The evolution of armament strength: Evidence for a constraint on the biting performance of claws of durophagous decapods

Performance data for the claws of six sympatric species of Cancer crabs con firmed a puzzling pattern reported previously for two other decapod crustac eans (stone crabs, Menippe mercenaria, and lobsters, Homarus americanus): A lthough biting forces increased, maximum muscle stresses (force per unit ar ea) declined with increasing claw size. The negative allometry of muscle st ress and the stress at a given claw size were fairly consistent within and among Cancer species despite significant differences in adult body size and relative claw size, but were not consistent among decapod genera. Therefor e, claw height can be used as a reliable predictor of maximum biting force for the genus Cancer, but must be used with caution as a predictor of maxim um biting force in wider evolutionary and biogeographical comparisons of de capods. The decline in maximum muscle stress with increasing claw size in C ancer crabs contrasts with the pattern in several other claw traits. Signif icantly, three traits that affect maximal biting force increased intraspeci fically with increasing claw size: relative claw size, mechanical advantage , and sarcomere length of the closer muscle. Closer apodeme area and angle of pinnation of the closer muscle fibers varied isometrically with claw siz e. The concordant behavior of these traits suggests selection for higher bi ting forces in larger crabs. The contrast between the size dependence of mu scle stress (negative allometry) and the remaining claw traits (isometry or positive allometry) strongly suggests that an as yet unidentified constrai nt impairs muscle performance in larger claws. The negative allometry of mu scle stress in two distantly related taxa (stone crabs and lobsters) furthe r suggests this constraint may be widespread in decapod crustaceans. The im plications of this performance constraint for the evolution of claw size an d the "arms-race" between decapod predators and their hard-shelled prey is discussed.