number and surface area. Because of their thinness, the gills usually are not selfsupporting.
For a strictly water-breathing fish this is not a problem because water
provides support for the gills. However, if such a fish finds itself out of water, the
gills are not supported anymore. The collapse of gills and the limited surface area of
other possible breathing organs are the reasons why most fishes ultimately choke
outside water, despite the abundance of oxygen in air. Only air-breathing fishes can
manage, and among their anatomical adaptations are thicker, self-supporting (but,
consequently, less numerous) gills, as well as modified swimbladders, mouths, gill
chambers, guts, or skin which have more blood vessels and are more gas-permeable
than in other species.
Some species of fishes have become so specialized for air-breathing that their
capacity for water breathing is impaired and they cannot meet all of their oxygen
demand even in normally oxygenated water. They must breathe in air at least once in
a while. Surprisingly for a fish, they will “drown” if they are forced to stay in water.
Among such fishes are the South American pirarucu Arapaima gigas (this fish can get
huge, at 4.5 m and 200 kg, and 95% of its large oxygen demand is provided by airbreathing),
the freshwater butterflyfish Pantodon buchholzi (popular in the aquarium
trade), the adult Lepidosiren paradoxa (a South American lungfish), the electric eel
Electrophorus electricus, the three-spot (=blue) gourami Trichogaster trichopterus,
and the climbing perch Anabas testudineus