Osmoregulation in juvenile Rhabdosargus holubi (Steindachner) (Teleostei: Sparidae)

Juvenile Rhabdosargus holubi (Steindachner), one of the commonest teleosts in south east African estuaries, are strong osmoregulators, showing little change in their internal osmotic concentration over an extremely wide salinity range. In 35‰ seawater the internal osmotic concentration is held at 370 mosmol/1. At a salinity of 1‰ the internal osmotic concentration falls to 216 mosmol/1 and at a salinity of 65‰ rises to 381 mosmol/1. When exposed to a new salinity the internal osmotic concentration does not change until after 10 h; this may be of considerable importance to fish living in areas subject to short term changes of salinity.