SUBTROPICAL TROPHIC CASCADES: HERMIT CRABS SKEW THE FUNCTIONAL ECOLOGY OF GASTROPOD ASSEMBLAGES

Richard Alexander was known for his work on marine predator-prey interactions and on biota that encrust or bioerode invertebrate skeletons. In celebrating his life and his work, hermit crabs and their use of gastropod shells link these two topics into a functional ecological framework. A three-year study within a subtropical intertidal environment (Puerto Penasco, Mexico) revealed that hermit crabs: (1) represented more gastropod taxa than what was living in the area, slightly inflating actual gastropod diversity; (2) increased the relative abundance of the gastropod shell resource, living snails comprised less than half of the shells; (3) consistently maintained gastropod shell diversity and relative-abundance patterns during the three year period while living gastropod taxa fluctuated in species and abundance; and, (4) altered the composition and diversity of shell-inhabiting biota.

The skewed retention of dead gastropod shells by hermit crabs along side the living snails, or in the absence of living counterparts, has implications for the gastropod fossil record. The diversity of gastropod species is elevated above that of the living snail if hermit crabs occur in the environment; the relative abundance of "gastropods" is primarily that of crustaceans, more than half of the shell resource in this subtropical habitat were represented by hermit crabs; hermit crabs consistently maintained these patterns such that a time-averaged gastropod deposit is more likely to represent a hermit crab death assemblage rather than the once-living snail; the actual number of live gastropod prey available for predators is a relatively small proportion of the total shelled prey; and, biota that attach to or erode shells flourish with hermit crab occupancy. Consequently, when hermit crabs dominate the shell resource, the functional ecology of the system shifts from herbivory (gastropod feeding mode) to scavenging and filter feeding by hermit crabs. Biota encrusting shells shift from autotrophic algae associated with grazing snails to heterotrophic biota associated with crab-occupied shells. Gastropods may start the trophic relay, but hermit crabs are a major component of this shell game, and by their activities, they diversify the functional role that gastropod shells play in marine communities.