Shell strength and carapace size in non-adult and adult male snow crab (Chionoecetes opilio)

Cannibalism occurs in snow crab and has been observed both in the wild and in the laboratory, but crabs larger than 60 mm CW are not killed by large m ale adult snow crabs either in the wild or in laboratory experiments. We te sted the hypothesis that cannibalism failed to occur as smaller snow crabs became large enough to resist the force of larger male snow crabs. Chela fo rce and resistance of the cuticle to loading were compared. Chela, merus, a nd carapace cuticles differed markedly in mechanical properties. The chela was hard compared to the merus which was the most flexible cuticle. Strengt h of the cuticle generally increased with size of crab, but decreased marke dly at molting and remained low for several months past molting. The mechan ical advantage (MA) of the chela was larger in adult than in non-adult snow crabs of similar size. The force of the chela was calculated from MA and p reviously documented size and contractile force of the closer muscle. Closi ng force at the first denticle on the dactyl of large male adult snow crabs was large enough to break the cuticle of non-adult and adult snow crab of any size. Only crabs less than 60 mm CW, however, were vulnerable to forces delivered at the tip of the dactyl of large adult, but not of large non-ad ult, snow crabs. Factors other than strength of the cuticle may explain siz e-selective cannibalism.