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 gastropodCerithium atratumwere recorded within and among the crab speciesMenippe nodifrons,Panopeus occidentalis,Eriphia gonagra, andCallinectes danae. Predation rate was more dependent on crab size than on crab species, and all predators consumed the largest individuals ofC. atratumavailable. Comparison ofM. nodifronsandC. danaepredation of the rocky shore gastropodsStramonita haemastoma,Morula nodulosa, andTegula viridularevealed that onlyM. nodifronsconsumedM. nodulosa, while both crabs preyed more heavily uponS. haemastomathan onT. viridula.M. nodifronscrushed larger individuals ofStramonita. (2) Prey subjected toM. nodifronsandC. danaepresented 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 byM. nodifrons. (4)M. nodifronsgenerally crushed its prey,C. danaeandE. gonagraused both peeling and crushing, andP. occidentalisgenerally 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 nondestructively preyed upon through the aperture. (5) Most shells used by the hermit crabPagurus 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. nodifronsshowed a clear preference for the gastropodC. atratumover the hermit crabP. 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.