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

Narahara, A.
Bergman, Harold
Laurent, P.
Maina, J. N.
Walsh, P. J.
Wood, C. M.
The tilapia Oreochromis alcalicus grahami is a unique ureotelic teleost, the only fish that lives in the alkaline hotsprings of Lake Magadi, Kenya. Physical conditions and fish behavior were monitored in the Fish Springs Lagoon area, a site where the tilapia were particularly abundant. Water Po2 and temperature fluctuated 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 38° C during the day, whereas pH remained constant at approximately 9.8. Field laboratory tests demonstrated that routine Mo2 (under normoxia) increased greatly from 27° C to 36° C (Q10 = 6.2) but then stabilized at a very high level (~34.5 μmol g-1 h-1) up to the lethal 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 approximately 16 Torr. Mo2 from water was well maintained down to a Po-2 of 60 Torr, below which it declined. Under such hypoxic conditions, the fish performed supplementary surface breathing when allowed access to air. Both the better oxygenated surface layer and air bubbles were inspired, resulting in significant uptake of O2. The Po2 threshold for surface breathing was 1.8-fold higher at 37.5° C than at 31° C. Surface breathing and voluntary entry of fish into air were observed in the field. The blood O2 dissociation curve at 30°-32° C was hyperbolic, with a high affinity (P50 = 6 Torr), low cooperativity (Hill coefficient = 1.18), and no Bohr effect over the extracellular pH range 8.2-8.6.
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