Effects of Artificially Elevated Water Temperatures on the Growth, Reproduction and Life Cycle of a Natural Population of Physa Virgata Gould

Abstract

Physa virgata Gould is the common pond snail of north central Texas. From two natural populations, one affected by heated discharge from a power plant in Lake Arlington, Texas, regular samples were collected from August 1972 until April 1974. Investigations of growth, reproduction, life cycle and the effects of heated effluent showed that each population had three generations/year. More northern physid populations have a simple annual pattern or at most two complete generations/year. Even without artificially evelated water temperatures the longer growing seasons which occur in southwestern USA allow P. virgata populations to achieve three generations/year. The autecology of these two P. virgata populations was remarkably similar even though one was in an area affected by thermal effluent. Observed differences include a more rapid rate of growth of the second generation in the discharge area (probably as a result of higher summer water temperatures) and a significant decrease in mean number of eggs per mass laid by the discharge population.