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

Abstract

The effect of salinity (6, 12 and 33‰) 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‰) or acclimated to an isosmotic environment of 12‰ was unimpaired by a reduction in ration size. Fish acclimated to a hyposmotic environment of 6‰ 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‰-acclimated fish and a reduction in the chloride cell exposure of 6‰-acclimated fish. A dietary related decrease in chloride cell exposure of 6‰-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‰-acclimated fish regardless of ration size. The weight of fish fed a 10% ration size after 84 days culture in 12‰ was significantly greater than that of fish held in 33 and 6‰. Fish fed a 5% ration size and acclimated to 12‰ exhibited a greater body mass than those in 33‰ but did not differ significantly from fish acclimated to 6‰. Regardless of ration size, the final body weights of fish in 33‰ and 6‰ 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‰ and fed low ration sizes experience a degree of osmoregulatory disequilibrium, the source of which is likely the reduction in chloride cell exposure.