The bony-tongued fishes make up the order Osteoglossiformes. This order is a relatively primitive or basal order of ray-finned fishes. The group contains a wide variety of fishes from the giant arapaima of the amazon the tiny featherfin knifefishes and freshwater butterflyfish. The classification of the order comes from a few morphological similarities. In particular, most fish in this group have a kind of bony-tongue. Often this tongue has teeth on it and is used to crush down against the teeth on the top of the mouth.
Many of the basal members of the ray-finned fishes have the ability to breath air. Osteoglossiformes have many members that breath air. Most of the fish known as arowana in the family Osteoglossidae breath air through a highly vascularized swim bladder. Additionally, the giant arapaima of the amazon is an obligate air breather, meaning that it must breath air to survive. A study by Don Stevens on the arapaima showed that while it does use its gills for respiration, in normal water conditions about 75% of its oxygen intake comes from the air. In very oxygen poor waters, the level approaches 100%.
There are relatively few fish that have the ability to produce electrical signals. The best studied of the Osteoglossiformes are the elephantfishes. In fact, they have become an ideal study specimen for research on sensory recognition. Elephantfishes (family Mormyridae) produce electrical signals in the caudal peduncle (the base of the tail). Other elephantfish can pick up these signals in sensory electroreceptors on their skin. Elephantfish are able to distinguish between species, and sexes using this sort of electrical communication.
Recently, there has been a lot of debate over the classification of the Osteoglossiformes. Most interent resources as well as published data have conflicting taxonomy. Sometimes the sister group, the mooneyes (classified here as the Hiodontiformes) are classified in this group. Additionally, the arapaima and African arowana are often given their own family, the Arapaimidae. In this classification system, we have stuck to the classification system of the Tree of Life web project. As the debate settles, we'll update the classification system appropriately.
Traditionally, this group is divided into two suborders, with the butterflyfishes and arowana making up one group and the knifefishes and elephantfishes make up the other. For simplicity, we list only the families.