Branchial osmoregulatory response to salinity in the gilthead sea bream,Sparus auratus

The branchial osmoregulatory response of gilthead sea bream (Sparus auratus L.) to short-term (2–192 hr) and long-term (2 weeks) exposure to different environmental salinities (5%, 15%, 25%, 38% and 60%) was investigated. A ‘‘U-shaped’’ relationship was observed between environmental salinity and gill Naþ,Kþ-ATPase activity in both long- and short-term exposure to altered salinity, with the increase in activity occurring between 24 and 96 hr after the onset of exposure. Plasma osmolality and plasma ions (sodium, chloride, calcium and potassium) showed a tendency to increase in parallel with salinity. These variables only differed significantly (Po0.05) in fish adapted to 60% salinity with respect to fish adapted to full-strength sea-water (SW). Plasma glucose remained unchanged whereas plasma lactate was elevated at 5% and 60%. Muscle water content (MWC) was significantly lower in fish adapted to 60%. Chloride cells (CC) were only present on the surface of the gill filaments and absent from the secondary lamellae. CC distribution was not altered by external salinity. However, the number and size of CC were significantly increased at salinity extremes (5% and 60%), whereas fish exposed to intermediate salinities (15% and 25%) had fewer and smaller cells. Furthermore, the CC of fish exposed to diluted SW became rounder whereas they were more elongated in fish in full-strength and hypersaline SW. This is consistent with previous reports indicating the existence of two CC types in euryhaline fish. At likely environmental salinities, gilthead sea bream show minor changes in plasma variables and the effective regulation of gill Naþ,Kþ-ATPase. However, at very low salinities both haemodilution and up-regulation of gill Naþ,Kþ-ATPase predict a poor adaptation most likely related to deficiency or absence of specific components of the CC important for ion uptake