King crabs (Family Lithodidae) are among the world's largest arthropods, having a crab-like morphology and a strongly calcified exoskeleton. The hermit crabs, by contrast, have depended on gastropod shells for protection for over 150 million years. Shell-living has constrained the morphological evolution of hermit crabs by requiring a decalcified asymmetrical abdomen capable of coiling into gastropod shells and by preventing crabs from growing past the size of the largest available shells. Whereas reduction in shell-living and acquisition of a crab-like morphology (carcinization) has taken place independently in several hermit crab lineages, and most dramatically in king crabs, the rate at which this process has occurred was entirely unknown. We present molecular evidence that king crabs are not only descended from hermit crabs, but are nested within the hermit crab genus Pagurus. We estimate that loss of the shell-living habit and the complete carcinization of king crabs has taken between 13 and 25 million years.

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The hypothesis that individuality is a derived trait in animals (Buss, '87, The Evolution of Individuality, Princeton, NJ: Princeton University Press; Michod, '99, Darwinian Dynamics, Princeton, NJ: Princeton University Press) can be further tested by a ''tree-based'' analysis utilizing a comparative methodology and recent phylogenies. We conducted a maximum parsimony analysis in which we mapped character states for clonality, coloniality, and mode of germline development onto four recent phylogenetic hypotheses (Peterson and Eernisse, 2001, Evol Dev 3:170-205). Clonality appears to be a shared primitive character for metazoans. Coloniality, on the other hand, is a derived trait found in relatively few phyla. The germline appears to have been derived at or near the origin of the first bilaterians. The stem-lineage metazoan thus appears to have been a clonal, acolonial organism that exhibited somatic embryogenesis. The stem-lineage bilaterian also was likely clonal and acolonial. Nevertheless, this lineage likely exhibited preformation, i.e., its germline was determined during embryonic development. In addition to supporting the hypothesis that the germline is a derived feature in animals, this analysis is relevant to current debates concerning the nature of the latest common ancestor of the bilaterians.

Pagurus longicarpus from two geographic locations were raised in the same environment in three species of gastropod shell. These shell species differed in shape and maximum size. Crabs in small, high-spired shells attained smaller sizes than those in large, low-spired shells. Further, the relative growth rates of male crabs showed differences related to shell differences. Males in small, high-spired shells produced relatively longer claws and greater right/left claw asymmetry than males in large, low-spired shells. These results show the close interaction between hermit crabs and utilized shells and may explain the geographic variation of P. longicarpus. Along the Atlantic coast, southern crabs are smaller and have relatively longer claws and greater right/left claw asymmetry than northern crabs. Southern crabs utilize small, high-spired shells almost entirely, whereas northern crabs utilize a high proportion of large, low-spired shells. Size and shape differences between geographic populations of P. longicarpus thus may be due to differences in inhabited shells.