Does claw morphology of the green crab Carcinus maenas vary in relation to its diet on rocky versus fine-sediment shores of southwest New Brunswick, Bay of Fundy, Canada?

• Claw morphology and diet were compared for green crabs between rocky and fine-sediment shores.
• No differences were found in the claw morphology between the two habitat types.
• Clear differences in diet not due to differences in hard-bodied prey
• Hard-bodied prey likely have greater impact on claw morphology than amount of soft-bodied prey.

Inter-specific variation in claw morphology of decapod crustaceans is related to the particular dietary niche different species occupy, but very few studies have investigated whether claw variation among populations of a same species is similarly related to diet. We set out to quantify differences in claw morphology between green crabs inhabiting rocky and fine-sediment shores in the southwest Bay of Fundy, Canada, and asked whether these were related to differences in the prevalence of hard- and soft-bodied prey in the crab's diet. We found that claw morphology did not differ between crabs from the two habitat types, but did significantly differ between crabs sampled from different rocky shores and different fine-sediment shores. The absence of significant differences in claw morphology between crabs from rocky and fine-sediment shores was somewhat surprising, given that we did observe significant differences in crab diet between the two habitats. However, these dietary differences were related to the prevalence of soft-bodied prey, and hard-bodied snails and mussels were the most prevalent prey items in the guts of crabs from both rocky and fine-sediment shores. Our results therefore suggest that the prevalence of soft-bodied prey in the diet of green crabs in nature does not affect the morphology of their claw. Given our findings, along with recently published patterns of phenotypic co-variance between green crab claw size and snail shell thickness, we hypothesize that consumption of hard-bodied prey may be more consequential to variation in green crab claw morphology than the diet as a whole. More field studies are needed to complement the rich tradition of lab-based research into the biochemical properties of the decapod claw, and in particular to elucidate the role of diet on intra-specific variation of this appendage in nature.