In many decapod-gastropod interactions, narrow apertures deter the shell entry techniques employed by crabs and crayfish. On the other hand, rotund shell morphology is best to deter shell crushing or shell swallowing predators such as fish. The snails I study are at least partially constrained in that additional defense against one predator is only possible at the expense of defense against the other predator. That is, there is a tradeoff. So, although there is adaptive plasticity of shell shape, there is a growth cost associated with the ability to be plastic.
Fish primarily eat large Physa whereas crayfish eat small Physa .
Here then is another tradeoff. Snails also address this problem with plasticity. When raised with crayfish, they grow faster than when raised in predator-free water. This results in snails getting quickly too big for crayfish to eat. When raised with fish, they grow slowly, resulting in sizes that are unappetizing to fish.