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The tilapia Oreochromis alcalicus grahami is a unique ureotelic teleost, that 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 Po-2 and temperature fluctuated more or less in parallel in a diurnal cycle from less than 20 Torr and less than 25 degrees C at night to greater than 400 Torr and 38 degrees C during the day, whereas pH remained constant at approximately 9.8. Field laboratory tests demonstrated that routine Mo-2 (under normoxia) increased greatly from 27 degrees C to 36 degrees C (Q(10) = 6.2) but then stabilized at a very high level (similar to 34.5 mu mol g(-1) h(-1)) up to the lethal temperature (similar to 42.5 degrees C), a pattern that was adaptive to the natural diurnal regime. The Po-2 threshold for survival during acute exposure (less than or equal to 1 h) was approximately 16 Torr. Mo-2 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 O-2. The Po-2 threshold for surface breathing was 1.8-fold higher at 37.5 degrees C than at 31 degrees C. Surface breathing and voluntary entry of fish into air were observed in the field. The blood O-2 dissociation curve at 30 degrees-32 degrees C was hyperbolic, with a high affinity (P-50 = 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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Physiological Zoology


© 1996 by The University of Chicago Press. The original publication is available at

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