REITZEL, A.M.*; HEYLAND, A.: Maternal investment and morphological plasticity: Insights from subtropical echinoids

Animals face a heterogeneous environment with respect to food. When feeding in an unpredictable environment, some animals are able to modify ingestive and digestive morphology and/or physiology in order to increase nutrient acquisition. Previous research has shown planktotrophic (i.e. feeding) echinoid larvae respond to the concentration of food in the environment by altering their phenotype to increase ingestion rates. In food scarce environments, the larvae elongate their arms to increase water clearance rates and therefore amount of food encountered. What has not been addressed in echinoid larvae is morphological plasticity with different degrees of maternal investment (i.e. egg size). A larger maternal investment results in a reduction in energy required from exogenous food to complete development. A decreased dependence on exogenous food may alter nutritional strategies. We tested for plasticity of larval arm length and stomach area in three subtropical echinoid species with different egg sizes (Mellita quinquiesperforata, 100 mm egg diameter; Clypeaster subdepressus, 150 mm; Leodia sexiesperforata, 200 mm) in three food environments (2, 6, and 8 algal cells per ml). Measurements of post-oral arm length and stomach cross-sectional area indicate that the expression of plasticity is dependent on maternal investment. Mellita and Clypeaster larvae both expressed plasticity of larval arms and stomach three days post-fertilization. Leodia larvae did not show significantly different larval morphologies over five days in different food treatments. These results suggest that there are costs and limits to phenotypic plasticity in obligate feeding echinoid larvae.