1EVOLUTIONARY FEATURES OF HYPOXIA TOLERANCE IN FISH OF THE AMAZON: FROM MOLECULAR TO BEHAVIORAL ASPECTS

Fish adaptation to hypoxia does not include, necessarily, the ability to tolerate hypoxia; however, it does include mechanisms they use to avoid hypoxi. Literature reviews on fish of the Amazon shows that they search for oxygen rich environments trying to reach air, water surface layer, or aquatic plant roots when facing hypoxia. While the oxygen lack is considered one of the main causes of air-breathing appearance among fish, the ability to breathe air is not necessarily related to the development of hyp xia tolerance. Additionally, some non air-breathing fish groups do tolerate hypoxia and are able to remain in hypoxic and anoxic environments. This paper will review the main adaptive mechanisms fish developed to survive hypoxic environments and try to correlate changes in innate behavior, morphology and anatomy with physiological and biochemical adaptive mechanisms under an evolutionary perspective. Considering that evolution of hypoxia tolerance is related to genetic mechanisms, we will discuss the studies of genes considered to be involved with hypoxia adaptation: LDH A*, B*, and C*. Data on LDH cDNA seem to be consistent with functional aspects of this isozyme system. Considering that the evolution of hypoxia tolerance in fish may have induced the whole group to adaptive radiation, we suggest it may have contributed to the biological fish diversity in the Amazon basin