The oldest characid fossil record comes from the late Jurassic period, more than 145 million years ago. Several ichthyologists now view the characins as representing the oldest and most ancient of freshwater fishes, and Boulenger considered them the ancestors of the naked catfish, barbs, eels and Knifefish.
The ancestor of all modern Characiformes appeared when the two southern continents of Africa and South America were part of the supercontinent called Gondwana (Lundberg 1993; Orti and Meyer 1997). The breakup of the two continents allowed the characins in what became South America to take full advantage of the potential [=empty or not fully-exploited] ecological niches, and with the exception of the marsupials in Australia, the characids underwent explosive radiation unlike any other in all the vertebrates. The diversification of the cichlid fishes in the African rift lakes is a more recent though much smaller-scaled example.
The divergence in ecological specialization among the characids is truly remarkable. With respect to feeding, many are predatory, several are herbivore, some survive by eating fish fins and scales, some are mud-eaters, and some sift plankton from the water; in other respects, some "hop" on the substrate, some leap from the water and glide, some jump out of the water and spawn on terrestrial vegetation, some swim with their head vertically downward, some are capable of air-breathing, and some exhibit nesting behaviours and parental care. No other order of fish possesses such diversity.
There are currently more than 1674 described species in approximately 270 genera within 19 Families (Nelson, 2006). Roughly 20% of the characin families with approximately 200 species occur in Africa; the remainder are in South America with a few having made their way into Central America, Mexico and southern Texas no later than 5 million years ago. The taxonomy of the characids is still changing and will continue to do so for many years due to increasing study of fish populations. For instance, there are currently some 304 species of aquarium fish in the two genera Hemigrammus and Hyphessobrycon, and all of these are now considered to be incertes sedes [meaning "of uncertain placement"]. Mirande (2009) has proposed a temporary revision of species into clades pending further phylogenetic study; those in Hemigrammus and Hyphessobrycon that have not been assigned to new genera are now deemed in the Hemigrammus clade of Characidae retaining the respective genus names for the present.
For the aquarist, what makes a fish a characid?
All characins are highly sensitive to water parameters and conditions; the water quality should be stable, and use of medications should be avoided unless absolutely essential. Salt should never be used in an aquarium with characids; with one or two exceptions, all species suffer stress with levels above 100 ppm [= .38 of one gram, or 1/6 of a level teaspoon, per gallon] and many cannot tolerate more than 60 ppm (Weitzman et al., 1996).
Several families within the characids provide suitable aquarium fishes, but there are three that have become very popular with home aquarists. The largest group are the tetra; this common name is an abbreviation of Tetragonopterinae, one of many families of characids but one that ironically is not closely related to most of the tetra species. The name derives from the Greek tetra (=four), gonia (=angle) and pteron (=wing). The second largest group is the pencilfishes, so called because of their long narrow profile; scientifically these are within the Nannostomini tribe of the subfamily Pyrrhulininae in the family Lebiasinidae. Third are the hatchetfishes, so named from their distinctive hatchet-shape keel, scientifically in the Gasteropelicidae family. Specific traits of these fishes are included in their respective profile.
Mirande, J. Marcos (2009), "Weighted parsimony phylogeny of the family Characidae (Teleostei: Characiformes)," Cladistics Volume 25, No. 6 (July 2009).
Nelson, Joseph S. (2006), Fishes of the World, 4th edition.
Weitzman, Stanley, Lisa Palmer, John R. Burns and Naercio A. Menezes (1996), "Breeding and Rearing Mimagoniates Species, Internally Fertilized Tetras," Tropical Fish Hobbyist Vol. 44, No. 12 (August 1996).