Predation on gastropods by shell-breaking crabs: effects on shell availability to hermit crabs

This study addressed the effect predation on gastropods by shell-breaking crabs has on shell availability to hermit crabs. (1) Variable rates of predation on the gastropod Cerithium atratum were recorded within and among the crab species Menippe nodifrons, Panopeus occidentalis, Eriphia gonagra, and Callinectes danae. Predation rate was more dependent on crab size than on crab species, and all predators consumed the largest individuals of C. atratum available. Comparison of M. nodifrons and C. danae predation of the rocky shore gastropods Stramonita haemastoma, Morula nodulosa, and Tegula viridula revealed that only M. nodifrons consumed M. nodulosa, while both crabs preyed more heavily upon S. haemastoma than on T. viridula. M. nodifrons crushed larger individuals of Stramonita. (2) Prey subjected to M. nodifrons and C. danae presented different critical sizes (S. haemastoma > T viridula = M. nodulosa. (3) The investment in shell material varied among gastropod species (M. nodulosa > T viridula > S. haemastoma) and was inversely related to their consumption rate by M. nodifrons. (4) M. nodifrons generally crushed its prey, C. danae and E. gonagra used both peeling and crushing, and P. occidentalis generally peeled. Crushing was usually lethal, while peeling attempts resulted in higher prey survival. Predatory strategy was dependent on relative prey size: small shells were crushed, medium-sized were peeled, and large ones were non-destructively preyed upon through the aperture. (5) Most shells used by the hermit crab Pagurus criniticornis (68%) in nature presented damage very similar to that recorded for gastropods preyed upon experimentally, indicating that this kind of predation makes new shells available to hermit crabs. (6) M. nodifrons showed a clear preference for the gastropod C. atratum over the hermit crab P. criniticornis, suggesting differential predation pressures in nature. In conclusion, shell availability to hermit crabs was demonstrated to be dependent on predator species and size, gastropod architectural defenses, and on the relative predator-prey size, which determined the predatory strategy and the damage inflicted on the shells.