Cobia Rachycentron canadum L. reared in low-salinity water: does dietary sodium chloride affect growth and osmoregulation?

Abstract

The effects of NaCl supplementation (0.0%, 2.5%,5.0%, 7.5% and 10.0% dry weight of a basal diet) ongrowth, gill histological alterations and osmoregula-tion of juvenile cobia reared in low-salinity water(5 g L1) were assessed. At the end of the experi-ment, gills were sampled for Na+,K+-ATPase activitydetermination and histological evaluation. In alltreatments, no mortality was observed. Resultsshowed that dietary NaCl supplementation did notalter growth. At the highest supplementations (7.5%and 10.0%), juvenile cobia showed higher feed intakeandfeedconversionratio.Na+,K+-ATPase activitywas higher in fish fed the diet without salt supple-mentation than in those fed with NaCl-supplementeddiets. The number of chloride cells significantlyincreased with increasing dietary salt level, being2.5-fold higher in fish fed with 10.0% NaCl supple-mentation (41 cells mm2) than in those from thenon-supplemented fed group (16 cells mm2). Thesefindings indicate that dietary salt supplementationstimulated chloride cell proliferation paralleled with a reduction in the gill Na+,K+-ATPase activity,suggesting a possible decrease in energy consumptionassociated with osmoregulation. However, the suggestedenergy sparing did not have a significant impact onjuvenile cobia growth.