Xenomystus

Originally classified in the genus Notopterus and reclassified in 1984, this is now the only described species of Xenomystus and has several interesting characteristics. Its swim bladder is specially adapted to act as an accessory breathing organ in a similar fashion to Polypteriids. It must rise to the surface to take a gulp of air periodically, often emitting a bell-like sound as it does so. X. nigri is often confused with the similar looking but far more aggressive Asian knifefish, Notopt...