Cannibalism causes size-dependent intraspecific predation pressure but does not trigger autotomy in the intertidal gastropod Agaronia propatula

Abstract

© 2015 The Author 2015. Published by Oxford University Press on behalf of The Malacological Society of London, all rights reserved. Autotomy of the foot is readily inducible in the predatory marine gastropod Agaronia propatula (Conrad, 1849), but the natural trigger of the autotomy response is obscure. Since cannibalistic predation has been observed in the species while interspecific predation on A. propatula has not, it was hypothesized that autotomy in A. propatula helps to defend against cannibalism. This hypothesis was tested in the present study. In our Costa Rican study population, autotomy as well as cannibalism occurred at significant rates; morphological indicators of foot regeneration suggested that 9-23% of the animals had autotomized previously, while about 5% of all observed predation attempts were directed at smaller conspecifics. However, autotomy in response to cannibalistic aggression was neither observed in the wild, nor did experimentally induced intraspecific aggression trigger autotomy. Successful cannibals generally were large and appeared to kill smaller conspecifics by suffocation in the metapodial pouch, but even then autotomy did not occur. These results refuted the hypothesis that A. propatula autotomizes to escape cannibalism. However, the size dependence of successful cannibalism on one hand, and the size spectrum of the population on the other, enabled a semiquantitative evaluation of the size-dependent intraspecific predation pressure in the population. The analysis indicated that in A. propatula different size classes represent 'ecological species' with distinct trophic roles. Since large A. propatula may actually reduce the total predation pressure on their heterospecific prey by cannibalizing smaller conspecifics, this size-dependent functional differentiation increases the complexity of the food-web around these snails.