Floating in Hydra Littoralis

Abstract

Starved or poorly fed Hydra will produce a bubble and float to the surface of the medium. Richly fed Hydra will not. The fraction of Hydra that will float is related to the degree to which they are crowded by other Hydra. Water which has been conditioned by either starved or well—fed Hydra will elicit floating. Excessively crowded animals are less effective in eliciting floating than are less crowded animals. Laboratory populations from which floating animals are periodically removed come to a steady state with approximately one—seventh as many animals as in confined populations with the same food supply. The implication of these phenomena is that Hydra in nature are relatively mobile within a lake. The evolution of the floating mechanism in Hydra does not require group selection, but it does illustrate the danger of an overemphasis on abundance as a criterion of adaptive success.