Effect of water alkalinity on the tolerance of juvenile pompano Trachinotus marginatus to the hypoosmotic environment
The aim of the present work is to evaluate the effect of salinity reduction with and without alkalinity reposition within the survival, Na+ K+ ATPase (NKA) and carbonic anhydrase enzyme activity, and gills histopathology in juveniles of pompano Trachinotus marginatus. Fishes of the control were maintained in salinity 34‰, while other two groups of fishes (all with four repetitions) had their salinity daily reduced with and without alkalinity reposition until salinity reached 0‰ (34 - 22 - 12 – 5 - 2,5 - 1 - 0‰). Animals with no alkalinity reposition began to die in the fourth day, when salinity was 5‰, while mortality for those fish with alkalinity reposition only began in the six day, when salinity was 1 ‰. The medium lethal time was of 5,3 days and 6,5 days, respectively for fishes without and with alkalinity reposition. It was observed hyperplasia of the branchial epithelium and hypertrophy and hyperplasia of the chloride cells after the salinity reduction. The number of chloride cells of the control treatment was smaller than others treatments with and without reposition in hiposmotic environment. Activity of NKA was greater when salinity was reduced to 2.5‰, and it was even higher in the treatment without alkalinity reposition. It was also seen higher activity of the carbonic anhydrase when salinity was reduced, but no difference was observed in regard to alkalinity. The results of this study show that alkalinity reposition increases the tolerance of juvenile pompano to low salinity.