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Cellular and Biochemical Characterization of Hyposmotic Adaptation in a Marine Teleost, Sparus sarba

Abstract Silver sea bream (Sparus sarba) were adapted to a hyposmotic environment of 6‰ for 5 or 21 day periods. Hyposmotic adaptation did not significantly alter serum Na+, Cl– or muscle moisture content. After 5 days in 6‰, chloride cell (CC) apical and fractional areas increased with no alteration in CC numbers. Elevated CC apical and fractional area was coupled with an increase in CC numbers after 21 days in 6‰. In fish adapted to 6‰, an increase was found in the area of CC cytoplasm occupied by mitochondria. Branchial Na+-K+-ATPase decreased after 21 days in 6‰, resulting in an uncoupled “typical” correlative relationship between this enzyme and CC numbers. Kidney Na+-K+-ATPase activity elevated after 5 days in 6‰ but was not significantly elevated after 21 days. In gill and kidney tissue, alterations were found in the activity of key metabolic enzymes after 5 days acclimation to 6‰, with few differences occurring after 21 days. Serum cortisol levels were unaltered by low salinity acclimation suggesting that an increase in the number of CCs found in 6‰-adapted S. sarba did not occur as a result of hypercortisolemia. Based on the present evidence, it seems possible that alterations in the form and function of the branchial epithelium may play an important role in the ability of S. sarba to acclimate to low salinity conditions, with renal assistance occurring during short term exposure. The response of S. sarba to low salinity adaptation appears to differ from the generally accepted teleostean model of response and offers insight into the as yet undefined hyperosmoregulatory strategies of estuarine marine migrant fish.