Shell Strength of Mud Snails (Ilyanassa obsoleta) May Deter Foraging by Diamondback Terrapins (Malaclemys terrapin)

Abstract

Diamondback terrapins (Malaclemys terrapin) do not eat the common and abundant mud snail (Ilyanassa obsoleta) even though terrapin diets are dominated by similarly sized gastropods. To resolve this paradox, we tested a structural defense hypothesis as the potential deterrent against predation. We compared resistance to compressive force of Ilyanassa and three invertebrates (Littorina irrorata,) Uca spp, and Callinectes sapidus) that terrapins commonly eat. Ilyanassa shells were 2-3 times more resistant to crushing than the other prey. High processing costs for mud snails (in terms of structural resistance to crushing) may deter predation by terrapins despite the low search costs and equivalent energetic returns relative to alternative prey items.