The smart folks would be wrong, however. Egg production is minimal in the presence of fish. It would have been a tidy answer if they grew slowly because they produced alot of eggs, but I believe the real answer is wedded to the functional ecology of yet another trait. Behavior I said above that fish and crayfish attack and selectively eat different types of Physa . Here I should point out that the predators also locate prey by different means.
Fish are visual predators that can locate snails anywhere in the water column.
Crayfish chemotactically search the substrate for prey. Both of these predator avoidance behaviors should be costly because each can prevent feeding. In addition, snails that crawl out of the water face a severe risk of death by dessication. As necessary as these costly behaviors may be to avoid fish, they seem like wasteful defenses against crayfish. All a snail needs to do to avoid crayfish is get off the substrate.