Predator induced phenotypic plasticity in a freshwater snail

Abstract: In this thesis I examined a phenotypically plastic defence in a common freshwater snail, Radix balthica. Adaptive prey defences may involve behavioural, life-history or morphological changes. Snails are important species in freshwater systems. Their role as grazers on periphytic algae makes them particularly important as they have a positive impact on macrophyte growth in lakes and ponds. The structuring role of predators, especially fish, on snails may therefore have a great impact on lake ecosystems. I have studied the occurrence of different shell shapes in this snail in nature. In laboratory studies, I have determined the degree of shell plasticity over many populations, the morphological reaction to multiple predators, costs of expressing plastic traits and the role of plasticity in a community context with other snails. My results show this snail to have different shell shapes in nature that can be attributed to the presence or absence of molluscivorous fish. In the laboratory, I established it to be a phenotypically plastic species with identical reaction norms irrespective if the population originated from a fish or fish free pond. In a test with multiple predators, fish and crayfish that differs in foraging strategies, I showed that this species produces different shell shapes, but that the main defensive change was directed to defend against fish predation and that shape determined the crushing resistance of the shell. I also showed it to be able to compensate differences in shell shape with shell thickness to maintain crushing forces of the shell that are an important factor for defence against fish. The costs for adaptive trait changes like morphology, shell thickness or behaviour seems to be relatively low in this species. Instead, costs mainly emerge from reduced growth rates and lower fecundity. In the competition with other snail species, R. balthica is a strong competitor that dominates the system with or without the presence of predator cues only. Under direct predation from fish, it depends on the higher crush resistance of a morphologically altered shell shape and persists together with another hard shelled species in this environment. The main conclusions from my thesis is that this species is plastic over a large geographical range, probably through high gene flow and that fish has been a key predator in the evolution of a plastic defence. It shows great versatility in many traits in the presence of fish and at relatively low costs. It is a strong competitor and due to the morphologically plastic shell, persistent in many environments.