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 (ṀO) as ambient PO falls. However, hypoxia tolerance is often associated with strong oxyregulation, not oxyconformation as described for M. uropthalmus. To resolve these inconsistencies, we measured ṀO, the ambient PO at which ṀO begins to decline as PO falls (P), and gill ventilation rate (f) in the Mayan cichlid. Variables were measured at 23, 28 and 33 °C and temperature sensitivity (Q) calculated for each function. ṀO at air saturation was 2.9 ± 0.2, 4.3 ± 0.4, and 5.9 ± 0.3 μmol O/g/h at 23, 28 and 33 °C, respectively. Ps were low at 2.6 ± 0.8 kPa, 3.2 ± 0.8 kPa and 4.7 ± 0.9 kPa at 23, 28 and 33 °C, respectively. Q values for ṀO were 2.56 ± 0.21 (23-28 °C), 1.89 ± 0.15 (28-33 °C) and 2.2 ± 0.1 (full temperature range of 23-33 °C), suggesting overall Qs typical for tropical freshwater fish. f was 39 ± 3, 45 ± 4, and 53 ± 6 breaths/min at 23, 28 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 P) typical of very hypoxia-tolerant fishes.