The Effect of Temperature and Feeding on the Intrinsic Rate of Increase of Three Species of Hydra

Abstract

The use of the fresh water polyp, hydra, as an experimental animal in many facets of biological research has greatly increased since the difficulties of mass culturing were overcome by Loomis (1953) and Loomis and Lenhoff (1956). The continual proliferation of the cells of the adult hydra results partially in the formation of new individuals through asexual budding, rather than an increase in the size of the adult. In addition, cells are continually being lost from the tips of the tentacles and from the peduncle and, under certain conditions, through formation of gonads and the release of their products. For this reason, in part, hydra is considered to be an immortal animal (Brien 1953). The concern of this study is only with the reproductive potential resulting from asexual budding. Estimates of the reproductive potential of hydra are relatively scarce. Loomis's study (1954) on an estimate for Hydra Jittoralis has been a step in this direction. On the other hand, consideration of the variability of the rate of increase among individuals of the same species and between species is completely lacking. Loomis (1954) has shown that the rate of increase for H. littoralis is a function of the environment. His examination of several environmental factors (certain ions, oxygen tension, pH, temperature, and nutritional status) indicates some effect on this population parameter. However, the relative significance of each factor interacting with each of the others is not clear. The question also arises as to whether there is any inherent difference in the intrinsic rate of increase between species of hydra and whether this might be related in some way to their conditions of existence in nature. This study was designed to examine in detail the intrinsic rate of increase for 3 species of hydra under 2 temperatures and 2 states of nutrition and to assess the interactions between these factors.