Interactive Effects of Food Availability and Salinity on Phenotypic Plasticity of Echinarachnius parma Larvae

Many marine invertebrate larvae exhibit plasticity of phenotypes associated with feeding, development, and growth. While we understand how larvae respond to single environmental variables, we know less about how multiple variables impact the expression of plasticity. Pluteus larvae of the sand dollar Echinarachnius parma lengthen feeding structures under low food levels but will allocate more energy to other structures when there is sufficient food. Larvae will also delay their hatching and subsequent rates of development under low salinity. I examined the morphological and developmental plasticity of E. parma larvae in response to varied salinity and food level combinations. Larvae were reared under constant low or high salinity and constant low or high algal food levels for ten days. Morphological measurements were collected from images of larvae and developmental stages were recorded. Food level, but not salinity, had a significant effect on feeding structure plasticity. As demonstrated for other echinoid species, E. parma larvae exhibit a tradeoff between food collecting and food processing structures. Larvae appeared to prioritize their phenotypic response to food level over exposure to low salinity. These results suggest that as organisms integrate their responses to multiple environmental stressors, one variable likely takes precedence