Characiformes is an order of ray-finned fish, comprising the characins and their allies. Grouped in 18 recognized families, there are more than two thousand different species, including the well-known piranha and tetras.
Temporal range: Early Cretaceous–Recent 
Characiformes form part of a series called Otophysi within the superorder Ostariophysi. Otophysi contains three other orders, Cypriniformes, Siluriformes, and Gymnotiformes. Characiformes forms a group known as Characiphysi with Siluriformes and Gymnotiformes. Characiformes is the sister group to the orders Siluriformes and Gymnotiformes, though this has been debated in light of recent molecular evidence.
Originally the characins were all grouped within a single family, the Characidae. Since then 18 different families have been separated out. However, classification varies somewhat, and the most recent (2011) study confirms the circumscribed Characidae as monophyletic. Currently, there are eighteen families, about 270 genera, and at least 1674 species. The suborder Citharinoidei, which contains the families Distichodontidae and Citharinidae, is considered the sister group to the rest of the characins, suborder Characoidei.
The oldest characiform is Santanichthys of the early Cretaceous (Albian stage) of Brazil. While all extant species are freshwater, this species was probably either brackish or marine. Many other fossils are also known. Characiformes likely first diversified during the Cretaceous period, though fossils are poorly known. During the Cretaceous period, the rift between South America and Africa would be forming; this may explain the contrast in diversity between the two continents. Their low diversity in Africa may explain why some primitive fish families and Cypriniformes coexist with them while they are absent in South America, where these fish may have been driven extinct. The characiforms had not spread into Africa soon enough to also reach the land bridge between Africa and Asia. The earliest they could have spread into Central America was the late Miocene.
Characins possess a Weberian apparatus, a series of bony parts connecting the swim bladder and inner ear. Superficially, the Characiformes somewhat resemble their relatives of the order Cypriniformes, but have a small fleshy adipose fin between the dorsal fin and tail. Most species have teeth within the mouth, since they are often carnivorous. The body is almost always covered in well-defined scales. The mouth is also usually not truly protractile.
The largest characin are Hydrocynus goliath and Salminus franciscanus, both of which are up to 1.3 metres (4.3 ft). The smallest size is about 1.7 centimetres (0.67 in) in the Bolivian pygmy blue characin, Xenurobrycon polyancistrus. Many members are under 3 centimetres (1.2 in).
Characins are most diverse in the Neotropics, where they are found in lakes and rivers throughout most of South and Central America. The red-bellied piranha, a member of the Serrasalmidae family within Characiformes, is endemic to the Neotropic ecozone. At least 209 species of characins are found in Africa, including the distichodontids, citharinids, alestiids, and hepsetids. The rest of the characins originate from the Americas.
A few characins become quite large, and are important as food or game. Most, however, are small shoaling fish. Many species known as tetras are popular in aquaria thanks to their bright colors, general hardiness, and tolerance towards other fish in community tanks.
African tetras (family Alestidae, formerly spelled Alestiidae) are a group of characiform fish exclusively found in Africa. This family contains about 18 genera and 119 species. Among the best known members are the Congo tetra, and African tigerfish.Brycinus
Brycinus is a genus of ray-finned fish in the family Alestiidae. Like other "African characids", they were formerly included in the Characidae but are actually somewhat more distantly related Characiformes.Like some other Alestiidae, they are called robber tetras due to their bold and rather carnivorous habits. They are not infrequently kept as aquarium fishes and in their requirements resemble the South American tetras of the Characidae. Unlike these, Brycinus are not well-suited to accompany delicate fishes however and are better kept with dwarf cichlids and similar small but robust companions.Characidae
Characidae, the characids or characins is a family of freshwater subtropical and tropical fish, belonging to the order Characiformes. The name "characins" is the historical one, but scientists today tend to prefer "characids" to reflect their status as a by and large monophyletic group at family rank. To arrive there, this family has undergone much systematic and taxonomic change. Among those fishes that remain in the Characidae for the time being are the tetras, comprising the very similar genera Hemigrammus and Hyphessobrycon, as well as a few related forms such as the cave and neon tetras. Fish of this family are important as food and also include popular aquarium fish species.These fish vary in length, though many are less than 3 cm (1.2 in). One of the smallest species, Hyphessobrycon roseus, grows to a maximum length of 1.9 cm.These fish inhabit a wide range and a variety of habitats. They originate in the Americas, ranging from southwestern Texas and Mexico through Central and South America. Many of these fish come from rivers, but, for example, the blind cave tetra even inhabits caves.Citharinidae
The Citharinidae, the lutefishes, are a small family of characiform fish. They are freshwater fish native to Africa, and are sufficiently abundant to be significant food fishes.They are deep-bodied, silvery fish, measuring up to 84 cm (33 in) in length and weighing up to 18 kg (40 lb). They are filter feeders.Crenuchidae
The Crenuchidae, South American darters, are a family of freshwater fish of the order Characiformes. The 12 genera include about 74 species, though several species are undescribed. These fish are relatively small (usually under 10 cm (4 in) in standard length) and originate from eastern Panama and South America. Both subfamilies were previously included in the family Characidae, and were placed in a separate family by Buckup, 1998. Buckup, 1993, revised all genera, except Characidium.Cypriniformes
Cypriniformes is an order of ray-finned fish, including the carps, minnows, loaches and relatives. This order contains 11-12 families, over 400 genera, and more than 4,250 species, with new species being described every few months or so, and new genera being recognized frequently. They are most diverse in southeastern Asia, and are entirely absent from Australia and South America.Their closest living relatives are the Characiformes (characins and allies), the Gymnotiformes (electric eel and American knifefishes) and the Siluriformes (catfishes).Distichodontidae
The Distichodontidae are a family of African freshwater fishes of the order Characiformes.Two evolutionary grades are found in this family; micropredators (predators of very small organisms like aquatic insect larvae) and herbivores have a nonprotractile upper jaw and a deep to shallow body, while carnivores have a movable upper jaw and an elongated body. Although the herbivores primarily feed on plant material, these species often have omnivorous tendencies. The carnivores include specialized fish-eaters (genus Mesoborus), fin-eaters (Belonophago, Eugnathichthys and Phago) and species that will feed on both whole fish and fins (Ichthyborus). The fin-eaters attack other fish, even ones that are much larger, where they bite off pierces of fins with their sharp teeth.The fish in Distichodontidae vary greatly in size among species, with the smallest micropredators being less than 8 cm (3.1 in) in length, and the largest herbivores can reach up to 83 cm (33 in).Erythrinidae
The Erythrinidae are a family of fishes found in rivers and other freshwater habitats from Costa Rica south as far as Argentina. They are common and are caught with hooks by fishermen, partially because of their voracious behaviour. They are sometimes called trahiras or tarariras.
The Erythrinidae include cylindrical fish with blunt heads, and prey on other fish. They can reach lengths up to 90 cm (35 in). Some species can breathe air, enabling them to survive in water low in oxygen, and even to move over land between ponds.Freshwater hatchetfish
The freshwater hatchetfishes are a family, Gasteropelecidae, of ray-finned fish from South and Central America. The common hatchetfish is the most popular member among fish keeping hobbyists. The family includes three genera: Carnegiella (four species), Gasteropelecus (three species), and Thoracocharax (two species).Headstander
A headstander is any of several species of South American fish, including Anostomus ternetzi, Anostomus anostomus (family Anostomidae) and members of genus Chilodus from the family Chilodontidae. The name derives from their habit of swimming at a 45° angle, head pointed downwards, as if "standing on their heads".Hemigrammus pulcher
Hemigrammus pulcher is a semi-popular aquarium species, also known as the pretty tetra, garnet tetra or black wedge tetra. In the wild, the species is found near Iquitos in the Peruvian Amazon, and probably also in Brazil and Colombia.Hyphessobrycon
Hyphessobrycon is a genus of freshwater fish in the family Characidae. These species are among the fishes known as tetras. The genus is distributed in the Neotropic ecozone from southern Mexico to Río de la Plata in Argentina. Many of these species are native to South America; about half a dozen species are from Central America and a single species, H. compressus is from southern Mexico.All small fishes, the Hyphessobrycon tetras reach maximum overall lengths of about 1.7–9.6 cm (0.7–3.8 in). There is great anatomical diversity in this genus. They are generally of typical characin shape, but vary greatly in coloration and body form, many species having distinctive black, red or yellow markings on their bodies and fins. These species are generally omnivorous, feeding predominantly on small crustaceans, insects, annelid worms and zooplankton. When spawning, they scatter their eggs and guard neither eggs nor young.Lebiasinidae
The pencil fishes are a family (Lebiasinidae) of freshwater fishes found in Costa Rica, Panama, and South America. They are usually small and are known as ornamental fishes in aquaria, including popular fishes such as the various pencil fish and the splashing tetra.
Lebiasinids are small, cylindrical fish, ranging from 2 to 18 cm (0.8 to 7.1 in) in adult length. They prey on insect larvae, especially those of mosquitos. The family includes the voladoras (genera Lebiasina and Piabucina), mostly found in highlands of the north Andes, the Guiana Shield and Central America, but the other species are mainly lowland fish inhabiting the Orinoco, Amazon and Paraguay River basins, and rivers of the Guianas.Ostariophysi
Ostariophysi is the second-largest superorder of fish. Members of this superorder are called ostariophysians. This diverse group contains almost 8,000 species, about 28% of known fish species in the world and 68% of freshwater species, and are present on all continents except Antarctica. They have a number of common characteristics such as an alarm substance and a Weberian apparatus. Members of this group include fish important to people for food, sport, the aquarium industry, and research.Parodontidae
The Parodontidae are a family of fresh water fish of the order Characiformes. The three genera include about 32 species, though several are undescribed. These fish are generally benthic and live in mountain streams of eastern Panama and South America. It was formerly considered a subfamily of the family Hemiodontidae.Piranha
A piranha or piraña (, , or ; Portuguese: [piˈɾɐ̃ɲɐ], Spanish: [piˈɾaɲa]), a member of family Characidae in order Characiformes, is a freshwater fish that inhabits South American rivers, floodplains, lakes and reservoirs. Although often described as extremely predatory and mainly feeding on fish, their dietary habits vary extensively and they will also take plant material, leading to their classification as omnivorous. In Venezuela, they are called caribes. They are known for their sharp teeth and powerful jaws.Pygocentrus
Pygocentrus is a genus of the piranha family Serrasalmidae. All species are native to tropical and subtropical South America. All the species are predatory, scavengers and may form large schools. The famous red-bellied piranha, Pygocentrus nattereri, is one of four species in the genus.Serrasalmus
Serrasalmus is a genus of piranhas. They are collectively known as pirambebas; the "typical" piranhas like the piraya piranha are nowadays placed in Pygocentrus. Like all piranhas, Serrasalmus are native to South America.
These fish are predatory, have sharp teeth and generally have a rhomboid shape. In some, the shape is more ovoid, particularly in old specimens. Some Serrasalmus species can exceed 20 in or 510 mm (S. manueli and S. rhombeus, according to OPEFE), placing them among the largest Serrasalmidae.Thayeria boehlkei
Thayeria boehlkei is a species of characin fish endemic to the Amazon river basin and Araguaia river, in Peru and Brazil respectively. The species is popular with aquarium hobbyists where it is traded under a variety of common names including blackline penguinfish, blackline thayeria, hockey-stick tetra, penguin fish and penguin tetra.