WILLIAMS, J. D.; MCDERMOTT, J. J.; Hofstra University, Hempstead, NY; Franklin and Marshall College, Lancaster, PA: Ecosystem engineering by hermit crabs: a review of hermit crab symbiont communities

Hermit crab use of gastropod shells provides a substrate for the establishment of diverse communities of invertebrates associated with paguroid hosts. A worldwide review of hermit crab symbionts has revealed over 500 invertebrates, among 15 phyla, living with over 175 hermit crab species. Approximately 20% of the symbionts are obligate associates, the rest are evenly split as facultative or incidental associates. Arthropods, polychaetes, and cnidarians, with 113, 97, and 94 taxa represented, respectively, exhibit the highest numbers of associates. Most arthropod and polychaete associates are epibiotic, endolithic, or free-living, while cnidarians and bryozoans are almost exclusively epibiotic. The symbiotic relationships between many associates and hermit crabs are complex; the nature of the symbioses can change depending on biological and environmental factors. For example, over 25 species classified as commensals are potential egg predators of hermit crabs. Hermit crabs also benefit from some symbionts (particularly cnidarians and bryozoans) through extension of shell apertures (alleviating shell-switching) and by providing protection from predators. Through their use and recycling of gastropod shells, hermit crabs are important allogenic ecosystem engineers in marine habitats from the intertidal to the deep sea. Historically, hermit crabs provided a niche for a number of groups (e.g., bryozoans, hydractinians, polydorids) that were already associates of living gastropods. Apparently, hermit crab shells initially supplied a substrate for settlement and then many of these associations were reinforced by enhanced feeding of symbionts. Trends in the diversity of hermit crab assemblages do not follow predicted patterns, but this may reflect a lack of sampling.