either perceived high or low (control) predation risk conditions. In a first experiment, we investigated differences in social association preferences with conspecific groups whereas in a second experiment we studied interindividual interactions within groups. In line with expected antipredator benefits of collective defences, snails raised in high-risk conditions displayed greater social preferences and engaged in more interindividual interactions (sociability) than controls. In snails raised under high-risk, thick-shelled individuals (i.e., individually defended snails) showed lower social preferences, suggesting that for them, the costs associated with sociality outweigh fitness benefits of collective defences. Counterintuitively, the opposite pattern emerged in low-risk snails. However, larger studies show that even when raised under low-risk conditions, thick-shelled individuals are generally larger. This makes them better competitors, which favours maximization of resource-related benefits from joining conspecifics. Lastly, snails raised in high-risk conditions displayed lower competitiveness towards conspecifics, likely to reduce conspicuousness to predators. Altogether, even in a gastropod species, social behaviour expression is plastically adjusted to both developmental environments and individual phenotypes.