Metabolic physiology of the Mayan cichlid fish (Mayaheros uropthalmus): Re-examination of classification as an oxyconformer.

The Mayan cichlid (Mayaheros uropthalmus) is a freshwater fish inhabiting warm, potentially hypoxic and/or brackish waters, in Mexico and Central America. Despite its description as highly hypoxia tolerant, M. uropthalmus has been classified physiologically as an 'oxyconformer', which would place it in a very small (and shrinking) category of fishes that purportedly cannot maintain oxygen consumption (MO2) as ambient PO2 falls. However, hypoxia tolerance is often associated with strong oxyregulation, not oxyconformation as described for M. uropthalmus. To resolve these inconsistencies, we measured MO2, the ambient PO2 at which MO2 begins to decline as PO2 falls (PCrit), and gill ventilation rate (fG) in the Mayan cichlid. Variables were measured at 23o, 28 o and 33 °C and temperature sensitivity (Q10) calculated for each function. MO2 at air saturation was 2.9 ±â€¯0.2, 4.3 ±â€¯0.4, and 5.9 ±â€¯0.3 µmol O2/g/h at 23o, 28o and 33 °C, respectively. PCrits were low at 2.6 ±â€¯0.8 kPa, 3.2 ±â€¯0.8 kPa and 4.7 ±â€¯0.9 kPa at 23o, 28o and 33 °C, respectively. Q10 values for MO2 were 2.56 ±â€¯0.21 (23-28 °C), 1.89 ±â€¯0.15 (28-33 °C) and 2.2 ±â€¯0.1 (full temperature range of 23-33 °C), suggesting overall Q10s typical for tropical freshwater fish. fG was 39 ±â€¯3, 45 ±â€¯4, and 53 ±â€¯6 breaths/min at 23o, 28o and 33 °C, respectively, and increase 2-3 fold in severe hypoxia at each temperature. Experiments employing hyperoxia up to 35 kPa indicate a strong 'hypoxic drive' for gill ventilation. Collectively, these data show that, in contrast to a previous characterization, the Mayan cichlid is a strong oxyregulator exhibiting attributes (e.g. very low PCrit) typical of very hypoxia-tolerant fishes.