Hermit crab predator avoidance based on shell type: Suberites domuncula symbiont or gastropod shell

Abstract

Hermit crabs are ecosystem engineers, decomposers, and a prey source for many larger predators such as the red rock crab, Cancer productus. All of these ecosystem factors are affected by interactions between species, especially predator-prey interactions. Some subtidal hermit crabs live in symbiosis with Suberites sponges that dissolve and replace the calcareous gastropod shell the hermit crab initially inhabits and continues to grow with the hermit crab. Sponges produce anti-predatory chemicals which could protect the hermit crab. Despite these anti-predatory defenses, some nudibranchs (e.g. Doris montereyensis) specialize in eating Suberites domuncula, a sponge species that often lives in symbiosis with hermit crabs. Predatory avoidance behavior is quantified by the time it takes the hermit crab to come out of its shell (in this study referred to as emergence time), after it has retracted into its shell in response to a stimulus. Emergence time gets shorter when running away is more beneficial to its survival, such as when a hermit crab has a weak shell, or a slow and shell crushing predator is near. It was hypothesized that Sponge shelled hermit crabs would have a longer emergence time when exposed to C. productus, and a shorter emergence time when exposed to a D. montereyensis, compared to gastropod shelled hermit crabs. Emergence time of hermit crabs was measured after being dropped from 20cm above the water surface of three treatment tanks: C. productus, D. montereyensis, and control, to explore the effects of having a sponge shell on predatory avoidance behavior of hermit crabs. Hermit crabs inhabiting sponge shells were found to have shorter emergence times overall but responded similarly to gastropod shelled hermit crabs: longer emergence time when exposed to C. productus than D. montereyensis or control. Knowledge of symbiotic and predator-prey interactions are crucial in understanding food webs and ecosystem dynamics.