Effects of anoxia on the distribution, respiratory strategies and electric signal diversity of gymnotiform fishes

This paper explores the relationship between dissolved oxygen concentration and the distribution and electric signal diversity of 64 species of gymnotiforms from the Tefé region of the upper Amazon basin. Seventeen species are able to tolerate protracted periods of anoxia in inundated várzea floodplains or in terra firme swamps. The majority do so by breathing air—either with specialist accessory air‐breathing organs or via their gills. An assemblage of 38 species of gymnotiforms which are unable to tolerate hypoxia undertake lateral migrations from well‐oxygenated river channels into and out of the várzea floodplain in response to oxygen availability. These have evolved behavioural adaptations to avoid hypoxic water. While there is a mix of tone‐ and pulse‐type electric organ discharges (EODs) in species that live only in permanently well‐oxygenated habitats, 16 out of the 17 species that live in anoxic habitats have pulse‐type EODs. The tone‐type signals may have less flexible and perhaps greater overall energetic demands that impose handicaps in habitats where oxygen is a limiting factor. Many tone‐type species also have more active swimming behaviour which could impose further energetic demands.