Characiformes
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Characiformes
Temporal range: Late Cretaceous (Turonian) to present Possible Cenomanian occurrence
Diversity of American characiforms from the Uberaba River basin
Scientific classification Edit this classification
Kingdom: Animalia
Phylum: Chordata
Class: Actinopterygii
(unranked): Otophysi
Order: Characiformes
Regan, 1911[1]
Type species
Charax gibbosus
Suborders

Characiformes /ˈkærəsɪfɔːrmiːz/ is an order of ray-finned fish, comprising the characins and their allies. Grouped in 18 recognized families, more than 2000 different species are described, including the well-known piranha and tetras.[2] Characins are most diverse in the Neotropics, where they are found in lakes and rivers throughout most of South and Central America. At least 209 species of characins are found in Africa, including the distichodontids, citharinids, alestids, and hepsetids. The rest of the characins originate from the Americas.[2]

A few characins become quite large, and are important as food or game.[2] Most, however, are small shoaling fish (or forage fish). Many species commonly called tetras are popular in aquaria because of their bright colors, general hardiness, and tolerance towards other fish in community tanks.[3][2]

Description

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Characins possess a Weberian apparatus, a series of bony parts connecting the swim bladder and inner ear.[2] Superficially, the Characiformes somewhat resemble their relatives of the order Cypriniformes, but may 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.[4]

The largest characins are Hydrocynus goliath (the goliath tigerfish of Alestidae),[5] Salminus brasiliensis or Salminus franciscanus (golden dourado of Bryconidae),[6][7][8] and Hoplias aimara (a traíra or wolffish of Erythrinidae),[9] all of which are over 1 m (3.3 ft) long. Many members are under 3 cm (1.2 in),[2] and the smallest in size is about 1.4–1.7 cm (0.55–0.67 in) in the Bolivian pygmy blue characin, Xenurobrycon polyancistrus.[10][11]

Taxonomy

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The Characiformes form part of a series called the Otophysi within the superorder Ostariophysi. The Otophysi contain three other orders, Cypriniformes, Siluriformes, and Gymnotiformes.[2] The Characiformes form a group known as the Characiphysi with the Siluriformes and Gymnotiformes.[12] The order Characiformes is the sister group to the orders Siluriformes and Gymnotiformes, though this has been debated in light of recent molecular evidence.[2]

Originally, the characiformes 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.[13] Currently, 18 families, about 270 genera, and at least 1674 species are known.[13]

Citharinus congicus is a member of the most basal characiform lineage
Hydrocynus goliath, from Africa, is one of the largest species in the order
Diversity of characiforms from the Munim River basin
Diversity of large-sized characiforms from the Paraná River basin

The suborder Citharinoidei, which contains the families Distichodontidae and Citharinidae, is considered the sister group to the rest of the characins, suborder Characoidei.[12] This group has a very ancient divergence from the rest of the Characiformes, dating back to the Early Cretaceous or earlier, and it has been suggested that it be better treated as its own order, the Cithariniformes.[1]: 101 

Betancur et al (2018) & Melo et al (2022) recognise some infraordinal and superfamilial divisions but these are not recognised by Eschmeyer's Catalog of Fishes,[14][15][16] which the following classification is based upon (2025 edition):

Evolution

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The Characiformes likely first originated and diversified on the supercontinent of West Gondwana (composed of modern Africa and South America) during the Cretaceous period, though fossils from this time are poorly known.[2] During the Cretaceous Period, the rift between South America and Africa (which would form the Atlantic Ocean) was forming, which may explain the contrast in characiform diversity between the two continents; their low diversity in Africa may explain why some primitive fish families (like the polypterids) and the Cypriniformes coexist with them whereas they are absent in South America, where these fish may have been driven extinct through competition.[12] The characiforms had not spread into Africa soon enough to also reach the land connection between Africa and Asia.[12] The earliest they could have spread into Central America was the late Miocene.[12]

The earliest characiform fossils date back to freshwater deposits from the Late Cretaceous, from the Turonian of Uzbekistan (Bissekty Formation) and the Santonian of Hungary.[17][18] Other fossil teeth date back to the Cenomanian of Morocco, but it has been suggested that these teeth may be of early ginglymodians.[1] Previously, the oldest characiform was assumed to be Santanichthys of the Early Cretaceous (Albian Age) of Brazil. This presumably marine taxon was used as evidence of characiformes potentially having marine origins.[19] However, more recent studies indicate that Santanaichthys is likely a basal otophysan rather than a characiform. Similarly, Salminops from Spain and Sorbinicharax from Italy, previously also considered potential marine characiforms, are now thought to have no characiform affinities and are considered indeterminate teleosts. Given this, there is no paleontological support for characiforms having marine origins.[20][21] Two other alleged Eocene European characids, Prohydrocyon Piton, 1938 and Procharacinus Piton, 1938 from France, lack a Weberian apparatus and are very likely not characins, and instead may be related to the enigmatic Thaumaturus.[22][23]

Eurocharax Gaudant, 1980 is known from a fully-articulated specimen from the Oligocene of France, and appears to represent an estuarine taxon, although its phylogenetic position needs revision.[24][22]

Fossil taxa

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Lignobrycon ligniticus, a fossil characiform from the Oligocene of Brazil

Uniquely, Late Cretaceous characiform fossils are found significantly north of their modern distribution. Indeterminate characiform teeth are known from the Santonian of Hungary and Maastrichtian of France, which have a large, multi-cusped appearance reminiscent of African alestids.[31] Similarly, two Campanian freshwater characiform genera, Primuluchara and Eotexachara, are known from North America, with Primuluchara having a very wide distribution across Laramidia, ranging from Texas to as far north as southern Canada (Dinosaur Park Formation). It is likely that the warmer conditions of the Late Cretaceous allowed early characins to range farther north than the present day, with African characins colonizing Europe and South American characins colonizing North America. Early characins may have had some level of salt tolerance, allowing for such colonizations to take place.[32] Within their modern distribution, a number of modern South American characin families have their earliest occurrences in the Maastrichtian of Bolivia, with isolated teeth and skeletal elements identifiable to Acestrorhynchidae, Characidae, and Serrasalmidae.[1] Characins appear to have inhabited Europe into the Paleogene, with fossil teeth reminiscent of Alestes known from the Early Eocene of Spain.[33]

Phylogeny

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Below is a phylogeny of living Characiformes based on Betancur-Rodriguez et al. 2017[34] and Nelson, Grande & Wilson 2016, with the African clades being marked with Af;[35]

Characiformes
Citharinoidei

Distichodontidae Günther 1864 Af

Citharinidae Günther 1864 Af

Characoidei
Crenuchales

Crenuchidae Günther 1864 sensu Froese & Pauly 2001

Erythrinales
Alestioidea

Hepsetidae Hubbs 1939 Af

Alestiidae Cockerell 1910 Af

Erythrinoidea

Tarumaniidae de Pinna et al. 2017

Erythrinidae Valenciennes 1847

Serrasalmoidea

Serrasalmidae Bleeker 1859

Cynodontidae Eigenmann 1903

Hemiodontidae Bleeker 1859

Anastomoidea

Parodontidae Eigenmann 1910

Prochilodontidae Eigenmann 1909

Chilodidae Eigenmann 1903

Curimatidae Gill 1858

Anostomidae Günther 1864 sensu Nelson 1994

Characales
Lebiasinoidea

Ctenoluciidae Schultz 1944

Lebiasinidae Gill 1889

Chalceoidea

Chalceidae Fowler 1958

Iguanodectoidea

Iguanodectidae Eigenmann 1909

Acestrorhynchidae Eigenmann 1912

Characoidea

Triportheidae Fowler 1940

Bryconidae Eigenmann 1912

Gasteropelecidae Bleeker 1859

Characidae Latreille 1825 sensu Buckup 1998

References

[edit]
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Characiformes

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Characiformes is an order of primarily freshwater ray-finned fishes within the superorder Ostariophysi, characterized by the presence of an adipose fin (typically rayless), well-developed teeth in multiple rows that are often multicuspid and replaceable, and a body usually covered in cycloid scales, with most species exhibiting carnivorous or omnivorous feeding habits.[1][2][3] This diverse order encompasses approximately 2,150 valid species distributed across 29 families and 298 genera, with the vast majority inhabiting tropical and subtropical freshwater ecosystems in the Neotropical region (South, Central, and southern North America), as well as Africa south of the Sahara, though a few species occur in brackish or coastal marine environments.[3][2][4] Recent phylogenetic studies have revised the classification, splitting the former Characidae into four families (Characidae, Stevardiidae, Spintherobolidae, and Acestrorhamphidae), which collectively account for a significant portion of the order's diversity, including popular aquarium species like tetras, while other notable groups encompass predatory forms such as piranhas (Serrasalminae subfamily) and herbivorous headstanders (Anostomidae).[2][3][5] Characiformes exhibit a wide range of morphologies and ecologies, from small, schooling species under 5 cm in length to larger forms exceeding 1 m, such as the South American tigerfish (Hydrolycus scomberoides), and they play crucial roles in their ecosystems as both predators and prey, contributing to the biodiversity of rivers like the Amazon and Congo.[1][2] Their evolutionary origins trace back to the Late Cretaceous, around 95-100 million years ago, with fossil records indicating early diversification in Gondwanan freshwater systems.[1][3][6] Reproduction varies, including external fertilization in most species, though some exhibit internal insemination or substrate spawning, and many are important in aquaculture and the ornamental fish trade due to their vibrant colors and adaptability.[2]

Taxonomy and classification

Families and genera

The order Characiformes derives its name from the Greek word charax, meaning "pointed stake," a reference to the sharp, pointed teeth typical of many species within the order.[7] Following a 2024 phylogenomic reclassification, Characiformes currently comprises 27 families, approximately 300 genera, and over 2,400 valid species (as of 2025), representing a highly diverse group of primarily freshwater fishes.[8] Approximately 2,125 species inhabit Neotropical freshwaters, while about 209 species are endemic to African rivers and lakes, reflecting the order's Gondwanan origins.[9] The former family Characidae, now split into four families—Spintherobolidae, Stevardiidae, Characidae sensu stricto, and Acestrorhamphidae—collectively account for roughly 53% of the order's diversity, with Stevardiidae and Acestrorhampidae being the most speciose among them.[8] Taxonomic classification within Characiformes has evolved through extensive revisions, particularly regarding the delineation of families based on integrated morphological and molecular data. A key example is the separation of African genera from the traditionally broad Characidae into the distinct family Alestidae, supported by phylogenetic analyses that highlight differences in dentition, fin structure, and genetic markers. This reclassification, formalized in studies from the early 2000s, underscores the polyphyletic nature of earlier groupings and has refined the understanding of characiform diversity. More recently, the 2024 study has further refined Neotropical characins by splitting the paraphyletic Characidae. The major families of Characiformes exhibit varied ecological roles and morphological specializations, with representative examples including:
FamilyApproximate Species CountKey Genera and Notes
Alestidae131Hydrocynus (African tigerfish, predatory forms); African endemic, separated from Characidae via morphological and molecular evidence.[3]
Anostomidae150Anostomus (lemon tetras); headstanders with specialized grazing mouthparts.
Serrasalmidae103Pygocentrus (piranhas), Colossoma (pacus); includes herbivorous and carnivorous species with strong jaws.[3]
Ctenoluciidae4Ctenolucius (pike characins); elongated, predatory forms resembling pikes.[3]
Curimatidae~160Prochilodus (toothless characins); detritivores with reduced dentition.[3]
Spintherobolidae6Spintherobolus; small, Neotropical forms; one of four families from former Characidae.[8]
Stevardiidae365Hyphessobrycon (tetras, small schooling fishes); includes former Stevardiinae; dominant in Neotropical streams and rivers.[8]
Characidae204Various small characins; Neotropical, sensu stricto after 2024 reclassification.[8]
Acestrorhamphidae697Moenkhausia and others; large clade of former Characidae species, Neotropical.[8]
Other notable families include Bryconidae (large migratory fishes like Brycon), Chilodontidae (headstanders), and Distichodontidae (African butterfly fishes), contributing to the order's ecological breadth across freshwater habitats.[3]

Phylogenetic relationships

Characiformes belongs to the superorder Ostariophysi, specifically within the cohort Otophysi, where it forms part of the Characiphysi clade alongside Siluriformes (catfishes) and Gymnotiformes (knifefishes).[10] This positioning is supported by both morphological and molecular evidence, with the Weberian apparatus—a modified anterior vertebral chain connecting the swim bladder to the inner ear for enhanced hearing—serving as a key synapomorphy for Otophysi, including Characiformes.[10][11] Molecular phylogenies, derived from analyses of mitochondrial genes (e.g., cytochrome b) and nuclear loci (e.g., RAG1, rhodopsin), consistently recover Characiformes as monophyletic in most recent studies, though earlier datasets suggested potential paraphyly.[10] A 2023 phylogenomic study using ultraconserved elements (UCEs) and whole-genome data reinforced this monophyly while resolving internal relationships, identifying basal clades such as Ctenoluciidae (pike-characins) and Erythrinidae (trahiras) as early-diverging lineages sister to more derived groups like the Alestidae and Characidae.[12] Derived Neotropical clades, including the speciose Characoidei, dominate the phylogeny, with strong support (bootstrap >95%) for their position as a monophyletic radiation encompassing families like Bryconidae and Serrasalmidae.[13] These analyses highlight a gradient from predatory basal forms (e.g., Ctenoluciidae) to diverse herbivorous and omnivorous derived taxa, underscoring adaptive radiations in South American freshwaters.[10] Debates persist regarding the monophyly of Characiformes, particularly involving African taxa traditionally included within it, such as Citharinidae and Distichodontidae.[14] Some phylogenomic studies propose elevating these African lineages to a separate order, Cithariniformes, based on multi-locus data showing them as sister to a Characiformes-Siluriformes-Gymnotiformes clade, with divergence dated to ~170 million years ago.[10] This reclassification is supported by morphological distinctions (e.g., unique dentition and body elongation) and molecular signals from UCEs, potentially resolving long-standing paraphyly hypotheses where Gymnotiformes nested within Characiformes.[13] A 2024 multi-locus phylogenomic analysis further clarified relationships by demonstrating paraphyly of the traditional Characidae, reclassifying it into four families: Spintherobolidae, Stevardiidae (including former Stevardiinae), Characidae sensu stricto, and Acestrorhamphidae, with high posterior probability (>0.99).[8] Phylogenetic inferences also integrate extinct groups, such as the sabre-tooth characins (Cynodontinae), whose fossil record from the Late Pliocene informs higher-level relationships.[15] Total-evidence analyses combining morphological characters (e.g., dentary and premaxillary teeth) with molecular data place Cynodontinae (e.g., Hydrolycus and Rhaphiodon) within derived Characoidei, as sister to extant predatory lineages, supporting monophyly of the subfamily with consistency indices >0.84.[15] These findings imply that Characiformes' evolutionary history involved early Neotropical diversification, with extinct forms highlighting predatory adaptations in ancient biodiversity hotspots like the proto-Amazonian basins, where modern diversity hotspots (e.g., Guiana Shield) likely originated from vicariant events.[10][15]

Evolutionary history

Origin and diversification

Characiformes originated in the Late Cretaceous, approximately 100–66 million years ago (mya), within West Gondwana, the continental precursor encompassing present-day South America and Africa.[16] Molecular clock analyses calibrated with fossils indicate that the crown group of Characiformes emerged around 94–108 mya in the Late Cretaceous, with initial diversification occurring prior to the full separation of these landmasses around 90 mya during the Turonian stage.[16][17] This early radiation is supported by time-scaled phylogenies that reconcile molecular and paleontological data, highlighting the order's Gondwanan roots as a foundational event in otophysan evolution. A 2025 phylogenomic analysis confirms the crown Characiphysi originated around 94 Ma in West Gondwana, supporting a Neotropical cradle for the group with vicariance driving African-Neotropical disjunction.[17] The opening of the South Atlantic Ocean drove vicariance, splitting the ancestral Characiformes lineage into distinct Neotropical and African clades around 90–100 mya.[16] The Neotropical clade, which encompasses approximately 90% of all characiform species, underwent extensive adaptive radiation, particularly in Amazonian floodplains following the Eocene epoch (56–33.9 mya). In contrast, the African clades, including the basal Citharinoidei, diversified more modestly, serving as a relict lineage that underscores the biogeographic disjunction.[16] Phylogenetic analyses confirm these splits, with the African Citharinoidei as sister to the predominantly Neotropical Characoidei. Diversification rates accelerated markedly during the Miocene (23–5.3 mya), driven by the uplift of the Andes and subsequent riverine fragmentation, which created isolated habitats conducive to speciation. Fossil-calibrated molecular clocks estimate net diversification rates of 0.1–0.5 lineages per million years during this period, reflecting high speciation coupled with reduced extinction in dynamic Neotropical ecosystems.[16] Earlier pulses in the Oligocene (~30 mya) further contributed to this pattern, coinciding with mega-wetland formation and climatic shifts that facilitated ecomorphological divergence in major families like Characidae and Serrasalmidae. Within the superorder Ostariophysi, Characiformes represent an early-diverging lineage, splitting from Cypriniformes around 118.8–97.2 mya in the Early Cretaceous, as part of the broader Characiphysi clade (including Siluriformes and Gymnotiformes). This divergence, aligned with the fragmentation of Pangea, positions Characiformes as a key model for understanding Gondwanan biogeography in freshwater fishes, illustrating how vicariance shaped the distribution and evolution of ancient teleost groups.[16]

Fossil record

The fossil record of Characiformes is relatively sparse, particularly prior to the Miocene, with most discoveries consisting of isolated teeth, vertebrae, and scales rather than complete skeletons, reflecting taphonomic biases in freshwater deposits. The earliest definitive records date to the Cenomanian stage of the Late Cretaceous (ca. 95 Ma), represented by the genus Lusitanichthys from Portugal, including L. characiformis, which exhibits primitive otophysan features such as an incipient Weberian apparatus suggestive of early characiform affinities. Indeterminate characiform remains, including 44 vertebral centra, occur in the mid-Turonian (ca. 90 Ma) Bissekty Formation of Uzbekistan, indicating a Gondwanan origin with dispersal into Laurasian freshwater systems during the Turonian thermal maximum.[18] Further Late Cretaceous evidence includes indeterminate multi-cusped teeth from the Santonian (ca. 85 Ma) Csehbánya Formation at Iharkút, Hungary, and a tetra-like fish from the Campanian (ca. 75 Ma) Dinosaur Park Formation in Canada, expanding the known northern extent of the group. Major fossil taxa highlight both extinct lineages and early divergences within the order. Some early Cretaceous forms, such as those initially placed in Ellimmichthyidae, were misclassified as characiforms but are now recognized as belonging to the separate order Ellimmichthyiformes based on cladistic analyses. In South America, Paleocene records from the El Molino Formation in Bolivia include characoid teeth attributable to early Tetragonopterinae, while Eocene deposits yield genera like Bryconetes from the Ituzaingó Formation in Argentina and Paleotetra from the Santa Rosa Formation in Peru, demonstrating post-Cretaceous diversification in Andean basins. African Paleogene fossils, such as isolated teeth from the Eocene of Egypt and Tanzania, suggest parallel radiations on the continent, though fewer articulated specimens are known. North American Paleogene sites, including Eocene deposits in Montana, preserve characiform teeth and vertebrae, indicating a broader Holarctic distribution before modern restriction to tropical realms. Key fossil localities underscore the Gondwanan roots and vicariant history of Characiformes. The Eocene La Meseta Formation on Seymour Island, Antarctica, contains indeterminate teleost remains with potential characiform affinities among its diverse otophysan assemblage, supporting a southern high-latitude presence during warmer Paleogene climates. European Maastrichtian sites in southern France yield additional teeth from the Aix-en-Provence Basin, while South American Cenozoic lagerstätten like the Miocene Solimões Formation in Brazil provide more complete skeletons of basal taxa.[19] Despite these discoveries, significant gaps persist in the pre-Miocene record, with fewer than 50 described fossil species worldwide—only about 25 known exclusively from fossils—compared to over 2,300 extant species, implying substantial undersampling of early diversity and evolutionary transitions. This scarcity limits precise reconstructions of diversification rates but highlights the order's resilience through the Cretaceous-Paleogene extinction, with post-Paleocene fossils revealing increased morphological disparity tied to Neotropical river system development.

Physical description

Anatomy and morphology

Characiformes are ray-finned fishes (Actinopterygii) characterized by an elongate body plan, typically covered in scales, with a homocercal caudal fin and a single dorsal fin supported by soft rays.[20] As members of the Otophysi within the superorder Ostariophysi, they possess a Weberian apparatus consisting of modified anterior vertebrae and ossicles that connect the swim bladder to the inner ear, enhancing sound detection and auditory sensitivity.[21] The dorsal fin usually has 9-12 rays, while the anal fin is short to moderately long with fewer than 45 rays, and the caudal fin features 19 principal rays; the pelvic fin is abdominal with 5-12 rays.[20] Diagnostic morphological features of Characiformes include well-ossified teeth on the jaws and pharyngeal bones, often exhibiting a characidin pattern with multicuspid or conical forms adapted for carnivory in most species.[22] An adipose fin, a fleshy, non-rayed structure, is present in most species posterior to the dorsal fin, though absent in some like Gymnocharacinus bergi.[20] Scales are predominantly cycloid, though ctenoid or ctenoid-like scales occur in various taxa, covering the body except in rare scaleless exceptions.[23] The upper jaw is not protractile, and branchiostegal rays number 3-5, with typically three postcleithra and a first hypural separated from the centrum by a gap in adults.[20] Internal variations include accessory air-breathing organs in certain families, such as the highly vascularized swim bladder in Erythrinidae (trahiras), which facilitates atmospheric respiration in low-oxygen environments.[24] Sensory structures feature a lateral line system that is often decurved and sometimes incomplete, aiding in mechanoreception for navigation and prey detection.[20] In breeding males of many species, nuptial tubercles—epidermal protuberances—develop on the head, body, fins, and scales, serving reproductive functions.[25]

Size, coloration, and adaptations

Characiformes display a broad spectrum of body sizes, reflecting their diverse ecological niches. The smallest species, such as the pygmy characin Xenurobrycon polyancistrus, reach a maximum length of approximately 1.4 cm, while larger predatory forms like the giant tigerfish Hydrocynus goliath can grow to 1.5 m in length and weigh up to 50 kg.[26][22][27] Most characins, including common aquarium species like tetras, attain lengths of 5–20 cm, enabling their roles in mid-water schooling and foraging.[22] Coloration in Characiformes varies from silvery and cryptic to vibrant and iridescent, often linked to habitat and behavior. Many tetra species exhibit metallic sheen with bold patterns, such as the neon blue and red stripes of Paracheirodon innesi, which enhance visibility in clear waters for schooling and mate attraction.[28] In contrast, piranhas (Serrasalmidae) typically show subdued silver bodies with dark spots or bars for camouflage in vegetated rivers.[22] Sexual dichromatism is prevalent in at least six families, with males often displaying brighter hues, such as intensified yellows or reds, during breeding periods to signal readiness.[29] Adaptations in Characiformes include specialized visual systems and protective structures tailored to freshwater environments. Visual pigments, particularly rhodopsin (RH1) variants, exhibit spectral tuning with λmax values of 502–536 nm, shifting sensitivity toward red and green wavelengths via amino acid substitutions (e.g., at sites 261 and 292) to match the dim, tannin-stained light spectra of tropical rivers.[30] Unlike their gymnotiform relatives, which possess electric organs, Characiformes lack such structures but show precursors in myogenic tissues.[22] Predatory species like piranhas feature reinforced scale armor and an abdominal keel for defense against conspecific aggression, with thick, interlocking scales providing puncture resistance. Developmental morphology follows a typical teleost pattern, with larvae emerging with prominent yolk sacs for initial nutrition and undergoing metamorphosis marked by fin ray formation and scale development around 5–10 mm standard length.[31] This process supports rapid growth and transition to active swimming, aligning with the order's fusiform body plan for efficient locomotion in flowing waters.[32]

Distribution and habitat

Geographic distribution

Characiformes are predominantly distributed across freshwater systems in the Neotropical region, encompassing South America from the Amazon, Orinoco, and Paraná River basins southward to the La Plata system, as well as Central America extending northward into southern North America as far as Texas.[22][6] In Africa, the order is represented in tropical river systems including the Congo, Nile, and Zambezi basins, where it accounts for approximately 200 species across four families, comprising about 10% of the total Characiformes diversity.[9][22] Over 90% of the more than 2,300 extant species are endemic to the Neotropics, with 20 families restricted to this region, highlighting endemism hotspots in isolated basins such as the Amazon and Orinoco.[3] Fossil evidence indicates a broader historical range for Characiformes, suggesting an ancestral Gondwanan distribution that extended beyond current limits due to vicariance following continental drift.[6] Paleogene records from Europe, including Lower Eocene deposits in the Paris Basin and Messel Formation in Germany, document extirpated populations, while Eocene fossils from North America's Green River Formation further attest to a former Laurasian presence.[33][34] Late Cretaceous fossils from North America, such as those in Canada and Utah, reflect warmer climatic conditions that allowed northward expansion, with no post-Eocene records indicating subsequent extirpation.[35] Some Characiformes species, particularly small tetras like those in the genus Hyphessobrycon, have been introduced to Southeast Asia and Australia through the international aquarium trade, where they are commonly farmed or kept in captivity, though established wild populations remain limited. The African clade diverged from Neotropical lineages after the Gondwana split around 100 million years ago, contributing to the current disjunct distribution.[6]

Habitat preferences

Characiformes species inhabit exclusively freshwater ecosystems across tropical and subtropical regions, primarily in rivers, streams, lakes, and floodplains of the Neotropics and Africa, with only rare tolerances to brackish conditions observed in certain species such as Colossoma macropomum in estuarine areas.[36][37] These fish avoid marine environments entirely, thriving instead in continental systems where they form a dominant component of the ichthyofauna.[1] Preferred water parameters include temperatures of 23–30°C, reflecting their adaptation to warm tropical climates, and pH ranges from 5.0 to 8.0, accommodating acidic blackwater rivers to more neutral whitewater systems.[36][38] Many species, particularly in oxygen-poor habitats like stagnant floodplain pools, exhibit physiological tolerances to low dissolved oxygen levels (as low as 2–5 mg/L), with some Serrasalmus piranhas capable of supplemental air gulping to supplement gill respiration.[39][40] Microhabitat preferences vary widely within the order, enabling niche partitioning; small tetras (e.g., Hyphessobrycon spp.) often shoal in open-water columns of slow-flowing rivers and lakes, while herbivorous pacus (e.g., Piaractus spp.) favor vegetated shallows rich in aquatic plants for foraging and cover.[39] Rheophilic species from families like Anostomidae and Crenuchidae preferentially occupy rapids and high-velocity streams with rocky or gravel substrates, exploiting fast currents for feeding on drifting invertebrates.[41] In contrast, benthic or detritivorous forms seek muddy or sandy bottoms in lentic habitats.[42] Seasonal adaptations are pronounced in floodplain-associated species, which undertake migrations during high-water periods to exploit inundated forests and meadows for feeding and growth, often returning to main river channels as waters recede.[43] During droughts, many seek refugia in persistent riverbed pools or deep lakes, where reduced connectivity concentrates populations and influences community structure.[44]

Ecology and behavior

Feeding and diet

Characiformes display a broad dietary spectrum, encompassing carnivory, insectivory, herbivory, frugivory, and detritivory across their diverse families. Many species, particularly in Characidae and Cynodontidae, are predominantly carnivorous or insectivorous, preying on small fish, invertebrates, and aquatic insects. For instance, piranhas in the family Serrasalmidae, such as Pygocentrus nattereri, specialize in feeding on fish fins, scales, and flesh, using their sharp, interlocking teeth to shear tissue from prey.[45][22] In contrast, herbivorous and frugivorous diets prevail in genera like Colossoma within Serrasalmidae, where species such as C. macropomum consume fruits, seeds, nuts, and aquatic vegetation, contributing to seed dispersal and nutrient cycling in floodplain ecosystems. Detritivory is common in families like Curimatidae and Prochilodontidae, where species ingest organic detritus, algae, and microorganisms from river bottoms, facilitating nutrient recycling in sediment-rich habitats.[46][22] Foraging behaviors in Characiformes vary with dietary specialization and habitat. Predatory species often employ schooling attacks, as seen in piranha groups that coordinate to overwhelm prey in shallow waters, enhancing capture success through collective action. Smaller characids, such as tetras in the genus Hemigrammus, exhibit opportunistic feeding, opportunistically consuming drifting insects, algae, and zooplankton while schooling in streams and pools. Plant-eating species, including pacus and anostomids, utilize specialized pharyngeal jaws for grinding tough vegetative material, allowing efficient processing of fibrous fruits and leaves that fall into rivers. These behaviors are supported by morphological adaptations, such as multicuspid teeth in omnivores or dagger-like canines in predators, which facilitate prey capture and handling.[47][48][22] Trophically, Characiformes occupy roles from apex predators to mid-level consumers and ecosystem engineers. Large predators like Hydrolycus spp. in Cynodontidae, reaching up to 1 m in length, act as top carnivores in Neotropical rivers, preying on fish and crustaceans with their sabre-like fangs. Mid-level consumers, such as many characid tetras, link primary producers and higher predators through insectivory and omnivory. Frugivorous species play key roles in nutrient cycling by dispersing seeds via defecation, promoting forest regeneration along riverbanks.[15][22] Many Characiformes undergo ontogenetic diet shifts, transitioning from planktivory in juveniles to more specialized adult diets. Early larvae and juveniles often feed on zooplankton and small invertebrates, as observed in species like Astyanax lacustris and Roeboides affinis, before shifting to larger prey such as fish or scales as body size increases and predatory capabilities develop. This progression reduces intraspecific competition and aligns with morphological changes in jaw and tooth structure.[49][50]

Reproduction and life cycle

Characiformes predominantly utilize external fertilization, releasing eggs and sperm into the aquatic environment during spawning events. Courtship behaviors in many species, particularly tetras of the family Characidae, involve dynamic displays such as rapid color changes and fin flaring to attract mates, as observed in Hyphessobrycon eques where males and females exhibit shifts to darker green hues during pre-spawning chases. While most lack post-fertilization care, some exhibit paternal or biparental guarding; for instance, in the erythrinid Hoplias malabaricus, males construct nests in flooded depressions and guard adhesive eggs (mean clutch size 8,197 ± 2,204) for up to six days, with pairs occasionally sharing duties.[2][51][52] Spawning in Characiformes is often seasonal, synchronized with flood pulses in tropical freshwater systems to enhance larval survival and dispersal, though some smaller species spawn year-round. Eggs are typically demersal and adhesive, attached to substrates like vegetation or nests, or pelagic in open water; clutch sizes vary widely by body size, ranging from 94–500 oocytes in diminutive forms to 5,000–10,000 or more in larger characids, reflecting trade-offs between fecundity and egg investment. In invasive populations, such as certain characids, high fecundity combined with multiple spawning bouts (e.g., fractional oocyte release) facilitates rapid population expansion.[53][54][55] The life cycle of Characiformes features rapid growth and early maturity, with many reaching sexual maturity at 6–12 months and sizes of 13–25 mm standard length, as seen in Pyrrhulina australis (L50 = 23.4 mm for females). Lifespans generally span 2–10 years, supporting multiple reproductive cycles in iteroparous species. Larvae hatch at 2.5–3 mm total length with yolk sacs, displaying species-specific pigmentation for camouflage, such as speckled patterns; yolk resorption and mouth opening occur within 3 days post-hatch, followed by metamorphosis to juveniles around 1–2 cm, marking the transition to active foraging.[56][51]

Conservation and relationship to humans

Human uses

Characiformes species play a significant role in the global aquarium trade, particularly small, colorful tetras harvested from the Amazon basin. The neon tetra (Paracheirodon innesi) is one of the most popular, with approximately 12-15 million individuals exported annually from Brazil as of the early 2010s, representing about 80% of the ornamental fish market in the Amazon state.[57] Wild collection in Brazil, primarily from the Rio Negro basin near Manaus, supports this industry but poses sustainability challenges due to overharvesting pressures on natural populations.[58] Initiatives like Project Piaba promote eco-friendly practices to ensure long-term viability of these fisheries.[59] In commercial fisheries, larger Characiformes provide important food sources, especially in South America. The tambaqui (Colossoma macropomum) is a key species in Amazonian aquaculture, with production reaching approximately 157,000 tons as of 2023, driven by its fast growth and high market demand.[60][61] Sport fishing targets predatory species like the dorado (Salminus brasiliensis), prized for its acrobatic fights and size, attracting anglers to rivers in Brazil and Argentina.[62] Other human uses include employing small Characiformes as bait in regional fisheries.[26] Piranhas (Serrasalmus spp.) hold cultural significance in Amazonian folklore, symbolizing both danger and resourcefulness, as highlighted in historical accounts like Theodore Roosevelt's expeditions.[63] In biomedical research, the Mexican tetra (Astyanax mexicanus) serves as a model organism for studying evolution, development, and traits like blindness in cave populations, akin to zebrafish but within Characiformes.[64] Escaped or released aquarium specimens have led to reports of Characiformes outside their native range, such as the black pacu (Colossoma macropomum) in Florida waterways since the 1960s, though populations are not established.[65]

Conservation status

Characiformes face significant threats from habitat loss, primarily driven by deforestation in the Amazon basin, where approximately 20% of forest cover has been lost since the 1970s, leading to riparian vegetation degradation and altered riverine ecosystems essential for these fishes.[66][67] Overfishing exacerbates population declines, particularly for commercially valuable species, while pollution from agricultural and urban effluents contaminates freshwater habitats.[68] Invasive species introduce competition and predation pressures, and hydropower dams fragment migration routes, especially for African endemics in rivers like the Congo.[69][70] According to the IUCN Red List, around 15% of assessed fish species globally, including many Characiformes, are threatened with extinction; a January 2025 assessment indicates that 24% of the world's freshwater fish species, including many Characiformes, are at high risk of extinction, with Neotropical representatives showing elevated risks due to these pressures.[68][71] For instance, Brycon orbignyanus, endemic to the Paraná River basin, is classified as Endangered owing to overexploitation and habitat alteration.[72] A substantial portion of species remain Data Deficient, reflecting inadequate monitoring and taxonomic uncertainties that hinder effective conservation.[70] African Characiformes, such as those in the Congo Basin, are particularly vulnerable to dam-induced fragmentation, with limited assessments underscoring regional biases in data collection.[73] Conservation efforts include the establishment of protected areas like Manu National Park in Peru, which safeguards diverse Characiformes habitats within a UNESCO World Heritage site, preserving over 80% of Peru's freshwater fish diversity dominated by this order.[74][75] Aquaculture programs for Colossoma macropomum (tambaqui) have expanded in the Amazon, reducing fishing pressure on wild stocks by providing sustainable alternatives for local communities.[76] Some rare species fall under CITES Appendix II listings to regulate international trade, though implementation varies by fishery management frameworks.[77] Recent initiatives, including 2025 DNA barcoding projects, aim to enhance traceability in ornamental fish trade, identifying mislabeled shipments and supporting enforcement against illegal harvesting.[78] Knowledge gaps persist, particularly in understudied African Characiformes diversity, where assessments are biased toward economically prioritized regions and overlook endemic species in remote basins.[79] Climate change poses additional uncertainties, with rising temperatures projected to disrupt migration patterns and range shifts in Neotropical and African populations, though empirical data on these impacts remain sparse.[80]

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