NOTES AND NEWS

AGONISTIC BEHAVIOR IN LEACH, 1815 (DECAPODA, ANOMURA)

BY

BRIAN A. HAZLETT Division of Biological Sciences, the University of Michigan, Ann Arbor, Michigan 48109, U.S.A.

Empty gastropod shells appear to be the primary resource limiting the population growth and individual fitness of hermit crabs (Hazlett, 1981). Ac- cordingly, competition for that resource has been hypothesized as the basis of agonistic behavior in hermit crabs (Bach et al., 1976; Hazlett, 1970). One test of this hypothesis would be to compare the levels of agonistic behavior in a that is not shell limited and a related, shell limited species. The Pagurus prideaux Leach, 1815, is always found with an in- dividual of the anemone palliata (Bohadsch) attached to its shell (Bour- don, 1955; Fox, 1965; Nafilyan, 1912). Indeed, the anemone may cover the shell entirely and its growth may provide additional protection as the crab grows (Faurot, 1910). Consequently, the crab does not need to acquire new gastropod shells as frequently as it grows and is thus much less shell limited than most other species. Extended coverage of the crab by its anemone is especially frequent in larger crabs (Faurot, 1910), the size range for which ap- propriate shells are in especially short supply (Grant & Ulmer, 1974; Vance, 1972). Observations were made on the intraspecific agonistic behavior of P. prideaux Leach and of P. bernhardus (Linnaeus, 1758) (a sympatric shell-limited species), to compare aggressive levels. Additional notes on crab-anemone interactions in P. prideaux were made. Methods. - Observations were carried out in the laboratory at the Station Biologique de Roscoff, Roscoff, France in October, 1979. Specimens of Pagurus bernhardus were collected by hand in the lower intertidal near the Biological Sta- tion. Specimens of P. prideaux were collected by dredge a few kilometers east of Roscoff. were held in monospecific communal tanks for just several days prior to observations and were fed to excess during that period. Groups of five crabs (either P. bernhardus or P. prideaux) were placed in culture dishes 23 cm in diameter with a thin sand layer on the bottom and sup- plied with running sea water. After a ten minute accomodation period, the 308

crabs were observed for 30 minutes and all encounters recorded. An encounter involved any behavior of one crab which resulted directly in retreat (increased inter-individual distance) by another. The execution of visual displays by either crab was also recorded (see Hazlett, 1968, for a description of the reper- toire of P. bernhardus). Seven groups were observed for each species and crabs were used only once. The crabs used were relatively small (7-9 mm cephalothorax length) and there was no difference in size between the two species in the crabs used (mean = 7.5 mm and 7.7 mm for P. bernhardus and P. prideaux respectively, t = 0.76, p > 0.10). In addition to the quantified observations on agonistic behavior, empty shells of the appropriate sizes were added to aquaria containing P. prideaux and their behavior towards these shells observed. - Results. When empty gastropod shells (without attached anemones) were placed in aquaria with groups of P. prideaux, two patterns of special interest were seen. A number of times, after exploratory manipulation of an empty shell, a small ( < 10 mm c.l.) crab would move into an empty shell. In more cases than I have observed in other species, the crab rather frequently moved back to its original shell after a few seconds. None of the larger ( > 11mm c.l.) individuals in the aquaria showed any interest in empty shells. In most cases in which a crab remained in the new shell, the crab transferred its anemone to the new shell. This involved about three minutes of rather unstereotyped pushing and light pulling by the crab's chelipeds around the old shell aperture/anemone base, after which the anemone released its basal attach- ment. The crab then held the anemone in its chelipeds, pressed it against the new shell, and the anemone reattached. The movements executed by P. prideaux in transferring A. palliata were markedly less stereotyped than those of Dardanus arrosor (Herbst) or Dardanus gemmatus (H. Milne Edwards) as they moved Calliactis anemones (Hazlett, 1972). While the above pattern usually followed occupation of a new shell, on three occasions, crabs entered new shells and after a few seconds walked away without their anemone. There were no obvious sudden moves by other crabs or the observer stimulating such behavior. Observations continued for at least 30 minutes in each case but the crab did not return to transfer its anemone. While this behavior may well be a laboratory artifact, it is surprising in light of the obligatory relationship between crab and anemone. The occurrence of an A. palliata on a shell occupied by a Pagurus alatus Fabricius, 1775, reported by Mainardi & Rossi (1969), (as P. excavatus (Herbst)) could be the result of the P. alatus finding a shell + anemone abandoned by a P. prideaux. Despite many hours of observation of communal holding aquaria, no shell exchanges or attempted shell exchanges (Hazlett, 1970) between individuals of P. prideaux were seen. These observations involved primarily small crabs which would change into empty shells when presented with them. Shell exchanges between P. bernhardus of this size were not at all uncommon.