Enzymatic production of small molecules attracting hermit crabs to simulated gastropod predation sites

Abstract

Predatory marine gastropods commonly capture and feed upon gastropod prey. This event is of importance to hermit crabs as the result is a shell available for occupation. Previous studies of gastropod predation sites (McLean, 1974, 1975; Rittschof, 1980) have shown that hermit crabs are attracted to the sites via chemicals released from the prey snail flesh. This study was conducted to further characterize the nature of the chemical signal and to investigate possible mechanisms of its production. Simulated gastropod predation sites were initiated and observed on ebb tide in the low intertidal zone in Sarasota Bay, Sarasota, Florida. Flesh from the muscular foot was used to construct three major types of gastropod sites: fresh flesh, flesh which had been frozen-thawed, and fresh flesh plus trypsin. The tissue was placed in dialysis bags to limit the diffusion of all but small molecules. The first hermit crabs came to the trypsin sites 16 ± 3 min post initiation, to freeze-thaw sites 33 ± 11 min, and to flesh sites after about 2 hr. The average number of crabs attending each type of site was 0.3, 11.6, and 12.6 for the flesh, freeze-thaw, and trypsin sites, respectively. The time of initial arrival of hermit crabs to the freeze-thaw and trypsin sites was similar to that observed at natural predation sites. These data provide indirect evidence that hermit crabs are responding to peptides produced by enzymatic degradation of proteins in gastropod flesh, and support the hypothesis that the protease(s) responsible for producing the signal peptides originates from the predatory gastropod. Finally, a tabulation is presented of all gastropod prey species (13 species in 12 genera) and hermit crab attendants (8 species in 4 genera) for which the pheonomenon has been observed.