Chronic effect of NaCl salinity on a freshwater strain of Daphnia magna Straus (Crustacea: Cladocera): A demographic study

Daphnia magna is mainly recognized as a freshwater cladoceran, but there are some strains that grow in brackish waters. The tolerance to salinity of a freshwater strain was assessed at NaCl concentrations of 0, 2, 4, 6, and 7 g L−1. The green microalga Ankistrodesmus falcatus was fed at optimal concentration (4×105 cells mL−1). Reproduction and survival were recorded in two experimental series: in the first one, 20 female neonates were individually studied for each treatment. In the second, cohorts of 10 female neonates were distributed in each of five replicates per treatment. In both cases, experiments were conducted over a full life-cycle. The determined 48-h LC50 for NaCl was 5.48 g L−1, but we recorded reproduction at up to 7 g NaCl L−1. The average clutch size, total progeny, number of clutches, and longevity were significantly reduced by the NaCl concentration (P<0.01); total progeny ranged from 467 to 25 neonates as edge values for NaCl concentrations of 0–7 g L−1. Inter-brood time was significantly higher for females grown at 7 g NaCl L−1 (3.9 days). The Life Table analysis demonstrates that average lifespan, life expectancy at birth, net reproductive rate and intrinsic rate of growth were also significantly reduced according to NaCl concentration. Based on the results for the two highest NaCl concentrations (6 and 7 g L−1), we conclude that the used D. magna strain was acclimated to develop satisfactorily under concentrations of up to 6 g NaCl L−1; however, the established salinity conditions reduced significantly reproduction and survival in this strain.