So you’re in danger of being eaten. Ever thought of growing big enough so that you can’t fit in your enemy’s mouth anymore? That seems to be what at least two species of freshwater snail are able to do.
When the snails are raised in an environment in which some of them get eaten by crayfish (their normal predator) they grow to 10 millimetres—too big for the crayfish to eat and much bigger than their normal 4mm (3mm is about 1/8 inch).
Is this natural selection in action? No, it is a direct, programmed response to the environment. Raise the same snails in a tank with no crayfish, and their descendants only grow to 4mm again. How big they grow depends on the source of the water in their tank. If it comes from somewhere in which crayfish have been eating snails, they will grow big. If the water comes from another tank which contained only snails, or only crayfish, or one in which crayfish and snails were together, but separated by a mesh, the snails stay small.
Since live crayfish in the water give no growth response, is it some chemical released by other snails being devoured that they react to?
To test this, researchers tried crushed snails in the tank—no growth response. They may be reacting to a compound produced by a reaction between a chemical in the crayfish and another in the snails as they are eaten.
Scientific American, May 1990, p. 13A.