OLSON, Christopher; FARRAR, Eugenia; ADAMS, Dean; Iowa St. Univ.; Iowa St. Univ.; Iowa St. Univ.: A Morphometric Shape Analysis of Phenotypic Plasticity in Plains Spadefoot Toad (Spea bombifrons) Tadpoles

Phenotypic plasticity may enable organisms to cope with unpredictable environments and resource availability. The feeding morphology and trophic niche of spadefoot toad (Spea bombifrons) larva is highly influenced by environmental conditions, resulting in omnivore morphs and carnivore morphs. We used landmark-based morphometric techniques to examine differences in shape among tadpoles collected from ponds differing in the abundance of fairy shrimp (Order: Anostraca). Morphological differences were present between ponds and trophic morphs, however site and trophic morph interacted, suggesting body shape does not change in the same way between ponds and trophic levels. Indeed, carnivores lacking fairy shrimp as prey showed enlarged jaw muscles compared to those with fairy shrimp or omnivores from either pond. Carnivores were rare in the pond lacking fairy shrimp, but the function of enlarged jaw muscles in a few individuals may allow them to engage in cannibalistic feeding behavior. When landmarks associated with jaw muscles were removed, differences persisted between ponds and trophic niches with no significant interactions, therefore the remaining changes in shape proceed in the same trajectory among trophic levels and sites. Omnivores from the two ponds have distinct morphologies while both morphs from the pond lacking fairy shrimp most closely resembled each other, despite large jaw muscles in carnivores, thus highlighting the difficulty in defining a “morph” in this species. Removal of jaw muscles from the analysis diminishes, but not eliminates, differences among the groups, suggesting ‘morphological integration’ in which different components of morphology covary with one another to act as a functional unit.