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Atypical respiratory Physiology of a recent air‐breathing fish: Pangasius

Air‐breathing in fish has evolved independently many times and is normally associated with marked reduction in gill size, often argued as an adaption to avoid oxygen loss to hypoxic water, while retaining a capacity for branchial ion and acid‐base regulation. This is not the case in the economically important, but sparsely studied teleost Pangasius (Pangasionodon hypophthalmus), which seems to have acquired air‐breathing during a recent a speciation event in the mid‐Miocene. Using a heavily vascularized and modified swim bladder, it has retained well‐developed gills that can provide for an almost five‐fold rise in oxygen uptake from normoxic water during swimming without access to air. Its gills, however, show considerable plasticity, such that the secondary lamella become almost completely embedded by an interlamellar when the fish are kept in normoxic water, becoming fully exposed in hypoxic water. The blood and isolated hemoglobin of Pangasius have extremely high oxygen affinities with P50′s of a mere 4‐6 mm Hg at 30oC, but there is no evidence for a Root effect and the red cells are devoid of adrenergic stimulation of the sodium‐proton exchanger. Moreover, as an additional difference to other air‐breathing fishes, Pangasius shows efficient extracellular pH regulation and can be found growing in strongly hypercapnic water with PCO2 in excess of 40 mm Hg. Based on these unusual attributes in an air‐breathing fish, we will present data on the regulation of cardiac and ventilator responses to hypoxia from fish acclimated to different temperatures and oxygen regimes.