MCALISTER, J/S; University of North Carolina at Chapel Hill: Environmental heterogeneity and the evolution of plasticity in Panamanian echinoid larvae

Theoretical models predict that phenotypic plasticity should evolve in response to changes in environmental heterogeneity. To test the hypothesis that historical changes in heterogeneous food resources are associated with the evolution of plasticity, I studied the expression of morphological plasticity of food collection structures in larvae of echinoid Echinoderm �geminate species pairs� found off the coasts of Panama. Closely related species of sea urchins in the genera Diadema, Echinometra, and Eucidaris formed when the Panamanian Isthmus raised 2.8-3.1 million years ago and created two different food level environments: variably high food levels due to strong, seasonal nutrient upwelling in the eastern Pacific versus constant low food levels due to little nutrient upwelling in the western Caribbean. In response to food limitation, larvae of several other echinoid species elongate a food collecting ciliated band by lengthening larval arms during early larval development. For this study, larvae from multiple full-sib pairings of each species were reared in one of two different replicated food environments (1 and 5 algal cells/microliter) for 10 days post fertilization. 3-D morphological measurements of individual arm and body lengths were made for over 3600 larvae. My prediction was that Pacific species larvae would express greater arm and ciliated band length plasticity than Caribbean species larvae, in response to heterogeneous environments, as found in the eastern Pacific. This study will contribute to our understanding of the effects of food level heterogeneity on the evolution of phenotypic plasticity and the degree to which plasticity varies among related taxa.