Sublethal toxic effects of cyanobacteria and nonylphenol on environmental sex determination and development in Daphnia

In many cladoceran zooplankton, females divide their reproductive effort among females, males, and resting eggs depending on a suite of environmental conditions. Having alternative life-history strategies allows cladocerans such as Daphnia to achieve high reproductive rates asexually when conditions are favorable and to produce offspring sexually for survival when the environment becomes unsuitable. Because of their rapid reproduction, Daphnia and other cladocerans are useful model organisms in aquatic toxicology. Bioassays using Daphnia serve two functions, to estimate the ecosystem level risk posed by specific chemicals and to quickly detect contaminated waters. Bioassays examine effects in terms of life history features such as growth, survival, and asexual fecundity. We studied the effects of toxic stress on Daphnia galeata mendotae grown under conditions in which it produces female, male, and ephippial offspring. Fecundities in terms of the three types of offspring showed different sensitivities to chemical stress. The stressors were a toxic strain of the cyanobacterium Microcystis aeruginosa and nonylphenol, a plasticizer, which is a surfactant that interferes with endocrine function in vertebrates. Production of resting eggs and female offspring were affected in both experiments, but the experiments showed different dose responses. Production of males was less sensitive. Exposure to nonylphenol also produced a characteristic developmental abnormality at environmentally relevant concentrations. Life table analysis showed that stress during the sexual phase of the life cycle of Daphnia reduces short-term population growth and as well as affecting the stock of dormant individuals.