Date of Award
Master of Science
Most models of the impacts of climate change on the distributions of animals have focused on limits to thermal tolerances of individual species. Such "bioclimatic envelope" models do not consider the importance of interactions among species, each of which may respond to climate change in its own way. Hermit crabs (Paguridae) cannot exist without shells produced by gastropods. Thus, their ranges are expected to depend not only on their own physiological tolerances, but also on tolerances of gastropod species that produce shells of suitable sizes for growing crabs that use successively larger shells. To assess their potential importance to range shifts of hermit crabs, I characterized these commensal relationships over a large area of the northern Bering Sea in May to early June. Of 1539 hermit crabs collected, Pagurus rathbuni comprised 55%, P. trigonocheirus 44%, and Labidochirus splendescens 1%, with only four individuals of three other Pagurus species. Species richness for all living gastropods in my study area was 26 species, of which 18 were used by hermit crabs. P. rathbuni used shells of mostly moon snails (Naticidae); only 7 to 19% used whelk shells (Buccinidae) in the first four size classes, and 34% in the largest size class. P. trigonocheirus also used shells of mainly moon snails, but its use of whelk shells ranged from 18 to 44% in the first four size classes, and 70% in the largest size class. Densities of P. rathbuni and P. trigonocheirus varied independently of each other (r2 = 0.08, p = 0.09, N = 36 stations). Other studies suggest that hermit crabs obtain most of their shells from gastropods that have recently died, and that such empty shells are available for a relatively short period before being buried in sediments; thus, available shells should resemble the local pool among living gastropods. Correlation of P. rathbuni densities with densities of living gastropods with suitable shells was weak (r2 = 0.18, p < 0.01, N = 36 stations), while there was no correlation for P. trigonocheirus (r2 < 0.01, p = 0.59). Density patterns of hermit crabs within the five size classes did not correspond to those of living gastropods with suitable shells (randomization tests of independence, all p < 0.002). These results suggest that in the northern Bering Sea, initial effects of climate change on hermit crab distributions will depend more strongly on factors other than concurrent effects on dispersion of gastropods.
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