FEED INTAKE AND SATIETY IN THE SEA URCHIN Lytechinus variegatus  

Sea urchins are in high demand for both commercial seafood production and research applications. Aquaculture provides the opportunity to increase production of sea urchins while reducing many of the risks associated with declining sea urchin fisheries. For sea urchins, understanding nutritional requirements will be of primary importance in the development of efficient culture strategies.

Feed intake is the primary determinant of nutrient and energy acquisition for most metazoans. In sea urchins, feed intake proceeds until one or more requirements are met. The level at which an organism stops feeding is satiation. Factors potentially affecting satiation range from volume satiation (fullness) to nutrient satiation (when an organism consumes to a nutritional intake target), or to energy satiation (the point at which enough energy, regardless of the source, is consumed). Evidence from field observations of sea urchins show that urchins feed selectively, reducing feed intake even when food items are readily available. Thus, urchins are regulating 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 nutrient density. Nutrient density refers to the concentration of formulated feed in the agar-based diet. Wild collected urchins were placed in one of four treatments (n=5 urchins per diet treatment). Each day, agar cubes containing a formulated diet at various nutrient densities were weighed and placed in individual mesh cages in a recirculating raceway system during a six-day period. 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 nutrient density. Highest feed intake was observed in those consuming a diet containing 2% dry matter. Volume satiation was not demonstrated at the levels of dry matter inclusion in this study; however, we suggest these urchins experienced nutrient or energy satiation as nutrient content of the diets increased. We hypothesize that L. variegatus regulate feed intake in part based on dry matter, energy and/or macronutrient concentrations in the gel based diets. However, these parameters do not fully explain daily macronutrient intake, particularly at high nutrient density, suggesting other control mechanisms may be important.