Patterns in crab claw size: the geography of crushing

151.—Brachyuran crabs are important predators of molluscs, influencing both the distribu-tion and characteristics of their prey. Crab master-claw strength, as inferred from morpho-logical measurements, decreases in a general way from a peak in the tropical Indo-West-Pacific to progressively lower values in the Eastern Pacific, Western Atlantic, and Eastern Atlantic, with temperate crabs having the smallest claws. Many genera of large-clawed crabs found in the tropical Pacific and Indian Oceans are poorly represented or missing in the tropical Atlantic, especially West Africa. Metabolic limitations and competition are likely to be important determinants of claw size in crabs. From analyses of gastropod shell archi-tecture, I have suggested that low intertidal snails from open rocky surfaces in the tropi-cal Pacific and Indian Oceans are more predator-resistant than their ecological counterparts in the tropical Atlantic, and that temperate gastropods are less armored than tropical ories (Vermeij, 1974, 1977). These inferences were experimentally con-firmed using brachyuran crabs as predators. In Guam, crabs were offered local living snails as well as exotic shells spiced with local hermit crabs (Vermeij, 1976). Pre-liminary data have also been presented sug-gesting that Indo-West-Pacific crabs have somewhat larger, and therefore presumably more powerful, claws than Caribbean congeners (Vermeij, 1976). In the present contribution, I shall review the importance of crabs as predators of mol-luscs, and then show that the size of crab crusher claws increases in parallel with the predator-resistance of prey molluscs