The handling of salt by the neotropical cultured freshwater catfish Rhamdia quelen

The jundiá (Rhamdia quelen) is a Siluriform native to South America, intensely cultivated in Southern Brazil, and which has been increasingly considered as a potential substitute for the culture of exotic species. Due to this interest, there is a constant need of methods to reduce parasite infestations common to the species. The use of sodium chloride (NaCl) is one of such practices. However, not much is known about the effects of NaCl on the osmotic homeostasis of this and other commercially relevant tropical freshwater species. This study thus aimed at evaluating the effect of adding NaCl (5, 15, or 25 g/L) to the water for 1 h, on the osmoregulation, and capacity for muscle tissue water regulation of the jundiá. In addition, muscle slices were subjected in vitro to isosmotic (control, animal in fresh water) and hyper-osmotic (120, 150, and 180% of control) salines, for evaluation of their capacity to control their water content. Some changes have been observed, when fishes submitted to 25 g/L were compared to freshwater controls: plasma ions and osmolality increased, the last from 260±5 to 419±12 mOsm/kg H₂O; plasma glucose increased from 47.8 to 92.8 mg/dL, and muscle water content decreased from 80 to 76.7%; plasma cortisol levels remained unchanged (~150 ng/mL). Branchial carbonic anhydrase activity also increased in 25 g/L, probably as a response to the metabolic acidosis that develops upon severe saline challenges. In vitro experiments indicated a high capacity for water content regulation, even when muscle slices were exposed to an 80% increase in osmolality. Exposure of jundiás for 1 h to 25 g/L was indeed harmful to their osmotic homeostasis, also affecting sensory and motor functions (observed as loss of barbel function, and swim impairment). However, the high degree of euryhalinity shown both in vivo and in vitro by this freshwater siluriform is coherent with its occasional presence in dilute estuarine waters and to the positioning of its family within a group of seawater-related Siluriforms. Thus, a reasonable amount of salt (15 g/L) for an intermediate time period (1 h) could be employed against parasites of the jundiá, while still being harmless to both the jundiás and humans that consume its flesh.