Acute salt exposure of the freshwater Characiformes: Pacu (Piaractus mesopotamicus, Holmberg 1887), Tambaqui (Colossoma macropomum, Cuvier 1818), and their hybrid "Tambacu"

The use of salt in the management and culture of freshwater fishes is a quite common practice, either to fight infection or to possibly reduce the stress of transport, through reduction in the metabolic cost of hyper-regulation. Increased salinity, depending on the salt concentration used and the species, can be in itself a stress factor. This study aimed at evaluating and comparing the physiological effects of exposure to increased salinity in P. mesopotamicus (Pacu), C. macropomum (Tambaqui) and their hybrid (Tambacu), as well as to test the resistance of the hybrid when compared to its two parent species. Fishes (n = 38 each) were submitted to 5, 15, and 25 psu for 1 h. Osmolality, ions, and glucose were assayed in plasma, tissue hydration levels were quantified in gills and muscle, carbonic anhydrase activity (CAA) was assayed in gills and kidneys, and the expression of heat shock proteins (HSP70) was evaluated in muscle. All 3 fishes showed similar disturbance in osmotic homeostasis when exposed to 25 psu; the tambaqui displayed higher glycemia than the other fishes, in 15 and 25 psu. In this salinity the fish showed increased osmolality, Na+, Cl- and Mg2+ concentrations, as well as a reduction in muscle hydration. The tambacu showed lower potassium and higher magnesium in 25 psu than the other fishes. CAA was variably sensitive to salinity in the two species and the hybrid. No effects of salinity on tissue HSP70 expression were observed. Tambacus are similar in robustness to their parent species. The markers investigated showed that the three fishes tolerate exposure for 1 h at 10 psu; this concentration can be used for prophylactic or anesthetic purposes, but should not be exceeded, as it can offer significant homeostatic challenges to these fishes. Statement of relevance: Rare studies on the comparative physiology of parent species and their hybrid, with commercial interest. (C) 2016 Elsevier B.V. All rights reserved.