Hermit Crabs

There are 800 known species in the super family Paguroidea (Hazlett, 1981). They are commonly found in coastal tidal areas and exist in both terrestrial and aquatic habitats. Being detritovores, food is abundant in the crab's surroundings, and predators from birds to larger crabs also influence the crab's life. Hermit crab's shells are their main defense, thus finding appropriate shells is a great mitigating factor of surivial value in this species. This site focuses on the hermit crab behavior of choosing an appropriate shell for defense and growth, in the context of external influences such as limited shell availability, competition, predation, and varying shell quality.

Crabs and Shells

Much work has been done on hermit crab's distinctive use of exogenous shelter. Hermit crab's abundancy and vitality make them perfect specimens for lab work. There is a significant body of literature focusing on shell selection, morphology, and phylogentic history (Hazlett, 1981).

Shell Selection

It is clear that hermit crabs do not enter shells at random, but select shells according to shape, shell opening and weight. A variety of mechanisms such as shell rapping, olefactory signals, and visual ques have been shown to transfer information about the quality of a shell. Hermit crab's not only find empty shells from its original organism (usally gastropods), but also intraspecies agression for shells occupied by other crabs. A more dominant crab is able to intiate a shell switch with a subordinate crab who may gave a higher quality shell.

Tinbergen's Questions

Shell selection in hermit crabs can be categorized among Tinbergen (1967) 4 example of crucial topics in animal behavior research, which are: mechanism, adaptive value, phylogeny, and ontogeny. Although the fossil record is vague on specific hermit crab behaviors, we can trace the successful behavior of exogenous shell preferences back to the cretaceous period (Maclaughlin et al., 2004). Hermit crab's prevalence as well as their long zoological history shows the adaptive value of the trait, and the fact that it is their main system of protecting themselves and their young indicates that this trait heavily influences survival value. We can also examine the high selective pressure through a hermit crab's lifecycle to find a shell to protect it as well as accomodate its growth. Additionally, we can directly observe individual hermit crab mechanism of competition and agnostic behavior surrounding shell aquisition, which is crucial for obtaining the optimum shell fit and quality.

Our Approach

The studies we reference are based on a broad range of species inside the Paguroidea. Yet, we consider their shell selection behavior to be similar among species, thus we have collapsed the research under the general heading of 'hermit crabs'. Although we recognize that different species will slightly vary on shell selection, we think the underlying behavior is similar enough to ignore interspecies differentiation.