Increasing Temperatures Enhance Hypoxic Encounters in the Amazon: Consequences for Air-Breathing Fish

To cope with frequently hypoxic conditions in the Amazon, many fish refer to breathing air. In order to avoid a possible loss of oxygen taken up in the air-breathing organ to hypoxic water through the gills in many of these fish, gill surface area is reduced. Gills, however, are multifunctional organs, so that breathing air affects not only gas exchange, but also other physiological functions typically located at the gills, for example, ion regulation, acid-base regulation, or nitrogen excretion. Elevated temperatures cause an increase in metabolic activity, reduce oxygen solubility in water, and therefore are expected to stimulate air-breathing. This may potentially increase the risk of predation at the water surface. An increased reliance on aerial respiration may also stimulate progressive reduction of the gill surface area, which would affect other functions located at the gills.