Life-history responses of salinity-tolerant and salinity-sensitive lineages of a stenohaline cladoceran do not confirm clonal differentiation

The adaptability of freshwater organisms to brackish conditions is important to understand the resilience of freshwaters to saline intrusion, a phenomenon that may affect vulnerable freshwater habitats. Bearing this in mind, this study intended to assess if there are genetically determined differences in the tolerance of stenohaline cladocerans in 21-day exposure scenarios to sodium chloride (NaCl, as a proxy for salinity). The objective was to compare the reproductive output of salinity-tolerant (brackish) versus salinity-sensitive (freshwater) Simocephalus vetulus genotypes, obtaining reaction norms for quantitative phenotypic traits (survival, fecundity, fitness). We found intra-specific (i.e., clonal) variability in the life-history responses of S. vetulus populations, but no evidence that the most resistant clones (brackish) were best suited to cope with salinity. Surprisingly, brackish genotypes were the most affected in terms of fecundity when exposed to sublethal levels of NaCl, which could be viewed as a potential trade-off mechanism. Although differences in tolerant and less tolerant genotypes were small, they may be important in genotype and species sorting in colonization or recolonization events in freshwater systems facing salinization.