Associations between shell morphology and land crab predation in the land snail Cerion

Abstract

1. The land crab Gecarcinus lateralis is a significant predator of the abundant Bahamian land snails of the genus Cerion. The crabs typically 'scissor' the cylindricalshells in half or break the lip and peel 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 Bahamas and Florida Keys show that about 8% (range:0-44%) of the snails have 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 strenght as the ability to withstand compressive forces, 10 morphotypes were found that exhibited mean relative strenghts of between 30 and 300 newtons. Feeding trials with one adult crab showed that snails whose shells could wothstand compressive forces of >95N were safe from individual predator. 3. Multiple linear regression analyses showed that both shell size (lenght and width) and shell wall thickness were the ultimate determinants of shell strenght. Ribs strenght the shell by contributing to wall thickness and also by increasing overall shell width. The thickened adult shell lip and collabral ribs provide effective protection from attack by peeling.