Ecomorphological adaptation to oxygen deficiency in Amazon floodplains by serrasalmid fish of the genus Mylossoma

Serrasalmids of the genus Mylossoma are obligate gill‐breathers that are encountered in the floodplain lakes of Amazonia, even when the oxygen concentrations there are below 0 . 5 mg l −1 . It was shown by experiments that fish of these species are capable of utilizing the oxygen‐rich surface layer of the water for respiration, in order to survive periods of habitat‐induced hypoxia. This so‐called aquatic surface respiration entails an increase in locomotory activity and an ecomorphosis involving the formation of a dermal extension on the lower jaw, that apparently has a hydrodynamic function for using the surface layer for gill respiration; when the water is aerated, it retrogresses to its original size. Histological examination showed that the extension is formed by edematous processes in the stratum spongiosum.