A fish in extreme alkalinity

Tilapia grahami (Boulenger), a small cichlid fish, inhabits extremely alkaline water where the total CO2 concentration exceeds 200 meq.L(-1), osmolality is about 600 mosm.L(-1), pH ranges between 9.6 and 10.5 and temperature may be 43 degrees C. Venous blood drawn anaerobically by heart puncture had pH values as high as 8.4 at 35 degrees C. A 10-15-fold bicarbonate concentration gradient exists across the gills of Tilapia. It is suggested that the uniquely high and variable blood pH results when blood in the gills are exposed to water in which free CO2 can hardly exist. Blood CO2 must be correspondingly low resulting in pH values hitherto unknown to occur in vertebrates.