Acute Effects of NaCl and Na 2 SO 4 on Daphnia menucoensis Paggi, 1996 and Moina eugeniae Olivier, 1954 (Crustacea, Cladocera)

Abstract

Despite the fact that halophilic cladocerans are relatively scarce, the neotropical endemic species Daphnia menucoensis and Moina eugeniae are frequent in temporary saline lakes of the semiarid centre and the north of Patagonia (Argentina). Although aspects of its biology in natural conditions are already known, there is a lack of ecophysiological information to help explain its geographical and temporary distribution. Considering that Cl- and SO42- are the predominant anions in La Pampa lakes, the aim of this study was to determine the effects of increasing NaCl and Na2SO4 concentration on neonates of both species, using acute bioassays, and to compare the results with previous information. The medium was prepared using demineralized water and different concentrations of pure salts of NaCl and Na2SO4 of analytical quality as well as a mixture of both salts; six concentrations for D. menucoensis (0.5; 1; 5; 10; 15 and 20 g.L-1) and four for M. eugeniae (1; 5; 10 and 15 g.L-1). Four replicates were performed, with five neonates per treatment. The bioassays took 48 h, without food or renewal of the medium. The survival of D. menucoensis was virtually zero in treatments with 15 or 20 g.L-1, but survival was higher at lower concentrations. Survival in treatments with salt mixtures differed, but was much lower at 10 g.L-1. The survival of M. eugeniae was zero in all treatments of 10 g.L-1. In previous bioassays with salts obtained from the natural environment, the tolerance of the neonates of both species was greater. D. menucoensis survived up to 24 g.L-1, while M. eugeniae survived up to 31 g.L-1. The lower tolerance observed in this study might be because analytical-grade salts were used. The absence of other electrolytes that are necessary to maintain a relatively stable internal environment of the neonates might explain the limited survival at concentrations higher than 10 g.L-1.