HOW MANY DEAD SNAILS ARE ACTUALLY DEAD HERMITS? A BEHAVIORAL STUDY OF CRUSHING PREDATION ON GASTROPODS AND HERMIT CRABS

Crushing predation on shelled organisms is among the few biotic interactions that can be observed in the fossil record. Mollusks are common research subjects, as their shells can record fatal attacks (crushed shells), nonfatal attacks (repair scars), and anti-predator defenses (such as shell thickness and ornament). Gastropods, in particular, have inspired much of the major research on durophagy. However, since the Mesozoic advent of hermit crabs, snails are not the only organisms that routinely inhabit gastropod shells.

The presence of hermit crabs means that crushed or peeled shells cannot automatically be attributed to predation on snails. How much fossil shell damage was incurred on hermit crabs? When presented with snail and hermit prey, which do predators prefer?

To explore the interactions among snails, hermits, and crushing predators, I performed behavioral trials using live Nucella lamellosa snails, Pagurus granosimanus hermits in N. lamellosa shells, and durophagous Cancer productus crabs at Friday Harbor Laboratories in Washington, USA. Two crabs were each presented with sets of four prey items (two snails and two hermits) of similar size and shell quality, and the behavior of predators and prey was observed as the crabs foraged, attacked, and consumed the prey. A total of 27 trials were initiated, with the crabs feeding in 17 of those trials.

The crabs exhibited no initial preference for either prey type, but second prey choice was significantly likely to be the same as the first choice (Fisher’s exact test, p = 0.037). Handling times were significantly longer for snail prey compared to hermits (Wilcoxon Rank Sum test, W = 146, p = 0.009). This suggests that hermits are less costly to consume; however, the crabs only consumed the hermits’ soft abdomens, so much less edible flesh was gained compared to snail kills. Behaviorally, the crabs were attracted to the hermits’ motion. When given chase, the hermits would flee, but otherwise did not avoid the crabs.

The lack of prey preference suggests that crushing predation on hermits may be proportional to that on snails; i.e., predation rates are a function of crab abundance, not prey preference. Further research into the crab-hermit-snail dynamic is necessary to determine the accuracy and applicability of this conclusion across environments and through time.