Evolutionary ecotoxicology of salinity tolerance in Daphnia pulex: Interactive effects of clonal variation, salinity stress, and predation.

摘要

Evolutionary ecotoxicology addresses effects of toxic chemicals in an ecological context and considers the potential evolutionary responses of organisms following exposure to toxins. Despite decades of research, the effects of salinity stress in freshwater systems, partly from road salt pollution, on a keystone species, Daphnia pulex, in its interaction with predators have received very limited attention. In this study, I quantified D. pulex clonal variation in response to salinity stress and the lethal and non-lethal effects of Chaoborus (a dominant planktivore in fishless ponds). Behavioral, morphological, and life history responses of two D. pulex clones, known to differ in salinity tolerance, were quantified in the presence/absence of Chaoborus chemical cues (kairomone) under different salinity levels. I predicted that kairomone would induce both clones to develop neckteeth structure, mature later at larger sizes, and give birth to fewer but larger neonates. Also, I predicted that the clone with stronger salinity tolerance would show stronger responses for these traits as well as higher escape efficiency from Chaoborus predation. I found some support for my predictions. Elevated salinity generally weakened some responses of both clones. However, clonal variation in salinity tolerance did not significantly mediate the effects of salinity on the responses. Additionally, I found that both clones reproduced more but smaller neonates, in contrast to my predictions. In an April field survey of 21 roadside ponds in Southeast Michigan, USA, D. pulex relative abundance was found to be negatively associated with increasing salinity, suggesting that community structure and Daphnia persistence may be impacted by freshwater salinization. Laboratory assays of clones collected from a subset of the surveyed ponds, provided evidence of local adaptation of D. pulex to pond salinity levels. Clones collected from ponds with higher salinities exhibited stronger salinity tolerance. Overall, my study revealed the negative impacts of salinity stress on D. pulex populations and its responses to Chaoborus. My work also suggests that intraspecific variation and the evolution of salinity tolerance may mediate some of the impacts of salinity stress on Daphnia populations.;Keywords: Daphnia pulex, Chaoborus, zooplankton, predator-prey interactions, non-lethal predator effects, escape behavior, life history, clonal variation, phenotypic plasticity, salinity stress, and road salt.