Serrasalmid

The Serrasalmidae (serrasalmids) are a family of characiform fishes native to freshwater habitats of South America. They include more than 90 species. The name means "serrated salmon family", which refers to the serrated keel running along the belly of these fish. Fish classified as Serrasalmidae are also known by these common names: pacu, piranha, and silver dollar. These common names generally designate differing dental characteristics and feeding habits.

Description

Serrasalmids are medium- to large-sized characiform bony fishes that reach about long, generally characterized by a deep, laterally compressed body with a series of midventral 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''.

Evolution

Taxonomy

The Serrasalmidae were until, recently, classified as a subfamily of the Characidae, with their taxonomic relationships uncertain at the time. More recent studies have found them to belong to a wider clade of South American characoids, being most closely related to the Hemiodontidae. The Serrasalmidae are relatively well understood, and agreement is wide on the genera and species included.

The family is classified as follows:

* Family Serrasalmidae Bleeker, 1859
** Subfamily Colossomatinae Kolmann ''et al.'', 2021 (lowland pacus)
*** Genus '' Colossoma'' Eigenmann & Kennedy, 1903
*** Genus '' Mylossoma'' Eigenmann & Kennedy, 1903
*** Genus '' Piaractus'' Eigenmann, 1903
** Subfamily Myleinae Eigenmann, 1903 (upland pacus)
*** Genus '' Acnodon'' Eigenmann, 1903
*** Genus '' Mylesinus'' Valenciennes, 1850
*** Genus '' Myleus'' Müller & Troschel, 1844
*** Genus '' Myloplus'' Gill, 1896
*** Genus '' Ossubtus'' Jégu, 1992
*** Genus '' Paramyloplus'' Norman, 1929
*** Genus '' Prosomyleus'' Géry, 1972
*** Genus '' Tometes'' Valenciennes, 1850
*** Genus ''Utiaritichthys'' Miranda Ribeiro, 1937
** Subfamily Serrasalminae Bleeker, 1859 ( piranhas)
*** Genus '' Catoprion'' Müller & Troschel, 1844
*** Genus †'' Megapiranha'' Cione ''et al.'' 2009 (fossil; Late Miocene)
Summary of the paper
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*** Genus '' Metynnis'' Cope, 1878
*** Genus '' Pygocentrus'' Müller & Troschel, 1844
*** Genus '' Pygopristis'' Müller & Troschel, 1844
*** Genus '' Serrasalmus'' Lacepède, 1803 (=''Pristobrycon'' Eigenmann, 1915)

Fossil record

The fossil record, particularly for piranhas, is relatively sparse. Most known fossils are from the Miocene. The earliest definitive serrasalmid fossil teeth are known from the Late Eocene-aged (~38 mya) sediments of the Santa Luca Formation in Bolivia. Teeth resembling those of pacus have been recovered from the late Maastrichtian-aged El Molino Formation, which could potentially suggest a Late Cretaceous occurrence for the family, but these teeth show significant differences from modern serrasalmids, and their assignment to the family is uncertain. 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'').

Distribution

Serrasalmids inhabit all major and some minor Atlantic river systems in South America east of the Andes, but have been introduced to other areas. Species range from about 10°N latitude south to about 35°S latitude.

Ecology

The diets of the various serrasalmid fishes include seeds, fruits, leaves, and 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 in the family other than piranhas are primarily herbivorous. In contrast, piranhas have been long believed to be strict carnivores. Many species change diets depending on age and resource availability.

The primarily carnivorous piranha group comprises the genera '' Catoprion'', '' Pristobrycon'', '' Pygocentrus'', '' Pygopristis'' and '' Serrasalmus'', but based on phylogeny also the mainly herbivorous (although with omnivorous tendencies) '' Metynnis''. The remaining primarily herbivorous species can be divided into two groups based on ecology and, to some extent, phylogeny: '' Colossoma'', '' Mylossoma'' and '' Piaractus'' are mainly found in relatively slow-moving waters, and feed extensively on fruits, nuts and seeds, playing an important role as seed dispersers. '' Mylesinus'', '' Myleus'', '' Ossubtus'', '' Tometes'' and ''Utiaritichthys'' are found in fast-flowing sections of rivers, and mainly feed on aquatic plants, especially Podostemaceae. '' Myloplus'' mostly feed on plant material and some of its species are phylogenetically related with the previous group, but this genus includes species of both slow and fast-flowing waters.

Relationship to humans

Many serrasalmids 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.

References

{{Taxonbar, from=Q18372588

Fish of South America
Extant Miocene first appearances
Characiformes families
Taxa named by Pieter Bleeker

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