SPONGE/SHELL SWITCHING BY HERMIT CRABS, PAGURUS IMPRESSUS

Although hermit crabs are cited as obligate users of gastropod shells, some also use portable sponge shelters. At Dog Island, Florida, in the NE corner of the Gulf of Mexico, the hermit crab Pagurus impressus occupies sponges, commonly found near shoreline in January. However, testing of P. impressus in conspecific groups and in a mixed-species assemblage (with P. longicarpus, P. pollicaris, and Paguristes hummi) confirmed that individuals of P. impressus do not prefer sponge shelters. Hermit crabs in sponges sometimes abandoned them for shells and those in shells rarely switched into sponges. Individuals of P. impressus, especially juveniles, may occupy sponge shelters due to competition from more aggressive species and/or shell-scarcity. Additional key words: sponges, Porifera, Decapoda, symbiosis, Gulf of Mexico Most pagurid hermit crabs live in empty gastropod shells, but certain species use sponges which grow on gastropod shells. The shells may be empty or occupied by either the living snail or a hermit crab. As the sponge overgrows and encloses the shell, the hermit crab uses less of the shell, occupying instead a spiralshaped chamber within the sponge and maintaining a sculpted opening to the outside (Fig. 1). These sponges, referred to in the literature as "mobile" sponges (van Soest 1993), are the most common sponges in some sandy or muddy bottom habitats (van Soest 1993). "Portable" sponge, a better name for a sponge transported by a living snail or hermit crab, will be used to refer to such sponges in this paper. This association between pagurid and portable sponge is distinct from that of hermit crabs in the genus Cancellus, which typically do not occupy shells, but instead use a variety of shelters including pieces of rock or coral, or crevices and natural chambers in other kinds of sponges (Mayo 1973). Associations between hermit crabs and portable sponges are known from the Western and Northern Pacific (Benedict 1900; Hart 1971), Chilean coast (Desqueyroux 1972), Central Atlantic (Diaz et al. in press), Eastern Atlantic (Vosmaer 1933; van Soest 1993), Scotland (Scott 1885), and Korea (Sim 1990). The best-known association involves the sponge Suberites domuncula, a Mediterranean-Eastern Atlantic species (van Soest 1993). The Florida hermit-crab sponge, as it is commonly called by local biologists (Rudloe 1984, 1991), is a compact portable sponge of uncertain taxonomy found in the NE corner of the Gulf of Mexico (Sandford 1994). It grows on gastropod shells of several species (Wells 1969; Sandford, unpubl. data), but most commonly uses the buccinid whelk Cantharus cancellarius as a substrate (Sandford 1994 and unpubl. data). Sponges are found growing on all sizes of shells (Sandford, unpubl. data), including large shells with a thin overlying sponge layer and small shells embedded in larger sponges (Fig. 1). Several hermit crab species may inhabit the sponge, but only Pagurus impressus Benedict 1892 is common in sponges found near shoreline in January, when they are relatively abundant and when most field work over the past 8 years has been done. Daily shoreline surveys for 2 weeks in July 1993 found few sponges, most in poor condition. The association between sponges and hermit crabs has been largely unstudied since first described by Wells (1969). Little has been reported on behavior of pagurids in sponge shelters, except for conjectures on possible benefits. Vosmaer (1933) assumed a mutualistic relationship between Suberites domuncula and hermit crabs, with the sponge gaining food and oxygen, and the crab benefitting from long-term protection in a living, growing shelter. Hart (1971), commenting on Pagurus stevensae from British Columbia and the sponge Suberites ficus, mentions that a hermit crab would not have to change shelters, implying that hermit crabs in living sponges might benefit from reduced e ergy costs and predation risks associated with a need to locate and contest larger shells. Such shell-switching by hermit crabs has been well documented (Wilber This content downloaded from 157.55. 39.105 on Fri, 07 Oct 2016 04:29:17 UTC All use subject to http://about.jstor. org/terms