Garza, Mark Isaac (2005-12). Predator induced defenses in prey with diverse predators. Master's Thesis. Thesis uri icon

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 developmental pathways. 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.
  • 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
    developmental pathways.
    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.

publication date

  • December 2005