Goldilocks and the Three Shells : When the Home is Just Right for the Hermit Crab Pagurus granosimanus
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Hermit crabs use gastropod shells as temporary homes. These shells protect their vulnerable, uncalcified abdomens from predation and competing crabs. Previous studies have shown that a lack of adequate housing significantly reduces growth, reproduction and feeding rates. In addition, hermit crabs are known to use a combination of tactile, visual, and chemosensory cues to examine new shells, and preferences for these shells are often species-specific. Here I used a rapid prototyping technique to explore the morphological preferences of hermit crabs in the absence of all other cues.The hermit crab Pagurus granosimanus (Stimpson) was given a binary choice between two shells of identical shape but different size. Three gastropod shells shapes from the Oxystele, Turritella, and Haustellum genera were tested. Shells were scaled at 1%intervals and 3-D printed in plaster infiltrated with cyanoacrylate, to mimic natural shell material. In all three shell shapes, hermit crabs preferred their natural shell size over enlarged or undersized shells, with the exception of oversized Turritella shells. Hermit crabs were capable of detecting a difference of just 12.7 urn in shell aperture size. There was no clear relationship between hermit crab mass and natural shell mass. This study suggests that tactile cues such as shell aperture size play a more important role in shell selection than mass. In addition, this study shows that crabs are capable of very fine distinctions in shell morphology. Taken together these results help explain why hermit crabs can be so picky during shell selection.