Survival and life history responses of Simocephalus vetulus (Müller, 1776) genotypes exposed to sodium chloride: does environmental context matter?

Abstract

Cladocerans are typically viewed as freshwater organisms with importance as regulators of primary production. However, populations of these filter-feeders can be found in brackish ecosystems, facing regular or sporadic inputs of seawater. According to climate change predictions, saline intrusion may affect vulnerable freshwater habitats. Nonetheless, populations adapt to local conditions and this may confound our ability to assess noxious effects of salinity. Bearing this in mind, our goal was to analyze the halotolerance of different genotypes of Simocephalus vetulus differing in their original environmental context (brackish versus freshwater populations). Clonal lineages were established from three extant populations: one from a brackish lagoon and two from freshwater systems. To assess short-time sensitivity of 23 genotypes to NaCl, they were exposed to a range of concentrations from 2 to 5gL−1 in standard 48-h acute assays and to a fixed concentration of 6 g L−1 in 12-h survival time (ST) trials. Life-history responses were analyzed in extreme genotypes, whose fitness was compared along a gradient of growing NaCl concentrations. While EC50 values demonstrated an overlap of the sensitivity of genotypes to NaCl, ST supported the idea of population differentiation. All tolerant genotypes (in terms of ST) to high levels of NaCl, except one, originated from the brackish population. On the contrary, life-history assays of extreme genotypes showed that fitness under saline stress was overall unrelated to environmental context. Furthermore, lifehistory experiments showed that tolerance to salinity is present in freshwater populations, thus demonstrating their potential to adapt to salinity increases.