| Feed intake is the primary determinant of nutrient and energy acquisition for most metazoans. Satiety regulates feed intake and may have cascading effects on nutrient acquisition 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 many simpler animals with less complex physiological processes. Sea urchins, with relatively simple digestive and nervous systems, provide an ideal model for the study of fundamental drivers of feed intake. Understanding the process of satiety in this primitive deuterostome can provide insight into satiety mechanisms in these and more complex taxa.;Evidence from laboratory and field observations indicate that sea urchins feed selectively, reducing feed intake even when food items remain available. In sea urchins, feed intake may be modified by volume satiation (fullness), nutrient satiation (consumption to a nutrient intake target), energy satiation (consumption to an energy target regardless of the source), or some combination thereof.;In the present study large L. variegatus were fed to excess formulated diets that varied in both food and nutrient density. Feed intake varied inversely with food density. Volume satiation was detected at the lowest levels of dry matter inclusion, during which sea urchins consumed ca. 1/3 of their body wet weight/day. However, sea urchins primarily sated to the dry matter content of the feed, consuming ca. 1.4 to 2.1% of their live 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 when offered a diet that is complex, indicating some level of protein leveraging. However, intake parameters cannot be fully explained by macronutrient intake. Evaluation of variable patterns of daily feed intake (amount consumed) indicated that daily intake variation occurs largely within an individual over time, and secondarily, between individuals within a population. Overall, variation in feed intake was highest when food density was lowest, and variation decreased with increasing food density. Macronutrients affected patterns of feed intake, and this complex interaction deserves further evaluation. |
