Reference : Biol. Bull.. 149 : 205-213. (August, 1975) AN ANALYSIS OF SHELL OCCUPATION BY TWO SYMPATRIC SPECIES OF HERMIT CRAB. I. ECOLOGICAL FACTORS K. A. MITCHELL Department of Zoology, The University of Glasglou', Glasglow, G12 8QQ, Scotland, U.K. To protect itself from its environment, a hermit crab must have a gastropod shell (Reese, 1969; Vance, 1972a) and where numbers of gastropod shells are limited, there is evidence that this limitation reduces the number of hermit crabs an environment can support (Hazlett, 1970; Provenzano, 1960; Reese, 1969; Thompson, 1903). In addition to acting as a limiting factor, the type of shells present in any one place can also determine what size or species of hermit crab is present in that area. Reese (1969, p. 346) states that: "in Hawaii the availability of shells seems to limit the size of individuals in the population", while Markham (1968) found that occupation of small shells by the European hermit crab, Pagnrits bcrnhardns, restricted the growth of these crabs. Examples of resource partition-ing of hermit crab species on the basis of two different types of shell have also been studied. Grant and Ulmer (1974) and Wright (1973) both found that shells covered by Hydractinia echinata were preferred by one out of two sympatric species while Vance ( 1972b) showed that, of the three rocky intertidal species of the San Juan Islands, Washington, one species preferred short light shells whereas the other two species preferred comparatively taller and heavier shells. In the present study experiments were performed on two species of hermit crabs, Pagitnts longicarpus and Clibanarius vittatits, which were equally abundant in the intertidal zone at Beaufort Harbor, North Carolina. This area had no empty gastropod shells, thus suggesting that shells were a limiting factor in this area and that resource partitioning of the shells may explain the coexistence of the two hermit crab species. The experiments were designed to answer the following questions. (1) Are shells a limiting factor in this area? (2) Does resource partitioning take place between P. longicarpus and C. vittatnsf (3) If resource partitioning does take place, what factors maintain the partitioning? MATERIALS AND METHODS All animals used in this experiment were taken from the intertidal zone of an area approximately 100 meters long at Beaufort Harbor, North Carolina, next to the Duke University Marine Laboratory dock. This area has a mixture of sand and mud as a substrate and has many living gastropods buried in it. All experiments were conducted during the summer months and no difficulty was encountered in obtaining specimens of either species of hermit crab in this area. Three series of experiments were conducted. The first series tested the effect of shell limitation on the hermit crabs' occupation of shells, while the second tested the ability of one hermit crab species to obtain a preferred species of shell from the other hermit crab species. The third series of experiments tested the relation between substrate preference of both hermit crab species and the natural occurrence of two living gastropod species on different substrates. 205
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