Predator-induced morphological defences in a freshwater snail

The ability of a certain genotype to express different phenotypes depending on variations in the environment is called phenotypic plasticity. Predator induced defences are among the most studied and wide spread examples of phenotypic plasticity. For example, in freshwaters, predators are constantly shaping communities with their presence and a plethora of defensive adaptations in prey have evolved. In this study, I have analyzed if the freshwater snail Radix balthica show any induced morphological defence when exposed to the molluscivoruos fishes, crusian carp (Carassius carassius) and tench (Tinca tinca). Two treatments were used; without predator and with a non-lethal predator (fish cues), to examine if the snails exposed to predators changed their shells in size, thickness and shape. The study contained snails from both northern Sweden (Umeå) and from southern Sweden (Lund). The experiment conducted on the northern snails of R. balthica did not confirm my hypothesis that snails threatened by a predator (fish cues) express inducible defences. However, there were tendencies pointing in that direction. In contrast, the R. balthica snails from southern Sweden, exposed to fish cues from tench, showed a strong response. Shells were thicker and showed a more rounded shape, i.e. a wider shell and lower apex in the presence of fish compared to in the absence of fish. The rounder shells are more resistant to crushing by molluscivorous fish, due to the fact that the crushing force is more evenly spread over the shell. Thickness and size of the shell also influences the crushing resistance. Shell crushing fish obviously have a strong effect on gastropod communities and are therefore likely to drive the evolution of induced morphological defence in their prey.