Printed in Great Britain SHORT COMMUNICATIONS Na+ AND Cl ~ EFFLUXES AND IONIC REGULATION IN TILAPIA GRAHAMI, A FISH LIVING IN CONDITIONS OF EXTREME ALKALINITY

The osmoregularity ability of the cichlid Tilapia grahami enables this fish to live in conditions of extreme alkalinity (Reite, Maloiy & Aashaug, 1974; Maloiy et al. 1978). In this paper we describe the ionic balance of T. grahami from Lake Magadi (East African Rift Valley) which has a pH value of around 10, temperature in the range 30-40 °C and an osmotic pressure of about 500 mOsm, the principal inorganic ions being sodium carbonate and bicarbonate (Table 1). We also describe ionic balance of fish in water with a high NaCl concentration and in fresh water. Fish were netted in pools at the edge of the lake and that day the air temperature was 26 °C, water temperature was 30 °C and water pH estimated using BDH pH papers was between 9-10. The fish were transported to Nairobi and kept in tanks of constantly aerated Lake Magadi water at 20 °C. After an initial high mortality the remaining fish survived for many months under laboratory conditions. One group of fish was used to determine body water and ions (Table 1) while others were injected with known amounts of 22Na and MC1 to determine effluxes of these ions (Table 2)