Summary
We have examined the oxygen-affinity of the fresh, whole blood from representatives of 40 genera of Amazonian fishes. When all the air breathnig fishes and all the water breathing fishes were considered, no differentiation in oxygen affinity could be distinguished, although in two cases of closely related species i.e. Osteoglossum, a water breather, versus Arapaima, an air breather, and Hoplias, a water breather, versus Hoplerythrinus, an air breather; the air breathers have bloods with lower oxygen affinity than do the water breathers. Significant differences in the oxygen affinity were fround when fishes were considered by habitat: the oxygen affinities were in general correlated with available environmental oxygen and relative oxygen demand of the fishes.