Respiratory Physiology of the Lake Magadi Tilapia (Oreochromis alcalicus grahami), a Fish Adapted to a Hot, Alkaline, and Frequently Hypoxic Environment

Abstract

The tilapia Oreochromis alcalicus grahami is a unique ureotelic teleost, the only fish that lives in the alkaline hotsprings of Lake Magadi, Kenya. Physical condi- tions and fish behavior were monitored in the Fish Springs Lagoon area, a site where the tilapia were particularly abundant. Water Po2 and temperature fluc- tuated more or less in parallel in a diurnal cycle from less than 20 Torr and less than 25* C at night to greater than 400 Torr and 380 C during the day, whereas pH remained constant at approximately 9.8. Field laboratory tests demonstrated that routine Mo, (under normoxia) increased greatly from 270 C to 360 C (Qo = 62) but then stabilized at a very high level (-~34.5 jgmol g-' h-1) up to the le- thal temperature (- 42.5 " C), a pattern that was adaptive to the natural diurnal regime. The Po2 threshold for survival during acute exposure (!; 1 h) was approx- imately 16 Torr. Io, from water was well maintained down to a Po2 of 60 Torr, below which it declined. Under such hypoxic conditions, the fish performed sup- plementary surface breathing when allowed access to air. Both the better oxygen- ated surface layer and air bubbles were inspired, resulting in significant uptake of 02. The Po2 threshold for surface breathing was 1.8-fold higher at 3 7.50 C than at 310 C. Surface breathing and voluntary entry offish into air were observed in the field. The blood O, dissociation curve at 300 -320 C was h3perbolic, with a high afinity (P50o = 6 Torr), low cooperativity (Hill coeficient = 1. 18), and no Bohr effect over the extracellularpH range 8.2-8.6.