In order to estimate metabolic demands of desert pupfish for conservation purposes, we measured oxygen consumption in fish acclimated to the ecologically relevant temperatures of 28 or 33°C. For these experiments, we used fish derived from a refuge population of Devils Hole pupfish (Cyprinodon diabolis). Measurement of routine oxygen consumption (V O2,routine ) revealed some 33°C-acclimated fish (10% of 295 assayed fish) periodically exhibited periods of no measurable oxygen consumption despite available ambient oxygen tensions that were above the critical P O2 . We call this phenomenon paradoxical anaerobism. The longest observed continuous bout with no oxygen consumption was 149 min, although typical bouts were much shorter. Fish maintained normal posture and ventilation rate (>230 ventilations per minute) during paradoxical anaerobism. Fish rarely demonstrated a compensatory increase in oxygen use following a period of paradoxical anaerobism. In contrast, only one out of 262 sampled fish acclimated at 28°C spontaneously demonstrated paradoxical anaerobism. Muscle lactate concentration was not elevated during periods of paradoxical anaerobism. However, the amount of ethanol released by the 33°C-acclimated fish was 7.3 times greater than that released by the 28°C acclimation group, suggesting ethanol may be used as an alternative end product of anaerobic metabolism. Exposure to exogenous ethanol, in concentrations as low as 0.1%, produced periods of paradoxical anaerobism even in 28°C-acclimated fish.

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The Devils Hole pupfish, Cyprinodon diabolis, exists in an isolated cavern administered by Death Valley National Park. Only 65 adults were counted in a recent census, down from over 500 fish in 1996. Limiting factors for population recovery are high temperatures that average 33°‐34° C in the main pool and low levels of dissolved oxygen during much of the year. A refuge population of C. diabolis propagates well in our laboratory at 28° C but less so at 33° C. The critical PO2 of 33° C acclimated fish (8.39 ± 0.16 kPa) is significantly higher than that of the 28° C acclimated fish (6.49 ± 0.14 kPa). Further, the 33° C acclimated fish produce 7.3x more ethanol suggesting a large anaerobic component of total metabolic rate. We further investigated the effect of limiting oxygen levels on developing eggs. Oxygen consumption (VO2) in air saturated water prior to hatching increased as the heartbeat appeared but remained stable thereafter, even when the larva was obviously active in the egg capsule. VO2 increased markedly when the larva emerged suggesting the egg capsule is a barrier for oxygen diffusion. Surprisingly, critical PO2 of eggs was even higher than that of adults indicating an even greater reliance on anaerobic metabolism. We suggest that the reliance on anaerobic metabolism in the presence of available oxygen is a larval adaptation that persists in the adults as paradoxical anaerobism.