Shell crushing resistance of alien and native thiarid gastropods to predatory crabs in South Africa

The successful invasion of freshwater and coastal lakes of South Africa by the recently introduced thiarid snail Tarebia granifera may be due in part to release from predatory pressure. This study aimed to determine the comparative vulnerability of T. granifera and the widespread native aquatic thiarid Melanoides tuberculata to predation. These species also account for many thiarid invasions in the Americas, Europe and parts of Africa. We quantified the shell crushing resistance of these snails, as well as the maximal shell crushing capability of native freshwater crab predators, Potamonautes sidneyi and P. perlatus. Using an Instron isometric transducer, we showed that Tarebia granifera shells were significantly stronger than Melanoides shells, and exceeded the crushing strength we documented for both potential predatory crabs. The greater shell strength of Tarebia granifera was due to shape, sculpture and thickness characteristics. Shell strength of Melanoides, however, remained within the range of crushing strength of their potential predators. Assuming crushing to be the main form of crab predation on snails, we inferred T. granifera to be less vulnerable to durophagous attack and that their population growth is thus not limited by predation pressure