Most species wear empty snail shells as portable houses. Hermit crabs host a great diversity of symbiotic associates: excluding parasites, over 515 species of animals in 14 phyla have been found in association with hermit crabs (though 232 of the species, including all representatives of four of the phyla, are thought to be incidental associates). The taxa exhibiting the highest number of associates are Arthropoda (126), Polychaeta (105), and Cnidaria (100). Pagurus longicarpus and Pagurus pollicaris have the most diverse assemblages in our area, and the fifth and sixth most diverse in the world, having each been found in association with more than 40 species. In addition, hermit crabs are known to be parasitized by over 130 animal species, mostly barnacles and isopods. 17 hyperparasites have been found on hermit crabs.
Williams, J.D. & McDermott, J.J. (2004) Hermit crab biocoenoses: a worldwide review of the diversity and natural history of hermit crab associates. Journal of Experimental Marine Biology and Ecology 305(1): 1-128.
McDermott, J.J., Williams, J.D., & Boyko, C.B. (2010) The unwanted guests of hermits: A global review of the diversity and natural history of hermit crab parasites. Journal of Experimental Marine Biology and Ecology 394(1-2): 2-44.