Choice among food levels by an aquatic grazer ( Physa gyrina Say)

Abstract

Laboratory populations containing 15 snails each were set up in five replicate trays to examine how an aquatic gastropod, Physa gyrina , would disperse and feed among three discrete feeding stations containing different food levels of 1, 5, and 10 particles of spinach. These unrestricted snails dispersed non-randomly among the three food levels, their densities increasing with the number of food particles. Their relative feeding frequencies at these food levels also closely matched the relative amounts of food available as intact and damaged (with rasped holes) particles. In comparison, the control populations which were subdivided into groups restricted to feeding stations and unable to disperse from fixed food levels, ate and damaged more of their food and their feeding densities poorly matched the relative amounts of food available at the different food levels. This experiment demonstrates that an aquatic grazer can distinguish among quantitative levels of a single food source and adjust its feeding densities to conform to the available food levels.