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 male adult snow crabs either in the wild or in laboratory experiments. We tested the hypothesis that cannibalism failed to occur as smaller snow crabs became large enough to resist the force of larger male snow crabs. Chela force and resistance of the cuticle to loading were compared. Chela, merus, and carapace cuticles differed markedly in mechanical properties. The chela was hard compared to the merus which was the most flexible cuticle. Strength of the cuticle generally increased with size of crab, but decreased markedly at molting and remained low for several months past molting. The mechanical 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 previously documented size and contractile force of the closer muscle. Closing 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-adult, snow crabs. Factors other than strength of the cuticle may explain size-selective cannibalism.