Cellular and biochemical characterization of hyposmotic adaptation in a marine teleost, Sparus sarba

Silver sea bream (Sparus sarba) were adapted to a hyposmotic environment of 6 parts per thousand for 5 or 21 day periods. Hyposmotic adaptation did no t significantly alter serum Na+, Cl- or muscle moisture content. After 5 da ys in 6 parts per thousand, chloride cell (CC) apical and fractional areas increased with no alteration in CC numbers. Elevated CC apical and fraction al area was coupled with an increase in CC numbers after 21 days in 6 parts per thousand. In fish adapted to 6 parts per thousand, an increase was fou nd in the area of CC cytoplasm occupied by mitochondria. Branchial Na+-K+-A TPase decreased after 21 days in 6 parts per thousand,, resulting in an unc oupled "typical" correlative relationship between this enzyme and CC number s. Kidney Na+-K+-ATPase activity elevated after 5 days in 6 parts per thous and but was not significantly elevated after 21 days. In gill and kidney ti ssue, alterations were found in the activity of key metabolic enzymes after 5 days acclimation to 6 parts per thousand, with few differences occurring after 21 days. Serum cortisol levels were unaltered by low salinity acclim ation suggesting that an increase in the number of CCs found in 6 parts per thousand-adapted S. sarba did not occur as a result of hypercortisolemia. Based on the present evidence, it seems possible that alterations in the fo rm and function of the branchial epithelium may play an important role in t he ability of S, sarba to acclimate to low salinity conditions, with renal assistance occurring during short term exposure. The response of S, sarba t o low salinity adaptation appears to differ from the generally accepted tel eostean model of response and offers insight into the as yet undefined hype rosmoregulatory strategies of estuarine marine migrant fish.