Acute temperature and oxygen stress among genotypes of Daphnia pulex and Simocephalus vetulus (Cladocera, Daphniidae) in relation to environmental conditions

Abstract

The interactions among the genetics, physiology, and ecology of Daphnia pulex and Simocephalus vetulus inhabiting an ephemeral pond were studied. Electrophoretic analysis of four loci revealed high genetic diversity and genotypic coexistence within each species. In a total of 227 D. pulex individuals, 92 unique genotypes were found, and for 112 S. vetulus individuals, there were 18 unique genotypes. Daphnis pulex inhabited the pond early in the summer, but later there was a shift to S. vetulus as the primary cladoceran inhabitant. Biweekly sampling through the open water period allowed us to monitor environmental parameters such as water temperature and oxygen saturation, as well as the condition of the animals under study by assessing egg number, lipid content and ovary development, carapace length, and haemoglobin content. In laboratory experiments, S. vetulus was found to be more tolerant of both high temperature and low oxygen saturation than D. pulex. In addition to these interspecific tolerance differences, there were genotypic differences within each species in temperature tolerance. We have related these laboratory-determined genetic and physiological differences within and between species to the temporal variation in environmental conditions experienced directly in the pond. Thus, the rapid changes in pond conditions appear to play a large role in determining the species and genotypes able to exist in such an unpredictable environment.