Adaptations to hypoxic environments in the erythrinid fish Hoplias microlepis

Synopsis

The respiratory and metabolic adaptations of Hoplias microlepis were studied to determine mechanisms which enable this species to persist in habitats in Panama that may become hypoxic or anoxic for extended periods of time. This fish can produce energy anaerobically and survive for as long as 3.30 h in anoxic water. Anaerobiosis results in the accumulation of metabolic end products, including lactate, and requires repayment of an O₂ debt. Following anoxia exposure, fish (37.51–269.77 g) repayed a mean O₂, debt of 38.0 ml O₂ during a recovery period which averaged 10.6h. Acclimation to hypoxia for 16–25 days increased routine metabolic rate ($\dot{V}{_{\text{O}}{_2}}$) from 54.0 to 73.2 ml O₂ kg^-1 h^-1 and raised the activities of both aerobic and glycolytic enzynes in white muscle and gill tissues, but did not affect enzyme levels in heart, liver, or brain, or fish anoxia tolerance or recovery times. Increases in $\dot{V}{_{\text{O}}{_2}}$, and in both aerobic and anaerobic enzyme activities in fish acclimated to hypoxia may relate to the higher ventilation rate and greater swimming activity of these fish during the period of acclimation, increased anaerobic capacity, and the coupling of the aerobic processing of the end products of anaerobiosis. In hypoxic water, H. microlepis uses aquatic surface respiration (ASR) to capture O, present in the uppermost layer of water. This and its anoxia tolerance are the two most important components of its adaptation to external oxygen limitation. Our study suggests that, in its habitat. H. microlepis may make periodic trade-offs between the need to increase its metabolic activity in order to carry out ASR and producing energy anaerobically and incurring an oxygen debt while remaining in deeper, hypoxic water.