Osmoregulation in three species of Ambassidae (Osteichthyes: Perciformes) from estuaries in Natal

Whole blood osmotic regulation was evaluated in three co-occurring, estuarine species of Ambassis exposed to ambient salinities from fresh water (0,13%.) to 53%°/00. Blood molality of all three species, acclimated to fresh water, showed significant increases over the range of ambient salinities from fresh water through 5%« and stabilized only in the range 18%°-35%.). Osmotic concentrations of all three species rose abruptly at salinities above 35%. and no species survived direct transfer into ambient salinities above 53%°/00 A. productus, collected in fresh water, required 24 h prior acclimation at 18%. for survival in sea water (35%°). A. gymnocephalus acclimated in sea water showed the least tolerance of the three species to low salinities and experienced a 42% decrease in blood osmotic concentration when exposed to fresh water whereas the decrease for A. natalensis and A. productus was only 20% and 23% respectively. Histological investigation of Ambassis kidneys indicated that all three species have structurally advanced kidneys of the mesonephric type common to most teleosts which spend a proportion of their lives in a hyposmotic medium. Osmotic regulatory characteristics of Ambassis species are discussed in relation to their distribution in estuaries