Evolutionary trait found in the
anabantid family of
fish. (Anabantids include the
betta,
gouramis and
paradise fish.) The
labyrinth organ is basically a
rudimentary lung. These fish have the labyrinth organ in *addition* to
gills.
This organ allows the fish to survive in MUCH poorer quality water than usual. High chemical buildup, wrong temperature, or too much algae growth can cause the water to become foul and highly oxygen-poor. Most fish would suffocate in water like this, but the anabantids, as needed, will surface and gulp in a breath of pure air from the surface of the water.
It is the labyrinth organ that makes bettas so successful in small fishbowls. the leading cause of death among the other popular 'bowl' fish, goldfish, is lack of oxygen, but the betta can live in a small, non-aerated bowl quite happily, since it is not restricted to gills only. (Paradise fish and gouramis CAN be kept in bowls as well, but the paradise fish are notorious jumpers, and gouramis grow to be quite large.)