Durophagous crabs successfully hunt hard-shelled prey by subjecting them to extremely strong biting forces using their claws. Here I show that, for a given body mass, six species of Cancer crabs (Cancer antennarius, Cancer branneri, Cancer gracilis, Cancer magister, Cancer oregonensis and Cancer productus) were able to exert mean maximum biting forces greater than the forces exerted in any other activity by most other animals. These strong biting forces were in part a result of the high stresses (740-1350 kN m(-2)) generated by the claw closer muscle. Furthermore, the maximum muscle stress increased with increasing mean resting sarcomere length (10-18 microm) for the closer muscle of the claws of these six Cancer species. A more extensive analysis incorporating published data on muscle stresses in other animal groups revealed that stress scales isometrically with the resting sarcomere length among species, as predicted by the sliding filament model of muscle contraction. Therefore, muscle or filament traits other than a very long mean sarcomere length need not be invoked in explaining the high stresses generated by crustacean claws.