Alterations in Na+-K+-ATPase activity and gill chloride cell morphometrics of juvenile black sea bream (Mylio macrocephalus) in response to salinity and ration size

The effect of salinity (6, 12 and 33 parts per thousand) and ration size (5 and 10% body ww day(-1)) on branchial chloride cell surface ultrastructural morphometrics and osmoregulatory response of juvenile black sea bream (Mylio macrocephalus) was assessed. The osmoregulatory performance of fish held in sea water (33 parts per thousand) or acclimated to an isosmotic environment of 12 parts per thousand was unimpaired by a reduction in ration size. Fish acclimated to a hyposmotic environment of 6 parts per thousand exhibited competent hyperosmoregulation when fed a 10% dietary ration size. Under such conditions chloride cell apical exposure was significantly elevated, however, branchial and renal Na+-K+-ATPase activity were unaltered. A reduction in ration size (5% body ww day(-1)) did not alter renal Na+-K+-ATPase activity, but did result in an elevation in branchial Na+-K+-ATPase activity of both 33- and 6 parts per thousand-acclimated fish and a reduction in the chloride cell exposure of 6 parts per thousand-acclimated fish. A dietary related decrease in chloride cell exposure of 6 parts per thousand-acclimated fish fed a 5% ration size appeared to result in a hypochloraemic response while serum Na+ levels were not significantly affected. Renal Na+-K+-ATPase activity was lower in 12 parts per thousand-acclimated fish regardless of ration size. The weight of fish fed a 10% ration size after 84 days culture in 12 parts per thousand was significantly greater than that of fish held in 33 and 6 parts per thousand. Fish fed a 5% ration size and acclimated to 12 parts per thousand exhibited a greater body mass than those in 33 parts per thousand but did not differ significantly from fish acclimated to 6 parts per thousand. Regardless of ration size, the final body weights of fish in 33 parts per thousand and 6 parts per thousand were not significantly different. Data suggest that M. macrocephalus juveniles may grow better under isosmotic conditions while fish acclimated to a hyposmotic environment of 6 parts per thousand and fed low ration sizes experience a degree of osmoregulatory disequilibrium, the source of which is likely the reduction in chloride cell exposure. (C) 1999 Elsevier Science B.V. All rights reserved.