Symbiosis with Sponge: Differential Predator Response with Hermit Crab Shell Types

Abstract

Hermit crabs are important marine detritivores, and characteristically use discarded gastropod shells to cover their exoskeleton. However, some species use toxic symbiotic sponge shells instead, where the sponge dissolves and replaces their gastropod shell. The impact these mutualistic sponges have on hermit crab ecology remains poorly understood. In the present study, the behavioral responses of gastropod-shelled (GSCs) and sponge-shelled (SSCs) hermit crabs to two different predators was examined, with the goal of determining if the type of hermit crab shell influences the duration of their predator avoidance response. GSCs and SSCs were individually dropped into experimental tanks containing either a known hermit crab predator or a sponge predator, and the time it took for crabs to emerge from their shells after being dropped (refuge time) was recorded. It was found that hermit crabs of both shell types spent more time refuging from the hermit crab predator (a red rock crab - Cancer productus) compared to the sponge predator (lemon nudibranch - Doris montereyensis) or control treatments. However, compared to GSCs, SSCs spent (non-significantly) less time refuging from the hermit crab predator, relative to the control. The difference between SSC and GSC predator response could indicate behavioral changes due to sponge protection, size, or hermit crab species, such as increased protection due to sponge toxicity or less room to shelter in the sponge due to smaller aperture size. Shorter refuge time in sponge shells could also result in less predation by hermit crab predators if the SSCs escape faster or the sponge acts as a toxic barrier. Further research, such as larger sample sizes or more species of hermit crabs, is needed to determine the significance and mechanism behind this altered behavior.