|Hermit Crab Sponges|
Photo 1: Two juvenile Pagurus impressus in hermit crab sponges, Dog Island, Florida.
Photo 2: A larger Pagurus impressus from Dog Island, Florida
in the opening of a bright orange hermit crab sponge. Several empty
sponges, each of which originally contained a hermit crab that has since
switched into another shelter, rest nearby.
Hermit crabs are a successful group of decapod crustaceans that typically live in empty snail shells. But some hermit crabs also live in "mobile" or "portable" sponges. To see a small rounded colorful sponge moving about on the ocean floor for the first time is an unusual experience, and it often takes several minutes before one realizes that there is a hermit crab inside, moving the sponge about from place to place.
Most sponges are sessile. They grow on a hard substrate and remain there for the duration of their life, a fact that partially explained why early naturalists often viewed sponges more like plants than animals. Hermit crab sponges, however, typically grow on snail shells, although some can grow on other substrates such as other mollusk shells or wharf pilings. The snail shell can either be empty, a living shell still occupied by the living snail, or a dead shell occupied by a hermit crab.
The sponge begins its development by thinly encrusting the shell. As it grows the shell becomes increasingly surrounded and covered by the sponge. Some living snails have been found with shells completely covered by a large sponge mass, with only the shell aperture containing the snail's foot visible, making locomotion difficult for the snail. A few sponges have been found with the shell, still containing a living snail, completely enclosed by sponge.
If the shell is used by a hermit crab the crab lives in the shell as the sponge overgrows it. However, as the sponge continues to grow the shell becomes increasingly buried within the sponge mass and eventually the crab leaves the shell and occupies a chamber inside the sponge. Some large sponges (> 10 cm long) have been found with a small shell (5 mm or <) embedded within. The hollow chamber within the sponge is coiled, conforming to the shape of the crabs abdomen. The sponge opening, through which the hermit crabs chelae and 2nd and 3rd pair of legs extend, is sculpted and shaped by the crab. Sometimes one finds a sponge on the beach, with a portal-like opening in it leading to a coiled inner chamber; this is a hermit crab sponge that has no crab inside it.
Hermit crab sponges are compact and slightly compressible and have a smooth waxy-like texture. They are usually colorful, occurring in shades of green, brown, or orange. Because these sponges occur in association with hermit crabs they can live in habitats with sand or mud bottoms, where other sponges would not be able to survive because of the lack of a hard substrate and the risk of burial by sediments. The sponge benefits by being moved from place to place by the hermit crab and such sponges typically are the most common sponges in sand or muddy bottom habitats; the benefits to hermit crabs living in sponges compared to those in shells, are less obvious.
Hermit crab sponges occur in many different locations throughout the world, and have been reported from the western coast of Scotland, the Irish Sea, southwest England, the Mediterranean and Aegean Seas, the west coast of Africa, South Africa, the Korean Seas, the Chilean coast, the Eastern and Central Atlantic and the Galapagos. Quite often the researchers reporting the occurrence of these sponges are sponge biologists who neglect to identify the species of the hermit crab inside the sponge. In almost every instance the sponge partner in a reported sponge/hermit crab association is Suberites sp. (Phylum Porifera, Class Demospongiae, Order Hadromerida, Family Suberitidae). Probably the best known of these is Suberites domuncula, a Mediterranean species first reported in 1792 [domunculus = "little house"].
Sponge/hermit crab associations are known from two locations in North
American waters. All of the hermit crab sponges reported in the Pacific
Northwest from the Vancouver Island area to the Aleutians are Suberites
sp. Over 12 different hermit crab species have been found occupying
sponges in this area.
The other location is the Central Atlantic from the Carolina's south to the Gulf of Mexico. The population in this region is interesting because all sponges reported to date have been found in widely scattered locations (several from deep water off the N. and S. Carolina coasts, a single specimen from the Dry Tortugas, a single specimen from near Venezuela, and many from the Dog Island/Apalachee Bay area in the NE corner of the Gulf of Mexico. The sponges from this latter area can often be found in shallow water near shore unlike nearly all hermit crab sponges elsewhere which occur in deep water (> 20 m). Another unique difference is that these sponges may not be suberitid sponges. They superficially resemble Suberites but they may belong to a completely different order. This sponge, which has been erroneously identified in the past literature, was identified as a new species, Spongosorites suberitoides (Order Halichondrida) in 1993 on the basis of two specimens (from N. Carolina and from Venezuela). In 1996, this species was redescribed by Sandford and Kelly-Borges on the basis of specimens from the Gulf of Mexico, near Dog Island, Florida, where the sponge is plentiful. In 2002, the correct taxonomic affinities of the sponge were revealed by genetic analysis. McCormack and Kelly, studying ribosomal DNA of Florida hermit crab sponges and making comparisons with other sponges showed that the sponge is indeed a suberitid sponge like all the other hermit crab sponges know from elsewhere in the world, albeit a suberitid with atypical spicules (oxeas rather than tylostyles). Their work resulted in the sponge being transferred into the sponge family Suberitidae and in its being given a new name Pseudospongosorites suberitoides.
Floyd Sandford a biology teacher at Coe College in Cedar Rapids, Iowa has been studying hermit crab sponges and the hermit crab Pagarus impressus, the most frequently occuring sponge occupant, since 1992. Coe College, located 1000 miles from the nearest ocean, is not the most opportune base for studies in marine biology, but Dr. Sandford and Coe students periodically visit Dog Island. Although hermit crab sponges elsewhere in the world occur in deep water, the sponges are easily visible from shore on Dog Island in January when Sandford and his students usually visit the island. At his study site, a north-facing bay at the eastern end of the island, Sandford and his students commonly sample hermit crabs occupying hermit crab sponges when he conducts belt transect surveys in the bay. Hermit crabs in sponges are also easily visible moving in the water when Sandford walks the shoreline during his daily shoreline surveys. When hermit crabs occupying a sponge switch into a shell, the sponge is left unoccupied and often is found stranded at shoreline or up on shore as the tide recedes. Hermit crab sponge abundance varies on an annual basis. They were commonly seen during transect surveys and shoreline surveys in 1992, 1996, and 1999, and were extremely abundant in 1994 and 1997, but were uncommon in 1993, and rarely found in 1995.
Studies to date have investigated preferences of different hermit crab species for sponge shelters, shell/sponge switching, and the identity of the gastropod species used as a substrate by the sponge. These studies have shown that in the Gulf population only certain hermit crab species commonly use sponge shelters, usually either Pagurus impressus or Paguristes hummi. Both are subtidal species but in the winter months individuals of P. impressus, many in sponges, move close to shore, and sponges -- both empty or with hermit crab occupants -- are often cast ashore by wave action or stranded by receding tides. All hermit crab sponges found near Dog Island have contained gemmules. Gemmules, reproductive structures that can survive adverse conditions such as desiccation or extreme cold, are not typically produced by marine sponges. Their common occurrence in hermit crab sponges may represent an adaptation to help counter the consequences of stranding on shore.
Shell/sponge switching experiments have shown that all four kinds
of switchs occur, i.e. shell to shell, shell to sponge, sponge to shell,
and sponge to sponge, but that hermit crabs in shells and those in sponges,
prefer to switch into other shells, not into sponges. Also, compared
to hermit crabs captured in shells, hermit crabs captured in sponges
are more likely to leave their shelter when other empty shelters are
For more information about hermit-crab sponges, consult the references below:
Sandford, F, and Kelly-Borges, M. (in press - will appear in 1997). Rediscription of the hermit-crab sponge Spongosorites suberitoides Diaz, Pomponi and Van Soest. J. Natural History.
Sandford, F, and Brown, C. 1997. Gastropod shell substrates of the Florida hermit-crab sponge, Spongosorites suberitoides, from the Gulf of Mexico. Bulletin of Marine Science, 61(2): 215-223.
Sandford, F. 1995. Shell/sponge switching by hermit crabs, Pagurus impressus. Invertebrate Biology, 14(1):73-78.
Sandford, F. 1994. The Florida Hermit-crab sponge , a little known "mobile" sponge from the NE corner of the Gulf of Mexico, and its hermit crab associations, 273-278. In: Sponges in Space and Time, RWM van Soest, TMG Van Kempen, and JC Braekman, (eds), Balkema, Rotterdam. Proceedings of the IVth International Porifera Congress, Amsterdam, the Netherlands, April 19-23, 1993, 515 pp.
Soest, RMW van. 1994. On the intimate relationships of molluscs and sponges. De Horen en Zijn Echo. Stichting Libri Antilliani, Zoolisch Museum Amsterdam, pp 71-75.
Soest, RWM van. 1993. Distribution of sponges on the Mauritanian continental shelf. In: Ecological Studies in the Coastal Waters of Mauritania, WJ Wolff, J. van der Land, PH Nienhuis, and PAWJ de Wilde (eds), Klewer, Belgium, 258: 95-10 6.
Vosmaer, GCJ 1933. The Sponges of the Bay of Naples - Porifera Incalcaria. Vol. 1. The Hague, Martinus Nijhoff. [Good historical accounting of the best known hermit crab sponge, Suberites domuncula, first reported in 1792]
Wass, ML. 1955. The decapod crustaceans of Alligator harbor and adjacent inshore areas of Northwestern Florida. Quart. J. Fla. Acad. Sci., 18(3):129-76 [Perhaps the first mention of hermit-crab sponges in the Dog island area]
Wells, HW. 1969. Hydroid and sponge commensals of Cantharus cancellarius
with a "false shell". Nautilus, 83(3):93-102 [Likely the first significant
discussion of hermit-crab sponges in the Dog Island area]