Brachyuran crabs are important predators of molluscs, influencing both the distribution and characteristics of their prey. Crab master-claw strength, as inferred from morphological measurements, decreases in a general way from a peak in the tropical Indo-WestPacific 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.