Assessing food-induced plasticity of digestive enzyme activity during echinoid larval development

Abstract

Growth and development of planktotrophic larvae are dependent on food availability within a nutritionally heterogeneous ocean. Insufficient food can reduce growth and delay metamorphosis, thereby decreasing survival and recruitment. Phenotypic plasticity allows an organism to adjust its phenotype (morphological, physiological, biochemical) to its environment. Planktotrophic echinoid larvae with low food grow longer arms to increase feeding capacity and with high food grow shorter arms to allocate more resources to faster development of other structures. In this study, we investigated if food-induced plasticity in the Pacific sand dollar, Dendraster excentricus, also involved the regulation of digestive enzyme activities during pre-feeding and early feeding larval stages. We measured protein-specific activities of protein, lipid, and carbohydrate digestive enzymes at 33 h post-fertilization (HPF; pre-feeding stages) and 96 HPF (feeding stages) to examine if algal presence influenced enzyme rates at these stages and if there was an interaction between pre-feeding stage and feeding stage algal presence. While all enzymes studied increased activities during development, there was no significant effect of pre-feeding algae levels on pre-feeding digestive enzyme activities for all three types of enzymes studied. Furthermore, there was no interaction between larval feeding status (pre-feeding vs. feeding stage) and the presence of algal food. Evidence of food-induced plasticity was observed during the 96 HPF feeding stage for esterase (lipid digestion) and amylase (carbohydrate digestion). Both enzymes exhibited an increase in protein-specific rates ~1.6-times greater for larvae reared with algae than for those reared without algae. Protease activities were similar for fed and unfed larvae. The differential responses to food during the larval stage likely reflect the initial biochemical composition of the egg, the composition of algal food, and the unique use of each substrate to sustain larval growth and development. Our results demonstrate a multidimensional food-induced biochemical plasticity response of larvae to their feeding environment which includes discrete temporal windows of sensitivity and different enzyme class-specific responses.