Summary
Blood oxygen affinities, erythrocytic nucleoside triphosphate concentrations (NTP) and other hematological parameters were measured in facultative air-breathing fish from the Amazon after acclimation to well aerated ("normoxic") and hypoxic water (PO2 = 125 to 135 and 20 to 25 mm, respectively). In the armored catfish Hypostomus sp, and Pterygoplichthys sp., hypoxia induces intermittent surfacing to gulp air and results in lower NTP levels, chiefly through significant decreases in guanosine triphosphate (GTF). The subsequent increases in blood O2 affinity appear adaptive to lowered time average internal O2 tensions. No similar changes were seen in the eel Synbranchus which breathes air almost continuously when kept in hypoxic water. The results are discussed in terms of their adaptive significance, and compared with data on temperate fish.