The Evolution of Satiety: Studies in the Sea Urchin, Lytechinus variegatus , Nutritional Model

Feed intake is the primary determinant of nutrient and energy acquisition for most metazoans. Satiety regulates feed intake and may have many cascading effects on diet and health. Satiety of many higher organisms involves complex processes and a variety of signaling molecules that are responsive to many factors associated with nutrient intake. However, the ability to sate is observed in a number of simpler animals with less complex processes. The ability to detect and react to food is essential to an animal's survival and, thus, provides evolutionary fitness. Sea urchins, with their relatively simple digestive and nervous systems, provide an ideal model to the study the fundamental drivers of feed intake and their contribution to satiety. Understanding the process of satiety in this primitive deuterostome will provide insight into satiety mechanisms in more complex organisms. In sea urchins, feed intake is presumed to proceed until one or more requirements are met. Factors potentially affecting satiation include volume satiation (fullness), nutrient satiation (consumption to a nutritional intake target), energy satiation (consumption to an energy target regardless of the source), or some combination thereof. Evidence from field observations of sea urchins show that urchins feed selectively, reducing feed intake even when food items are readily available. In the laboratory sea urchins also exhibit feed selection. Thus, urchins regulate feed intake, but the mechanism(s) controlling feed intake and satiety are not fully understood. In this experiment adult L. variegatus (25–35mm) were fed to excess an agar‐based formulated diet that varied in both food and nutrient density. Food density refers to the concentration of all components in a formulated feed and nutrient density refers to the specific macronutrient content in the agar‐based diet. Wild collected urchins were placed in one of nineteen treatments. Each day for 11 days, agar cubes, containing a formulated diet at various food and nutrient densities, were weighed and placed in mesh cages housing a single individual in a recirculating raceway system. After 24 hours, each cube was removed, rinsed with deionized water, blotted dry and weighed to determine the amount consumed. Feed intake varied inversely with food density. Volume satiation was demonstrated at the lowest levels of dry matter inclusion in this study, under which sea urchins consumed ca. one third of their body weight per day. Additionally, sea urchins demonstrated satiation to dry matter content of the feed, consuming ca. 1.95% of their weight in dry matter at food densities ranging from 1 to 18%. There is evidence that sea urchins demonstrate a diffuse target for protein intake, indicating some level of protein leveraging. However, intake parameters cannot be fully explained by protein and carbohydrate intake, particularly at high nutrient densities, suggesting other macro‐ or micronutrients may be important in regulating food intake. These same parameters affect feed intake in many mammalian species, implying similarities in regulation of feed intake across diverse taxa. Support or Funding Information This research was funded in part by an Alabama Academy of Science Research Grant and the Aquatic Animal Research Core (NIH NORC P30DK056336).