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Serrasalmids are medium to large-sized characiform fishes that reach about 1 meter (3.3 ft)long, generally characterized by a deep, laterally compressed body with a series of mid-ventral abdominal spines or scutes, and a long dorsal fin (over 16 rays). Most species also possess an anteriorly-directed spine just before the dorsal fin extending from a supraneural bone; exceptions include members of the genera Colossoma, Piaractus, and Mylossoma.
Most serrasalmids have about 60 chromosomes, ranging from 54 to 62.Metynnis has 62 chromosomes, as does Catoprion, Pristobrycon striolatus, and Pygopristis.
Serrasalmids inhabit all major and some minor Atlanticriver river systems in South America, but have been introduced to other areas. Species range from about 10° N latitude south to about 35° S latitude.
The diets of the various serrasalmid fishes include seeds, fruits, leaves, various invertebrate and vertebrate prey, as well as fish flesh, scales, and fins. To emphasize the diversity of diets, authors commonly highlight the fruit- and leaf-eating pacus and the highly carnivorous piranhas. Most non-piranhas in the family are primarily herbivorous. In contrast, it was long believed that piranhas were strict carnivores. Many species change diets depending on age and resource availability.
Serrasalmidae were recently classified as a subfamily of Characidae. The relationship of Serrasalmidae to other characiforms has yet to be determined. The taxonomy and systematics of piranhas and their relatives are complicated and much remains unsettled. Consequently, both species identification and phylogenetic placement of many taxa are problematic.
However, the ongoing classification of these fish is difficult and often contentious, with ichthyologists basing ranks according to characteristics that may overlap irregularly (see Cladistics). DNA research sometimes confounds rather than clarifies species ranking. Ultimately, classifications can be rather arbitrary.
Despite this, Serrasalmidae is relatively well understood, and there is wide agreement on the genera and species that it includes.
The fossil record, particularly for piranhas, is relatively sparse. Most known fossils are from the Miocene, although a few unidentified forms are considered Paleocene and two reportedly date to as early as the Late Cretaceous. Fossils of a living species of Colossoma from the Miocene have been described, suggesting a very conservative history for a specialized herbivorous fish. All serrasalmine genera had originated by the middle Miocene, with the possible exception of three of the four piranha genera (Pygocentrus, Pristobrycon, and Serrasalmus).
Many serraslmids are in demand as aquarium ornamentals, and several pacus, such as Piaractus and Colossoma, are economically important to commercial fisheries and aquaculture.
Piranhas are generally less valued, although they are commonly consumed by subsistence fishers and frequently sold for food in local markets. A few piranha species occasionally appear in the aquarium trade, and, in recent decades, dried specimens have been marketed as tourist souvenirs. Piranhas occasionally bite and sometimes injure bathers and swimmers, but serious attacks are rare and the threat to humans has been exaggerated. However, piranhas are a considerable nuisance to commercial and sport fishers because they steal bait, mutilate catch, damage nets and other gear, and may bite when handled.