Balancing macronutrient intake in cultured Lytechinus variegatus

Among aquatic organisms, nutritional requirements are historically assessed by evaluating formulated diets with changes in a single nutrient profile. These studies provide organisms with the option of regulating the total feed intake but do not enable individuals to regulate intake of specific nutrient ratios. An alternate experimental approach, known as the geometric framework (GF) for nutrition, tests whether organisms held under specific conditions have optimal target intake levels for particular nutrients and whether, when provided with diet choices, they will self-regulate nutrient intake to reach these specific targets. In this study, we used the GF to assess intake levels of dietary protein and carbohydrate in cultured adults of the sea urchin Lytechinus variegatus provided choices between diets varying in both dry matter and specific macronutrient concentrations. Adult urchins (ca. 120 g) held previously on formulated diets were offered moist gel-based diets in pairwise combinations. Diets varied in levels of fish meal isolate (as a protein source) and wheat starch (as a carbohydrate source) and in nutrient concentration (5 or 10% dry matter). Regardless of diet combination, individuals maintained an average dietary protein intake of ca. 0.047-0.061 g day− 1. Dietary carbohydrate intake was not regulated to a specific level and ranged from 0.042-.0136 g day− 1. These data suggest that large adult L. variegatus held in culture have a tightly regulated intake target for dietary protein but not carbohydrate. Regardless of nutrient ratios or macronutrient concentration, individuals adjusted intake patterns to defend this target. We suggest that the GF will be an important tool in evaluating macronutrient requirements in cultured sea urchins and other aquatic species.