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Scientific classification
The Characiformes are an order of ray-finned fish, comprising the characins and their allies. There are a few thousand different species, including the well-known piranha and tetras.

These belong within the superorder Ostariophysi, a group mainly distinguished by the presence of a Weberian apparatus, a series of bony parts connecting the swim bladder and inner ear. Superficially the Characiformes somewhat resemble their relatives the minnows. The most noticeable differences are the presence of a small fleshy adipose fin between the dorsal fin and tail, and the presence of teeth within the mouth, though there are exceptions to both of these. Barbels are always absent and the body is almost always covered in well-defined scales.

Characins are found in tropical lakes and rivers throughout most of South America, Central America, and central Africa. This and other evidence suggests they first diversified during the Cretaceous period, though fossils are poorly known. A few characins become quite large, and are important as food or game. Most, however, are small shoaling fish. Many species are popular in aquaria thanks to their bright colors, general hardiness, and tolerance towards other fish in community tanks.

Originally the characins were all grouped within a single family, the Characidae, which was included in the Cypriniformes. Since then about 15 different families have been separated out, including the following:

However, classification varies somewhat, and it appears the circumscribed Characidae are still polyphyletic.