PEDOMORPHOSIS IN THE SALAMANDER AMBYSTOMA-TALPOIDEUM - EFFECTS OF A FISH PREDATOR

We examined the expression of metamorphic and paedomorphic phenotypes of the salamander Ambystoma talpoideum in relation to a fish predator. We measured the effects of fish density and time of introduction on larval activity, growth, survival, and body size, and the percentage of larvae metamorphosing or becoming paedomorphic, i.e., maturing as larvae. In addition, a laboratory experiment measured refuge use by larvae according to varying amounts of chemical cues from fish. There was no effect of fish density or time of introduction on survival and growth rate of larvae, nor on final density of zooplankton. Percentage of individuals becoming paedomorphic or metamorphosing was positively related to mean body size in ponds. An analysis of covariance, after removing the effect of body size, revealed a significant effect of fish density on the expression of the paedomorphic phenotype, but not on that of metamorphosis. The proportional decline of paedomorphs was related to the increase of larvae. The laboratory experiment indicated that refuge use by larvae was proportional to the amount of fish stimuli. Activity of larvae in the ponds, however, was not affected by fish density, but rather showed that larvae became habituated to fish. Our experiment demonstrated that expression of the paedomorphic phenotype is reduced in the presence of fish, not by increasing the number of metamorphs as expected, but by increasing the number of individuals that remained as larvae. These results indicate that models of amphibian metamorphosis based on growth and mortality are not adequate to explain paedomorphosis. We propose that trade-offs with reproductive traits such as size and age at first reproduction must be incorporated into models predicting metamorphosis as an alternative life history pathway to paedomorphosis.