|dc.description.abstract||Phenotypic plasticity is an environmentally based change in phenotype and can be
adaptive. Often, the change in an organism's phenotype is induced by the presence of a
predator and serves as a defense against that predator. Defensive phenotypes are induced
in freshwater physid snails in response to both crayfish and molluscivorous fish.
Alternative morphologies are produced depending on which of these two predators snails
are raised with, thus protecting them from each of these predators' unique mode of
predation. Snails and other mollusks have been shown to produce thicker, differently
shaped shells when found with predators relative to those found without predators. This
production of thicker, differently shaped shells offers better protection against predators
because of increased predator resistance.
The first study in this thesis explores costs and limits to plasticity using the snailfish-
crayfish system. I exposed juvenile physid snails (using a family structure) to either
early or late shifts in predation regimes to assess whether developmental flexibility is
equally possible early and late in development. Physid snails were observed to produce
alternative defensive morphologies when raised in the presence of each of the two
predators. All families responded similarly to the environment in which they were raised.
Morphology was found to be heritable, but plasticity itself was not heritable. Morphology was found to become less flexible as snails progressed along their respective
In the second study, I raised physid snails with and without shell-crushing sunfish
and examined the differences in shell thickness, shell mass, shell size and shell
microstructural properties between the two treatment groups. Shells of snails raised with
predators were found to be larger, thicker and more massive than those raised without
predators, but differences in microstructure were found to be insignificant. I conclude that
the observed shell thickening is accomplished by the snails' depositing more of the same
material into their shells and not by producing a more complex shell composition.||en