ASSOCIATIONS BETWEEN SHELL MORPHOLOGY AND LAND CRAB PREDATION IN THE LAND SNAIL CERION

1. The land crab Gecarcinus lateralis is a significant predator of the abundant Bahamian land snails of the genus Cerion. The crabs typicall y 'scissor' the cylindrical shells in half or break the lip and peel b ack the shell to reach the animal which withdraws two or three whorls into the shell. Scars on shells of live adults at 73 sites in the Baha mas and Florida Keys show that about 8% (range: 0-44%) of the snails h ave survived attacks of this type. 2. An artificial crab claw was used to investigate the compressive force required to break Cerion shells of different morphotypes. Defining shell strength as the ability to wi thstand compressive forces, 10 morphotypes were found that exhibited m ean relative strengths of between 30 and 300 newtons, Feeding trials w ith one adult crab showed that snails whose shells could withstand com pressive forces of > 95 N were safe from this individual predator. 3. Multiple linear regression analyses showed that both shell size (lengt h and width) and shell wall thickness were the ultimate determinants o f shell strength. Ribs strengthen the shell by contributing to wall th ickness and also by increasing overall shell width. The thickened adul t shell lip and collabral ribs provide effective protection from attac k by peeling.