Haloplasticity of black seabream (Mylio macrocephalus): Hypersaline to freshwater acclimation

Black seabream (Mylio macrocephalus) were acclimated to various salinities (50, 33, 12 and 6 parts per thousand) for eight months. Acclimation of fish to 6 parts per thousand for eight months allowed successful adaptation to freshwater (0 parts per thousand) for a further 21 days without mortality. This is the first report of freshwater acclimation of a "true" marine fish for an acceptable experimental duration. Osmoregulatory and metabolic strat egies were characterized via alterations in branchial chloride cell (CC) nu mbers and surface ultrastructural morphometrics along with changes in serum chemistry, muscle moisture, liver glycogen and branchial, renal, hepatic a nd intestinal enzyme activities. Branchial CC numbers were elevated in 50 a nd 6 parts per thousand environments; however, freshwater acclimation resul ted in return to low numbers. Branchial Na+-K+-ATPase activity was generall y higher in 50 and 33 parts per thousand environments and exhibited a decli ning trend in 12 and 6 parts per thousand environments. Freshwater acclimat ion resulted in a marked elevation in branchial Na+-K+-ATPase activity. Ele vated CC exposure areas were typically found at salinity extremes. Serum Na +, Cl- and muscle moisture content did not vary between groups acclimated f rom 50 to 6 parts per thousand. Freshwater acclimation resulted in signific ant hyponatremia, hypochloremia and muscle hydration. Branchial ICDH activi ty was lowest in a 12 parts per thousand environment and highest at salinit y extremes. Renal Na+-K+-ATPase exhibited lower activity in 12 and 6 parts per thousand and was markedly elevated in 0 parts per thousand. Enzyme acti vities of both liver and kidney indicated elevated gluconeogenic activity i n freshwater-adapted fish. Total intestinal Na+-K+-ATPase activity tended t o decline in lower salinities; however, lowest activity was found in fish a dapted to 12 parts per thousand. Na+-K+-ATPase activities in different segm ents of the intestine may reflect the osmoregulatory role of this organ in varying salinities. The data indicated efficient homeostatic control in Myl io macrocephalus acclimated from hypersaline to freshwater environments and clearly demonstrates the haloplasticity of this marine fish species. (C) 1 999 Wiley-Liss, Inc.