Stylohates : A Shell - Forming Sea Anemone ( Coelenterata , Anthozoa , Actiniidae ) 1

Anatomy and cnidae distinguish two species of deep-sea actinians that produce coiled, chitinous shells inhabited by hermit crabs of the genus Parapagurus. The actinian type species, Stylobates aeneus, first assigned to the Mollusca, occurs around Hawaii and Guam with P. dofleini. Stylobates cancrisocia, originally described as Isadamsia cancrisocia, occurs off east Africa with P. trispinosus. pedal disk secretes a chitinous cuticle over the small mollusk shell which the pagurid had initially occupied and to which the small actinian had first attached, often extending the cuticular material beyond the lip of the shell (Balss 1924, Faurot 1910, Gosse 1858). This arrangement affords the crab mainly mechanical protection (Ross 1971). Carlgren (I 928a) described as a new genus and species Isadamsia cancrisocia (family Actiniidae), an actinian attached to a shell occupied by a hermit crab, from four specimens collected by the Deutschen TiefseeExpedition (1898-1899) at 818 m in the Pemba Channel off east Africa. The generic name recognizes two points of superficial similarity with Adamsia: (1) the sea anemone is oriented on the shell so that its mouth is beneath that of the crustacean; (2) the coiled shell, called by Carlgren (I928a, 1928b) a carcinoecium, is almost entirely produced by the actin ian. Carlgren (I 928a, 1928b) did not note that a shell-making actinian had been reported previously. Dall (1903: 61) described what he thought was a new genus and species of trochid gastropod, Stylobates aeneus, based on shells "of a flexible, horny consistency ... " "occupied by hermit crabs and completely covered by the extended basal membrane of a large Actinia" that had been collected at 220-436 fm (402-797 m) by the U.S. Fish Commission steamer Albatross in waters of the Hawaiian Islands. Sixteen years later, having seen a collection of similar but irregularly shaped shells, Dall (1919: 379 I Manuscript accepted 12 August 1980. 2 California Academy of Sciences, Department of Invertebrate Zoology, Golden Gate Park, San Francisco, California 94118. 3 Bernice P. Bishop Museum, Division of Invertebrate Zoology, Post Office Box 19000-A, Honolulu, Hawaii 96819. 4 California Academy of Sciences, Department of Invertebrate Zoology, Golden Gate Park, San Francisco, California 94118. MANY MEMBERS OF THE ORDER Actiniaria attach obligately or facultatively to gastropod shells inhabited by hermit crabs. Some of these partnerships seem to be strictly phoretic, the normally sedentary sea anemone being transported by the motile hermit crab (Ross 1971, 1974b). The relationships between other species pairs are mutualistic, the anemone gaining motility while protecting its associate from predation (Balasch and Mengual 1974; Hand 1975; McLean and Mariscal 1973; Ross 1971, 1974b; Ross and von Boletsky 1979). As the crustacean grows, it must move to increasingly larger shells. Some species of hermits detach their sea anemones from the smaller shells and place them on the larger, whereas in other associations, the anemones transfer themselves from shell to shell (Ross 1974a). The crustacean Pagurus prideauxi Leach need never change shells when in association with the cloak anemone, Adamsia palliata Bohadsch (family Hormathiidae), which wraps around the hermit's abdomen and grows as the crab does. In the process, the