Characiformes is a diverse order of ray-finned fish comprising the characins and their allies and with all extant species found in freshwater habitats. Characiformes is a large assemblage of about 2,000 species, including the well-known piranhas and tetras. Extant characiforms are found exclusively in freshwater environments in Africa and the Americas. The largest family is Characidae, the characins, with about 1,000 known species.
The taxonomy of Characiformes is not settled, with various subfamilies since 1994 being raised to the level of family or moved to different families and a large number of taxa being incertae sedis (of uncertain placement).
Characiformes includes many very colorful fish, some silvery, and quite a number are popular as aquarium fishes. In particular, the tetras are popular in aquaria thanks to their bright colors, general hardiness, and tolerance towards other fish in community tanks. Some characiforms become quite large and are important food fishes, such as species of Brycon. However, most are small shoaling fishes.
Ecologically, characiformes are important in food chains. Mostly carnivores, they consume smaller vertebrates and invertebrates, with some consuming algae and diverse plant matter, while being food for a diversity of fishes, birds, mammals, and reptiles. As such, Characiformes reflects the principle of bi-level functionality, advancing both various individual functions (survival and reproduction) while also providing larger functions for their ecosystems and for humans.
Overview and description
Characiformes belong to the Actinopterygii, a major taxonomic class (or subclass) of fish, known as the "ray-finned fishes," within which 96 percent of all fish species are placed. Characiformes further belong to the superorder Ostariophysi, and within that the series Otophysi. As members of Ostariophysi, characiformes have a swim bladder, which is usually divided into an anterior chamber and a posterior chamber, and typically possess a fright reaction elicited by an alarm substance, with the alarm substance released upon injury to the skin. (Members of Serraslamidae, such as piranhas, lack the fright reaction but have an alarm substance.) Members of Otophysi mainly are distinguished by the presence of a Weberian apparatus, a series of bony parts connecting the swim bladder and inner ear. Other orders in the series Otophysi are Cyriniformes (carps), Siluriformes (catfishes), and Gymnotiformes (American knifefishes) (Nelson 2006).
Characiformes is a large assemblage of species that are poorly known and with much morphological diversity (Nelson 2006). Nelson (2006) recognizes 18 families, 270 genera, and at least 1674 species; however, there has been much taxonomic revision with various subfamilies being raised to the level of families, and so forth.
Superficially, the Characiformes somewhat resemble their carp relatives of the order Cypriniformes. One noticeable difference is the presence of a small fleshy adipose fin between the dorsal fin and tail. The presence of well-developed teeth within the mouth is also often present as most (though not all species) are carnivorous. However, there are exceptions to both of these. Barbels are always absent and the body is almost always covered in well-defined scales. (The characid tetra Gymnocharacinus bergii lacks and adipose fin and scales are almost totally lacking in adults.) The upper jaw of characiformes usually is not truly protractile. A pelvic fin is present, with 5 to 12 rays, and the anal fin is short to moderately long, with fewer than 45 rays (Nelson 2006; Agbayani 2005).
The largest size is 1.4 meters 4.5 feet), reached in Hydrocynus goliath of the Congo, while the smallest size is about 13 millimeters (0.5 inches). Many members are under 3 centimeters (about 1 inch) in length (Nelson 2006).
While most are carnivores, some are herbivores, such those distichodontids (family Distichodontidae) of Africa with non-protractile upper jaws and the paradontids (family Parodontidae) of South America, the later of which have ventral mouths and teeth modified for scraping algae off rocks (Nelson 2006).
Distribution and habitat
Extant characiformes are found only in freshwater. They are most diverse in the Neotropics, where they are found in tropical lakes and rivers throughout most of South America, Central America, and central Africa. At least 209 species of characins are found in Africa, including the distichodontids, the citharinids, the alestiids (the African tetras), and the monotypic Hepsetidae, Hepsetus odoe. The rest of the characins originate from the Americas, being found in southwestern United States, Mexico, Central America, and South America, as far south as Argentina (Nelson 2006).
Characiformes forms a group known as Characaphysi with the orders Siluriformes and Gymnotiformes (Briggs 2005). Characiformes is considered the sister group to the orders Siluriformes and Gymnotiformes, though this has been debated in light of recent molecular evidence (Nelson 2006). The suborder Citharinoidei, which contains the families Distichodontidae and Citharinidae, is considered the sister group to the rest of the characiformes, suborder Characoidei (Briggs 2005; Fink and Fink 1981).
Nelson (2006) recognizes the following families within Characiformes:
- Suborder Citharinoidei
- Family Distichodontidae (distichodontids)—Africa
- Family Citharinidae (citharinids)—Africa
- Suborder Characoidei
- Family Parodontidae (parodontids)—eastern Panama and most of South America
- Family Curimatidae (toothless characiforms)—southern Costa Rica to northern Argentina
- Family Prochilodontidae (flannel-mouth characiforms)—South America
- Family Anostomidae (toothless headstanders)—Central America and South America
- Family Chilodontidae (headstanders)—northern South America
- Family Crenuchidae (South American darters)—eastern Panama and South America
- Family Hemiodontidae (hemiodontids)—northern South America to the Parana-Paraguay Basin
- Family Alestiidae (African tetras)—Africa
- Family Gasteropelecidae (freshwater hatchetfishes)—Panama and South America
- Family Characidae (characins)—southwestern Texas, Mexico, Central America, and South America
- Family Acestrorhynchidae (acestrorhnchids)—South America
- Family Cynodontidae (cynodontids)—South America
- Family Erythrinidae (trahiras)—South America
- Family Lebiasinidae (pencil fishes)—Costa Rica, Panama, and South America
- Family Ctenoluciidae (pike-characids)—Panama and South America
- Family Hepsetidae (African pikes)—Africa
Nelson (2006) recognizes 12 subfamilies within Characidae, which has over 962 species placed in 165 genera. However, other authorities raise subfamily Serrasalminae, within Characidae, to the family level as Serrasalmindae.
Since 1994, the taxonomy of the Characiformes has undergone substantial revisions, with many taxonomic groups formerly within the family Characidae raised to the family level. For example, the Characidae subfamily Alestiinae was promoted by some to the family level (Alestiidae) and the subfamilies Crenuchinae and Characidiinae were moved to the family Crenuchidae (Nelson 2006). Other fish families that were formerly classified as members of the Characidae, but which were moved into separate families of their own during recent taxonomic revisions (post-1994) include Acestrorhynchidae, Anostomidae, Chilodontidae, Citharinidae, Ctenoluciidae, Curimatidae, Distichodontidae, Gasteropelecidae, Hemiodontidae, Hepsetidae, Parodontidae, and Prochilodontidae. Likewise, as noted above, while Nelson (2006) recognizes the Characidae subfamily Serrasalminae (pacus, silver dollars, and piranhas), some authorities raise this to the family level Serrasalmidae.
Given the current state of flux of the Characidae, a number of other changes will doubtless take place, reassigning once-familiar species to other families. Indeed, the entire phylogeny of the Ostariophysans (fishes possessing a Weberian apparatus) has yet to be conclusively settled, and until that phylogeny is settled, the opportunity for yet more upheavals within the taxonomy of the Characiformes is considerable.
The oldest fossil considered to be a characiformes, with some reservation, 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, such as Paleohoplias and Tiupampichthys from South America nad Eocitharinus and Mahengecharas from Africa. Sorbinicharax is a fossil from the now extinct family Sorbinicharacide (Nelson 2006).
Characiformes likely first diversified during the Cretaceous period, though fossils are poorly known (Nelson 2006). 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 in Africa yet are absent in South America, where these fish may have been driven extinct (Briggs 2005). The characiforms had not spread into Africa soon enough to also reach the land bridge between Africa and Asia (Briggs 2005). The earliest they could have spread into Central America was the late Miocene (Briggs 2005).
- J. S. Nelson, Fishes of the World, 4th edition (Hoboken, NJ: John Wiley & Sons, 2006, ISBN 0471250317).
- Agbayani, E.. 2005. Characiformes. FishBase. (R. Froese and D. Pauly, editors). Retrieved December 5, 2008.
- Briggs, J. C. 2005. The biogeography of otophysan fishes (Ostariophysi: Otophysi): A new appraisal. Journal of Biogeography 32: 287–294. Retrieved December 5, 2008.
- Buckup, P. A. 1998. Relationships of the Characidiine and phylogeny of characiform fishes (Teleostei: Ostariophysi). Pages 123-144 in L. R. Malabarba, R. E. Reis, R. P. Vari, Z. M. S. Lucena, and C. A. S. Lucena (eds.). Phylogeny and Classification of Neotropical Fishes. Porto Alegre, Brazil: EDIPUCRS. ISBN 8574300357.
- Fink, S. V., and W. L. Fink. 1981. Interrelationships of the ostariohysan fishes (Teleosti). J. Linn. Soc. (Zool.) 72(4): 297-353.
- Nelson, J. S. 2006. Fishes of the World, 4th edition. Hoboken, NJ: John Wiley & Sons. ISBN 0471250317.
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