any of about8,000 species of bony fishes (bony fish) belonging to a group that includes the majority of freshwater fishes throughout the world. Familiar representatives of this group are the minnows (minnow), suckers (sucker), carps (carp), piranhas (piranha), electric eels (electric eel), and innumerable catfishes (catfish). Humans consume huge quantities of ostariophysans for food. Some of these fishes are also popular in tropical aquariums. A few harmful species can inflict painful injuries; some others serve as intermediate hosts for parasites of humans. Many of these fishes exhibit strange and fascinating behaviour such as nest building, oral incubation, egg laying in mollusk shells, walking and flying, air breathing, production of sound and electricity, and communication by chemical secretions.

      The largest order in superorder Ostariophysi is Siluriformes, containing the 35 recognized families of catfishes. The remaining 33 families in Ostariophysi are distributed among the orders Cypriniformes (minnows, carps, suckers, and other fishes), Characiformes (characins (characin), hatchetfishes (hatchetfish), pencil fishes (pencil fish), and others), Gymnotiformes (electric eels and other fishes), and Gonorynchiformes (the milkfish, beaked sandfishes, and others). These orders are divided into two groups, or series, based on the presence or absence of the Weberian apparatus, a bony connection between the swim bladder and the inner ear that enhances the perception of sound. Series Anotophysi, containing only the order Gonorynchiformes, does not possess a true Weberian apparatus. Series Otophysi is made up of the four remaining orders, the members of which possess a true Weberian apparatus.

General features

Size range and diversity of structure
 Most ostariophysans are small to moderate in size, from 2 to 30 cm (about 1 to 12 inches) long. Others rank among the giants of the freshwater world. The elegant mahseer (Cyprinidae) of Asia grows to 2 metres (about 7 feet) long and weighs 90 kg (200 pounds), and the wels, a Eurasian catfish (Siluridae), attains a length of 4.5 metres (15 feet) and a weight of 300 kg (660 pounds). The extent of morphological diversity is at least as great as that in any other group of living vertebrates.

 Ostariophysans abound in nearly all freshwater habitats, including subterranean caverns and those on all major landmasses and continental islands of the world, except for Greenland and Antarctica. A few live in brackish waters, and two families consist largely of marine species. Approximately 8,000 species are recognized, nearly one-fourth of all known species of bony fishes (bony fish). Their undisputed success may be attributed at least in part to two remarkable features: a sense of hearing more acute than that in any other group of fish and a warning system by chemical communication unique among fishes.

      Many moderate to large ostariophysans are utilized for food, and commercial fisheries harvest huge quantities of marketable species. The common carp (Cyprinus carpio), native to China, has been introduced nearly worldwide and is extensively cultured in the warmer regions. Other Chinese carp under cultivation include the grass carp (Ctenopharyngodon), silver carp (Hypothalmichthys), snail carp (Mylopharyngodon), and bighead carp (Aristichthys). Culture of the channel catfish (Ictalurus punctatus) is an important industry in the southern United States. Numerous ostariophysans provide sport fishers with recreation and food; several, such as the mahseers (several species of Tor) of Asia and the dorado (Salminus maxillosus) of South America, rank among the world's prized game fish.

      Among the most popular aquarium species are the characins (characin), tetras (tetra), rasboras (rasbora), danios (danio), barbs (barb), loaches (loach), and catfishes. Adaptability of many ostariophysans to aquarium life has resulted in their widespread use as experimental animals in scientific research. Foremost among these are the goldfish (Carassius auratus), the common carp, and the zebra fish (danio) (Danio rerio).

      In eastern Asia and parts of Europe, humans frequently become infected with liver flukes acquired by eating raw or imperfectly cooked fish. The carps, especially Ctenopharyngodon idellus, are the second intermediate host of the Chinese liver fluke (Clonorchis sinensis). Many cyprinids serve as intermediate hosts for the cat liver fluke (Opisthorchis felineus). Domestic animals similarly become infected with flukes and tapeworms.

      Some ostariophysans are pests or are potentially dangerous to people. The common carp is a nuisance in many localities in the United States. Introduced species such as the walking catfish (Clarias batrachus) pose a serious threat to the native fauna. In South America, on occasion, the piranha (Serrasalmus) voraciously attacks humans and domestic animals, and the diminutive candiru (Vandellia cirrhosa) can penetrate the urogenital openings of human bathers and cause intense pain and hemorrhaging.

Natural history

Reproductive cycle
 Like most fishes (fish), ostariophysans are either male or female throughout life; they do not change sex. Eggs develop in the ovaries of the female and spermatozoa (milt) in the testes of the male. In temperate zones most species breed in the spring, when water temperatures are rising and day lengths (photoperiods) are increasing. In tropical regions many fishes spawn throughout the year. All ostariophysans lay eggs; none gives birth to living young. Eggs are fertilized externally in the water in all species except auchenipterid catfishes (catfish) and the South American characins (characin) of the subfamily Glandulocaudinae, in which insemination is internal. Development is direct; newly hatched individuals do not pass through morphological changes (metamorphosis) but instead are miniature replicas of their parents. The age at sexual maturity depends on the species and relative body size. Individuals of many small species reproduce when only a few months old and rarely live more than one to a few years. Large species attain sexual maturity when several years old, and in captivity the common carp has been known to live more than 50 years.

      Distinct pairing occurs in most ostariophysans, and courtship behaviour in characiforms and cypriniforms often consists of elaborate displays by males in brilliant nuptial coloration. The eggs are heavier than water (demersal) and sink; most are sticky and adhere to the surface or to various objects. Characins and cyprinids generally deposit their eggs among aquatic plants, under stones and logs, or in shallow pits in gravel and sand. Among the many exceptions is the characin (Copeina arnoldi); the female actually leaps out of the water to lay her eggs on the undersides of overhanging leaves (or, in captivity, of aquarium covers), to which she clings, joined by the male, during egg deposition. The parents then splash water on the fertilized eggs during development. The female bitterling (Rhodeus sericeus) deposits its eggs in the gill cavity of freshwater mussels (mussel) by means of an elongated ovipositor, which she inserts into the mussel's incurrent siphon. Catfishes choose breeding sites in streams and ponds, generally in quiet water among plants or on mud, sand, gravel, or debris. The nest may be a simple circular depression (as in bullheads (bullhead) [Ameiurus]) or a tunnel-like affair in the bank (as in the channel catfish [Ictalurus punctatus]). Migrations comparable to those of salmon and eels (eel) are unknown among the ostariophysans, but the tendency to migrate occurs among suckers (sucker) (Catostomidae), which swarm upstream into small tributaries and spawn over gravel or sand bottoms, and in other riverine species such as the mahseer (Tor) and the African tigerfish (tigerfish) (Hydrocynus).

Parental care
      Although many species exhibit no parental care, nest building and egg guarding are widespread among ostariophysans. Some cyprinids, such as the chubs (chub) (Nocomis), build massive pyramidal nests of stones; they desert the nests once spawning is completed. Other species of breeding minnows (minnow) often swarm over these nests, and the mixing of eggs and sperm from different species frequently produces hybrids. The eggs of characins are commonly guarded by the male. Catfishes provide their eggs with considerable protection, either by guarding nests or by carrying the eggs with them. Oral incubation is practiced in sea catfishes (Ariidae); the male carries from 10 to 50 marble-sized eggs in the mouth cavity until hatching. The male continues to protect the hatchlings in his mouth even after the young have begun to feed independently. In certain species of banjo catfishes (Aspredinidae), the eggs are anchored to spongy tentacles on the underside of the female's abdomen. Some female callichthyid catfish carry eggs on the abdomen only for fertilization; others deposit their adhesive eggs in froth nests and guard them. The loricariid catfishes employ various methods; some lay adhesive eggs in cavities, others carry them under the lower lip, and a few deposit them on rocks, where they are cleaned, fanned, and guarded by the male.

      Ostariophysans with bright colours and gaudy patterns are popular among tropical fish fanciers; however, many other small species are somberly coloured, relying on this protective coloration for passive defense from enemies. Large carnivorous forms such as the African tigerfish and the South American piranhas (piranha) have powerful jaws and strong teeth, extremely effective weapons for defense as well as for offense. Most catfishes and some Old World cyprinids possess spines (hardened fin rays) in the dorsal and pectoral fins. The spines alone afford a considerable degree of protection; in addition, venom glands develop at the base of the spines in some bullheads and madtoms (madtom) of North America (Ictaluridae (ictalurid)), labyrinthic catfishes (Clariidae), and sea catfishes (Ariidae and Plotosidae). Painful but rarely fatal injuries result when the skin of a human victim is punctured and venom injected.

      Although a variety of freshwater fishes can generate an electrical (bioelectricity) charge, only two develop sufficient voltage to stun other animals, including humans—the electric eel (Electrophorus electricus) and the electric catfish (Malapterurus electricus).

Habitat and distribution
      Ostariophysans are the dominant fishes—by number and species—in virtually all types of freshwater habitats throughout the tropical, temperate, and subarctic regions of the world. Only a few species of the families Cyprinidae and Aspredinidae are known to inhabit low-saline or brackish waters. The only truly marine members of this superorder are the sea catfishes (Ariidae and Plotosidae) and the gonorynchiforms, which largely inhabit tropical and subtropical coasts. Some plotosids, however, live in fresh water.

      The upper regions of small mountain streams are characterized by steep gradients, waterfalls and rapids, and torrential currents. There live a variety of ostariophysans (Balitoridae, Sisoridae, Akysidae, Loricariidae, Astroblepidae), which exhibit fascinating structural adaptations, such as holdfast organs and specialized respiratory mechanisms. In river systems where the gradients are not steep, currents are slow, and quiet pools alternate with riffles, large numbers of characins, cyprinids, and suckers and other types of catfish are conspicuous elements of the fauna. In the sluggish waters of large rivers live large species of suckers, cyprinids (such as carp), and many catfishes generally characterized by environmental tolerances and nonrestrictive feeding habits. Ponds and lakes also support large populations of characins, cyprinids, catostomids, and siluriforms that prefer and are adapted to standing-water habitats. Although a few are benthic (bottom-dwelling) forms, most of the characins and cyprinids tend to live and feed in the middle and upper layers of the water column. Suckers, loaches (loach), and most catfishes are typically benthic animals and thus are highly adapted to such an existence. Catfishes are generally most active at night or under conditions of reduced light intensities.

      Among the most unusual habitats for fishes are those in subterranean waters, wells, and caves. A relatively large number of ostariophysans, belonging to unrelated families, present a striking example of convergent adaptation to life in more or less total darkness. The evolutionary trends have led to a reduction or loss of eyes, loss of pigment, and special development of certain sense organs, especially the lateral line system, to compensate for the loss of sight. Ostariophysans adapted to such a mode of life include six genera of cyprinids in Africa, the Middle East, and Java; a characin (Astyanax jordani) in Mexico; ictalurids (Trogloglanis and Satan) in the United States and Mexico (Prietella); six genera of pimelodids and trichomycterids in South America; and two genera of clariids in Africa.

Feeding habits
      The remarkable diversity of feeding habits among ostariophysans is associated with a fantastic variety of adaptations in mouth shapes and tooth types (especially in the order Characiformes), probably unsurpassed by any other group. At one extreme are certain cyprinids (such as Notropis atherinoides) with highly developed gill rakers that strain phytoplankton (minute plants) from the water. Mountain stream fishes (such as Gyrinocheilidae, Balitoridae, and Loricariidae) possess suckerlike lips for scraping algae from the rocks; their teeth are minute or entirely lacking. Because they devour large quantities of plants, herbivores such as the Chinese grass carp are used experimentally to control vegetation in weed-choked waters. Omnivores are especially common among the characins and catfishes. Suckers, long-snouted knifefishes (knifefish), many catfishes, and some minnows suck up mud and bottom debris, extract the nutriments, and eject the residue. Small carnivorous species consume insect larvae, small crustaceans, worms, mollusks, and other invertebrates. At the top of the food chain are the voracious predators, the most famous of which are the piranhas (piranha). Although modest in size, they have short, powerful jaws armed with razor-sharp teeth. These fearsome predators often occur in large schools and can quickly strip the flesh from their victims. Other fishes are their usual prey, but cattle and occasionally humans are also attacked. Probably the largest predatory ostariophysan is the tigerfish (Hydrocynus vittatus), which attains a weight exceeding 45 kg (approximately 100 pounds); its huge, sharp teeth and large, tunalike tail endow it with ferocity and speed. Parasitic habits are rarely found among bony fishes, but certain species of trichomycterid catfishes attach themselves to the gills of other fishes and feed on their hosts' blood.

Form and function

Distinguishing characteristics
Weberian apparatus and swim bladder
      The single character unique to the series Otophysi is the presence of the Weberian apparatus, a complex connection between the inner ear and the swim bladder. It is formed by the modification of the first four (or five) vertebrae immediately behind the skull, small portions of which have become separated and form a chain of four paired bones, or ossicles, named (from front to back) the claustrum, scaphium, intercalarium, and tripus. The first is in contact with a membranous window, or extension of the inner ear; the last touches the anterior wall of the swim bladder. The diverse modifications of the Weberian apparatus are diagnostic of orders and certain families. For example, the claustrum is absent in Gymnotidae. Although much remains to be learned about its functions, it is known to serve as a hearing organ. Changes in volume of the swim bladder due to sound waves in the water cause the ossicles to move and transmit pressure changes to the ear.

      The swim bladder varies in shape and size but typically consists of two, sometimes three, chambers. In bottom-dwelling fishes (fish) such as the Balitoridae, Cobitidae, and many catfishes (catfish), the posterior chamber is greatly reduced and the anterior one often more or less surrounded by a bony capsule. In some catfishes (Sisoridae), only the anterior chamber is present, and it may be encapsulated with bone. Gonorynchiforms, members of the series Anotophysi, have a type of rudimentary Weberian apparatus involving the first three vertebrae and one or more ribs.

Body covering
      The body covering is variable. Most cypriniforms and characiforms possess cycloid scales (scale) (smooth, overlapping scales more or less circular in shape). Exceptions are found among the Ctenoluciidae, Distichodontidae, and Citharinidae, which have ciliate, or ctenoid, scales (posterior margins of scales with fine teeth). Most catfishes have lost the scaly covering and are naked, but several families possess bony plates forming overlapping armour on the sides of the body (Doradidae, Callichthyidae, Loricariidae).

Fin spines and adipose fin
      Ostariophysans possess segmented, branched, flexible, soft rays in the fins, unlike the stiff spines of perchlike fishes. In some species, however, soft ray elements may fuse during development and give rise to a spinous ray (usually called a spine), commonly found in the dorsal and pectoral fins of most catfishes and in the dorsal and anal fins of some Old World cyprinids. The presence or absence of these spines may be diagnostic for genera and families.

      An adipose fin consists of a small to elongated fleshy or fatty structure without fin ray supports, located dorsally between the rayed dorsal fin and caudal (tail) fin. It is present in most ostariophysan fishes.

      Diverse morphological differences in the mouth region are related to the type of diet and to the modes of locating, capturing, and ingesting food. Barbels are short to filamentous, fleshy, fingerlike projections located at the corners of the mouth or on the snout and chin of many suctorial and bottom-feeding fishes (some minnows (minnow), loaches (loach), and catfishes). Barbels are highly sensitive to touch, and they bear numerous taste buds. Taste and touch probably function together in the selection of food before ingestion.

      Teeth may be present along the jaws, in the roof of the mouth, on the tongue, or in the pharynx, or they may be entirely absent. In the minnows (Cyprinidae) and suckers (sucker) (Catostomidae), the mouth is toothless, but an array of teeth is borne on a pair of branchial bones, the lower pharyngeals, located in the throat. In the minnows the pharyngeal teeth, arranged in one, two, or three rows, press or bite against a horny pad in the roof of the mouth. They have undergone specialization paralleling the diversity found in jaw teeth of other fishes. Vegetarians such as the carp have grinding, molarlike teeth; carnivores have pointed or hooked teeth. Suckers have numerous pharyngeal teeth aligned in a single row. Oral and pharyngeal teeth are of great value in classifying many families of ostariophysans.

Secondary sexual characteristics
      With the onset of the breeding season, many secondary sexual characteristics develop: size differences, nuptial coloration, enlarged and modified fins, breeding tubercles, and contact organs. These features are related chiefly to courtship and mating, but differences in size obviously play a role in guarding nests and care of the young; the sex that exercises parental care is usually the larger. Brilliant red, orange, yellow, green, and blue coloration may develop on various parts of the head, body, and fins, especially in the males. Some characins and cyprinids are among the most beautiful of all fishes. The male usually has larger and more brightly coloured fins than the female. In some characins (characin), the median and pelvic fins of the males may possess small hooks or contact organs, which aid in maintaining contact with the female during spawning. In the cypriniforms, breeding tubercles, or pearl organs (epidermal excrescences), develop on the head, body, and fins of males under the influence of sex hormones. The tubercles function in maintenance of body contact during spawning, in defense of nests and territories, and possibly in the stimulation of females during breeding.

      Sexual differences among the siluriforms are more marked in the highly specialized families. Pelvic fins of female ariid catfishes and, to a lesser extent, of ictalurid catfishes show specialized developments whose functions are not yet fully known. Some male loricariid catfishes develop elaborate dermal, branching growths and spines around the head; in others, the lower lip is enlarged to accommodate the transport of eggs.

Adaptations for locomotion
      The body of most ostariophysan fishes is more or less streamlined, taking the most efficient form for movement through water. In this highly diversified group, however, a large array of adaptations occurs. Lateral compression (flattened from side to side) is common, especially among characins and cyprinids that inhabit quiet, weedy lakes, ponds, and backwaters. Extreme examples are the flying hatchetfishes (hatchetfish) (Gasteropelecidae) and the knifefishes (knifefish) (Rhamphichthyidae and Apteronotidae). Depressed body form (flattened from top to bottom), especially in the head region, is widespread among fishes spending much time on or near the bottom or under rocks and similar objects (most catfishes) or among those inhabiting torrential mountain streams (Balitoridae, some Loricariidae). An elongated eel-like form has evolved in certain loaches (Cobitidae) and electric eels (electric eel) (Gymnotidae), fishes that live on soft, muddy, and sandy bottoms or in rock crevices.

      The common form of locomotion among ostariophysans is swimming by lateral undulations of the body, resulting from the contractions of muscles along the sides of the body and base of the tail. These undulating flexures culminate in a powerful back and forth sweeping of the caudal fin, which produces as much as 85 percent of the total thrust. Some fishes have departed from the normal horizontal swimming posture. The headstanders (headstander) (Anostomidae) move with the head pointing downward at a slant; some of the pencil fishes (pencil fish) (Hemiodontidae) assume a tail-standing position. Most bizarre of all are the upside-down catfishes (Mochokidae) of Africa, which can swim either in the normal position or inverted, with the belly uppermost; in one species, Synodontis batensoda, the coloration of the belly is darker than the back, a reversal of the usual pigmentation pattern. Displacement of the swim bladder toward the underside is a further adaptation to this unusual swimming behaviour.

      In fishes with specialized modifications of body form and habits, the fins are frequently modified and used for propulsion. The electric eels and knifefishes (Gymnotiformes) have lost the dorsal fin and, in some cases, the caudal fin. Slow forward and backward movements are made possible by undulations of an extremely long anal fin.

      Associated with locomotion is the need for maintaining position in the water, particularly in the rapid torrents of mountain streams. A variety of modifications have evolved that function as holdfasts, anchoring the fish to rocks or similar objects. The hill stream loaches (Balitoridae) of southeastern Asia possess a large ventral suction disk formed by the expanded pectoral and pelvic fins. Some of the mountain stream catfishes (Sisoridae) of Asia have an adhesive organ on the thorax (chest). Mountain-inhabiting catfishes of South America may use a suckerlike mouth (Loricariidae) or employ a combination of a disklike mouth and disklike paired fins (Astroblepidae) for adhesion to the surface.

Walking and flying
      A few ostariophysans have the capability to emerge from their aquatic abode and move over land, climb walls, or even glide or fly through the air. The walking catfish (Clarias batrachus), an exotic species in southern Florida, uses its pectoral fin spines as anchors to prevent jackknifing as its body musculature produces snakelike movements and can progress remarkable distances over dry land. Using suction disks and fins, the mountain stream catfishes (Sisoridae and Astroblepidae) can climb vertical rock walls above the water surface.

      The small hatchetfishes, or flying characins (Gasteropelecidae), of South America normally swim near the surface of the water but are capable of jumping clear and flying short distances. They vibrate enlarged pectoral fins rapidly back and forth by using highly specialized musculature on the shoulder girdle.

      Although gills (gill) are typical respiratory structures in fishes, many freshwater species occupy habitats where the oxygen may be depleted occasionally or where droughts may force them to live out of water temporarily. These fishes have evolved a variety of air-breathing organs, most of which are outgrowths or pouches from the pharynx, branchial (gill) chamber, or digestive tube. Some catfishes (Clarias and Heterobranchus) of Asia and Africa have branched respiratory structures extending above the gill chambers; others (Heteropneustes) have elongated, tubular, lunglike sacs extending backward as far as the caudal fin (tail). The electric eel is a mouth breather; gaseous exchange takes place through the wrinkled mucous membrane lining the mouth cavity. Some fishes actually swallow air into the lower part of the digestive tract, which then also serves as a respiratory structure. In the armoured catfishes (Doras, Plecostomus, Callichthys) of South America, the thin-walled stomach serves this function. The loaches swallow air into a reservoir-like bulge from the intestine and void the remaining gases through the anus.

Communication and sensory perception
      Sounds produced by ostariophysans are usually associated with the swim bladder. Minnows produce noises by expelling air through the pneumatic duct, which connects the swim bladder with the digestive tract, and the mouth; loaches do the same by expulsion through the anus. In several catfish families the expanded ends of a springlike mechanism (derived from modified portions of the fourth vertebra) are attached to the swim bladder. The contraction of muscles extending from the spring mechanism to the skull cause the springs and bladder wall to vibrate rapidly, producing a growling or humming noise. In other catfishes the rubbing or grating movements of the dorsal and pectoral spines produce sounds.

      The sense of hearing (sound reception) in the Otophysi is more highly developed than in any other fishes. The walls of the swim bladder are set in vibration by waves of underwater sound, and the Weberian ossicles then increase the amplitude of these vibrations, transmitting them to the internal ears. This combination is analogous to that of a hydrophone and endows these fishes with a remarkable sensitivity to sound. The normal frequency range detectable by otophysans is from 16 to 7,000 hertz (cycles per second); for some characins the maximum is 10,000 hertz. (For comparison, the frequency range of human hearing extends from about 20 hertz to about 20,000 hertz.)

      Among other functions, sound production and hearing in fishes may assist in bringing schooling fishes together; even more significant is the role of sound in reproduction. Experiments with North American cyprinids provide evidence that sounds are produced by both sexes and may serve for sexual recognition. A male is able to distinguish the calls of females of his own species from those of closely related species. Consequently, sounds may serve as isolating mechanisms in maintaining the genetic integrity of the species. For fishes living in muddy waters, sounds may be a vital communication link between individuals, especially in the breeding season. The combination of sound production and acute hearing is correlated with the dominant role of otophysans in fresh waters.

      Members of the order Gymnotiformes and of the siluriform family Malapteruridae possess the unusual capacity to generate electricity. The best known and most powerful of this group is the electric eel (Electrophorus electricus). The electrical organs, three on each side of the body, are derived from modified muscle tissue. The force of the discharge has been measured at 350 to 650 volts and can produce a current strong enough to stun animals as large as a horse or a human. The electric catfish (Malapterurus electricus) can deliver shocks up to 450 volts, but this power is apparently used only as a defensive measure. The electrical organ of this species, also derived from muscle tissue, consists of a specialized gelatinous coat of tissue that sheathes most of the body just under the skin.

      Other gymnotid eels and knifefishes (knifefish) (Gymnotus and other genera) produce currents of low voltage only, emitting a continuous series of pulses (from 35 to 1,700 hertz), which create an electrical field around the fish. When this field is broken, either by a moving animal or by inanimate objects in the vicinity, the fish can locate animals or objects, which otherwise would be difficult to see at night or in muddy water. Experiments indicate that electrical cues may also facilitate social interactions. Perception of electrical stimuli occurs in specialized electrical receptors in the skin, and portions of the brain are enlarged to process electrosensory information.

      Catfishes and other fishes living in muddy waters have relatively poor vision but possess chemosensory acuity. Lips, barbels, and most of the body are covered with innumerable taste buds. Experiments have proved that taste plays a leading role in the location of food by these fishes.

      Studies on the sense of smell have isolated odours emanating from mucus produced in the skin, from secretions of the gonads, and from other body parts. These odours, chemical signals called pheromones (pheromone), provide a means of communication between individuals of the same or different species. Certain minnows (Cyprinidae) can discriminate between the odours of at least 15 species of fishes belonging to eight different families. The social behaviour of bullheads (bullhead) (Ictalurus) and other ostariophysans is related to a system of communications using chemical signals. An individual not only recognizes individuals of other species but can identify and remember the identification of a particular individual of its own species after a time lapse of three weeks. Territorial and communal behaviour are evidently influenced by different pheromones.

Alarm substances
      In 1938 Austrian biologist Karl von Frisch (Frisch, Karl von) introduced an injured minnow (Phoxinus) into a school of the same species and observed that the school rapidly retreated and appeared very frightened. By experimentation he demonstrated that a chemical substance released from the lacerated skin produced a fright reaction when perceived through the nasal organs of other fishes. This “alarm substance,” secreted by specialized cells in the epidermis, is released only when the skin is injured. Alarm substances are present in almost all species of ostariophysans tested (except for a few species of Characidae, Hemiodontidae, Chilodontidae, and Rhamphichthyidae) and are absent in all non-ostariophysan fishes examined. Although the fright reaction appears to be important insurance for the individual against predation, the alarm substances are of greatest value among those species exhibiting social behaviour by warning other members of the school. Alarm substances and the fright reaction have contributed markedly to the biological success of the Ostariophysi.


Distinguishing taxonomic features
      Many characteristics are useful in classifying this large, diverse superorder—the nature of the body covering; presence or absence of barbels, fin spines, and adipose fin; modifications of mouth and fins; types of teeth. Less obvious but especially significant are numerous skull features, specializations of the Weberian apparatus in otophysans, configuration of the swim bladder, and fusions of vertebral elements.

Annotated classification
      This classification largely follows the work of American ichthyologists S.V. and W.L. Fink and R.P. Vari, Canadian ichthyologist J.S. Nelson, and Brazilian ichthyologist P. Buckup; it also includes the Gonorynchiformes as primitive ostariophysans. The smallest families are grouped for brevity or are included under a closely related family. Species numbers are given for representative families.

Superorder Ostariophysi
 Anterior vertebrae specialized and associated with anterior ribs, basisphenoid absent, orbitosphenoid present. 2 series, 5 orders, several dozen families, more than 1,000 genera, and about 8,000 species.
      Series Anotophysi
 Rudimentary Weberian apparatus involving the first 3 vertebrae and 1 or more ribs. Marine, brackish and freshwater. 1 order.

      Order Gonorynchiformes
 Primitive Weberian apparatus based on the first 3 vertebrae and 1 or more ribs, mouth small, jaws toothless, epibranchial organ present. 4 families, 7 genera, and about 37 species.

      Suborder Chanoidei

      Family Chanidae ( milkfish)
 Marine and brackish water (occasionally freshwater), tropical and subtropical Indian and Pacific. 1 species, Chanos chanos.

      Suborder Gonorynchoidei

      Family Gonorynchidae (beaked sandfishes)
 Marine, Indo-Pacific, rare in southern Atlantic. 1 genus with 5 species.

      Suborder Knerioidei

      Family Kneriidae
 Freshwater, tropical Africa and Nile River. 4 genera with 30 species.

      Family Phractolaemidae (snake mudhead)
 Freshwater, tropical Africa. 1 species, Phractolaemus ansorgii.

      Series Otophysi
 Swim bladder and internal ear connected by chain of ossicles (Weberian apparatus). All forms inhabit fresh water unless otherwise noted.

      Order Characiformes
 Mouth not protractile; jaws toothed. Characidae most generalized; other families have specialized skeletal structures, jaws, and teeth. 18 families with about 270 genera and 1,674 species. Cretaceous (about 136 million years ago) to present.

      Family Characidae (characins (characin))
 Tremendous morphological and ecological diversity. Many brilliantly coloured. Variable food habits. Many, including tetras and piranhas, are popular aquarium and food fishes. Size 2.5–150 cm (1–60 inches). Fresh to brackish waters; Africa, South and Central America. About 165 genera and more than 962 species.

      Family Erythrinidae (trahiras)
 Large mouths, canine teeth. Adipose fin; absent. Carnivorous. Food fishes. Size to 1.2 metres (4 feet). South America. 3 genera, 14 species.

      Family Ctenoluciidae (pike-characids)
 Elongate, pikelike body. Large mouth, canine teeth, scales ciliated, carnivorous, food fishes. Panama and South America. To 67.5 cm (27 inches) or more. 2 genera, 7 species.

      Family Cynodontidae (cynodontids)
 Large mouth, large canine teeth, long anal fin. Carnivorous, food fishes that inhabit South America. To about 65 cm (26 inches). 5 genera, 14 species.

      Family Acestrorhynchidae (acestrorhynchids)
 Elongate, pikelike. South America. 1 genus, 15 species.

      Family Crenuchidae (South American darters)
 Small, most less than 10 cm (4 inches). Panama and South America. 12 genera, 74 species.

      Family Alestiidae (African tetras)
 Africa. About 18 genera, 110 species.

      Family Hepsetidae (African pikes)
 Pikelike; large canine teeth; carnivorous. Food fishes. Size to 100 cm (40 inches), 55 kg (120 pounds). Africa. 1 species (Hepsetus odoe).

      Family Lebiasinidae (pencil fishes)
 Lateral line and adipose fin usually absent. Small to moderate-sized predators. South and Central America. 7 genera, 61 species.

      Family Gasteropelecidae (hatchetfishes (hatchetfish))
 Deep, strongly compressed body; pectoral fins with well-developed musculature. Capable of true flight. Insectivorous. Aquarium fishes. Size to 10 cm (4 inches). South and Central America. 3 genera, 9 species.

      Family Anostomidae (headstanders (headstander))
 Elongated snout; small mouth with folded or fleshy lips or sucking disk. Head-standing habits. Herbivorous. Aquarium and food fishes. Size to 40 cm (16 inches). South America. 12 genera, at least 137 species.

      Family Prochilodontidae (flannel-mouth characiforms)
 Predorsal spine, rough scales. South America. 3 genera, about 21 species.

      Family Curimatidae (toothless characiforms)
 Toothless jaws. Costa Rica to northern Argentina.

      Family Chilodontidae (headstanders)
 Specialized pharyngeal teeth. South America. 2 genera, about 8 species.

      Family Hemiodontidae (hemiodontid pencil fishes (pencil fish))
 Lower jaw toothless. Tail-standing posture. Herbivorous. Aquarium fishes. Size to 20 cm (8 inches). Family Parodontidae is similar. South and Central America. 5 genera, about 28 species.

      Family Parodontidae (parodontids)
 Panama and South America. 3 genera, about 21 species.

      Family Distichodontidae (distichodontids)
 Ctenoid (ciliate) scales. Africa. 17 genera, about 90 species.

      Family Citharinidae (citharinids)
 Deep-bodied, scales often denticulate (toothed), small mouth and teeth. Herbivorous. Aquarium and food fishes. Size to 0.9 metre (about 3 feet). 3 genera, 8 species.

      Order Gymnotiformes
 Body elongated; anal fin very long; electric organs present. 5 families, 30 genera and about 134 species. No fossil record.

      Family Gymnotidae (nakedback knifefishes)
 Carnivorous group that includes electric eels. Body eel-like and scaleless with powerful electric organs. Size to 2.75 metres (about 9 feet), weight to 22 kg (48 pounds). Mexico, Central and South America. 2 genera, 33 species.

      Family Rhamphichthyidae
 Body greatly compressed, scaled. Elephant-like snout, herbivorous, weak electrical powers. Size to 0.9 metre (about 3 feet). South and Central America. 3 genera, 12 species.

      Family Hypopomidae (bluntnose knifefishes)
 Teeth absent on oral jaws. Panama and South America. 7 genera, 16 species.

      Family Sternopygidae (glass knifefishes)
 Panama and South America. 5 genera, about 28 species.

      Family Apteronotidae (ghost knifefishes)
 Panama and South America. 13 genera, about 45 species.

      Order Cypriniformes
 Mouth toothless, protractile. Adipose fin rarely present. 6 families, about 3,270 species. Paleocene (about 65 million years ago) to present.

      Family Cyprinidae (minnows (minnow), goldfish, bitterlings (bitterling), barbs (barb), and carps (carp))
 Pharyngeal teeth in 1 to 3 rows. Some with 1 or 2 pairs of small barbels. Food habits variable. Food fishes of sport and commercial value; aquarium fishes. Size 2.5–250 cm (1 inch to more than 8 feet). Most in fresh but some in brackish water; Asia, Europe, Africa, North America. About 220 genera, 2,420 species.

      Family Catostomidae (suckers (sucker))
 Protractile, sucking mouth on underside of head. Detritus feeders. Food fishes. Size to 0.9 metre (about 3 feet). North America, Asia. 13 genera, 72 species.

      Family Gyrinocheilidae (algae eaters)
 Adaptations to fast currents include fleshy, suctorial mouth and inhalant-exhalant gill openings. Algae feeders. Size to 30 cm (12 inches). Inhabits mountain streams of Southeast Asia. 1 genus, 3 species.

      Family Psilorhynchidae (mountain carps)
 Size to about 8 cm (3.3 inches). Inhabits mountain streams in Asia. 2 genera, 6 species.

      Family Balitoridae (hill-stream loaches)
 Ventral sucking disk formed by paired fins. Freshwater, Eurasia. About 59 genera, 590 species.

      Family Cobitidae (loaches (loach))
 Wormlike; scales minute or absent; barbels 3–6 pairs. Intestine sometimes modified for aerial respiration. Mostly carnivorous. Aquarium fishes. Size to 30 cm (12 inches). Asia, Europe, Africa. About 26 genera, 177 species.

      Order Siluriformes (catfishes (catfish))
 Body naked or covered with bony plates; adipose fin usually present; pectoral and dorsal fins often with spines. Mostly omnivorous. About 2,500 species. Paleocene (some 65 million years ago) to present.

      Family Diplomystidae (velvet catfishes)
 1 pair of barbels; primitive Weberian apparatus. Size to 24 cm (about 9 inches). South America. 2 genera, about 6 species.

      Family Ictaluridae (ictalurid) (bullheads (bullhead), channel catfish, madtoms (madtom))
 Barbels 4 pairs; some with venom glands. Valuable food fishes (sport and commercial). Size to 1.7 metres (about 6 feet), 50 kg (110 pounds). Few enter brackish water. North America; widely introduced. 7 genera, approximately 50 species.

      Family Bagridae (bagrid catfishes)
 Similar to Ictaluridae but with elongated adipose fin. Food, aquarium fishes. Size to 0.9 metres (about 3 feet). Asia and Africa. About 18 genera, 170 species.

      Family Siluridae ( wels and glass catfishes)
 Body compressed; adipose fin lacking, anal fin very long; short dorsal fin (often lacking) without spine. Food; aquarium fishes. Size to 4 metres (about 13 feet), 300 kg (660 pounds). Asia, Europe, Africa. At least 11 genera, 97 species.

      Family Schilbeidae (schilbeid catfishes)
 Similar to Siluridae, but with adipose fin usually present and spine in dorsal fin. Food fishes. Size to 2.3 metres (about 8 feet), 110 kg (240 pounds). Asia and Africa. About 25 genera, 56 species.

      Families Amblycipitidae (torrent catfishes)
 Mountain streams of southern and eastern Asia. 3 genera, about 26 species.

      Families Akysidae (stream catfishes)
 Tuberculated skin. Mountain streams of Southeast Asia. 4 genera, at least 42 species.

      Family Amphiliidae (loach catfishes)
 Similar to Bagridae, but paired fins expanding horizontally for adhesion in fast currents. Size to 21 cm (about 8 inches). Africa. 12 genera, 66 species.

      Family Sisoridae (mountain-stream catfishes)
 Ventral surface flat; thorax with longitudinal plates or adhesive organ. Size to 30 cm (12 inches). Asia. 17 genera, at least 112 species.

      Family Clariidae (air-breathing catfishes)
 Long dorsal and anal fins without spines; adipose fin usually lacking. Treelike air-breathing organ. Food fishes. Size to 130 cm (51 inches). About 14 genera, about 90 species. The similar family Heteropneustidae has long, hollow air sacs. Asia, Africa; widely introduced elsewhere.

      Family Heteropneustidae (airsac catfishes)
 Pakistan to Thailand. 1 genus, 3 species.

      Family Cranoglanididae (armourhead catfishes)
 Large rivers. Asia. 1 genus, 3 species.

      Family Auchenoglanididae (auchenoglanidids)
 Africa. 6 genera, about 28 species.

      Family Austroglanidae (austroglanids)
 Southern Africa. 1 genus, 3 species.

      Family Erethistidae (erethistid catfishes)
 Southern Asia. 6 genera, 14 species.

      Family Pangasiidae (shark catfishes)
 Maximum length about 3 metres (about 10 feet). Southern Asia. 3 genera, 28 species.

      Family Chacidae (squarehead catfishes)
 Head broad, long, depressed, mouth terminal, wide. Eastern India to Borneo. 1 genus, 3 species.

      Family Malapteruridae (electric catfishes (electric catfish))
 Rayed dorsal fin lacking; spines lacking. Electric organs. Food fishes. Size to 1.2 metres (about 4 feet), 23 kg (50 pounds). Africa. 2 genera, 19 species.

      Family Mochokidae (upside-down catfishes)
 Bony shield on head and nape. Some swim upside-down. Food fishes. Size to 60 cm (24 inches). Africa. 11 genera, 179 species.

      Family Ariidae (sea catfishes)
 Nasal barbels lacking; oral incubation of eggs. Food fishes. Marine, a few entering fresh water. Tropical coasts, worldwide. About 21 genera, about 150 species.

      Family Plotosidae (eeltail catfishes)
 Lack adipose fin; long anal and caudal fins confluent. Marine, brackish and freshwater, Indo-Pacific. 10 genera, about 35 species.

      Family Doradidae (thorny catfishes)
 Overlapping plates cover sides of body. Intestinal modifications for aerial respiration. Aquarium fishes. Generally small, to more than 1 metre (3 feet). South America. About 30 genera, about 72 species.

      Family Auchenipteridae (driftwood catfishes)
 Internal insemination. Fresh and brackish water, Panama and South America. 20 genera, about 94 species.

      Family Heptapteridae (heptapterids)
 Superficially similar to Pimelodidae. Mexico to South America. About 25 genera, 175 species.

      Family Pseudopimelodidae (bumblebee catfishes)
 Wide mouth, small eyes. South America. 5 genera, 26 species.

      Family Aspredinidae (banjo catfishes)
 Adipose lacking; broad, flat head; large tubercles on naked body. Aquarium fishes. Size to 30 cm (12 inches). A few enter brackish waters and salt waters. South America. 12 genera, 36 species.

      Family Pimelodidae (long-whiskered catfishes)
 Similar to Bagridae but lack nasal barbels. Food, aquarium fishes. Size to 1.3 metres (about 4 feet), 65 kg (145 pounds). South and Central America. About 31 genera, at least 85 species.

      Family Trichomycteridae (candirus (candiru) and other parasitic catfishes)
 Operculum (gill cover) usually with spines. Many parasitic. Size to 10 cm (4 inches). The similar family Cetopsidae lacks opercular spines. Costa Rica, Panama, and South America. About 41 genera, 201 species.

      Family Nematogenyidae (mountain catfishes)
 Closely related to trichomycterids. Chile. 1 species (Nematogenys inermis).

      Family Cetopsidae (whalelike catfishes)
 Body naked, lacking bony plates. South America. 7 genera, 23 species.

      Family Callichthyidae (callichthyid armoured catfishes)
 2 longitudinal series of overlapping bony plates. Herbivorous aquarium fishes. South and Central America. 8 genera, about 177 species.

      Family Loricariidae (suckermouth armoured catfishes)
 Sucking mouth; 3 or 4 rows of bony scutes. Herbivorous aquarium fishes. Central and South America. About 42 genera, 230 species.

      Family Scoloplacidae (spiny dwarf catfishes)
 Body with 2 bilateral series of teethlike-bearing plates, 1 midventral series of plates. Maximum length about 20 mm (less than 1 inch). South America. 1 genus, 4 species.

      Family Astroblepidae (climbing catfishes)
 Mouth and fins modified for adhesion to rocks in mountain streams. Skin naked. Panama and South America. 1 genus, up to 54 species.

      Family Claroteidae (claroteids)
 Africa. 7 genera, up to 59 species.

Critical appraisal
      Ostariophysans are relatively primitive bony fishes. Based on molecular and morphological information, they are now understood to be closely related to clupeiforms, and both groups have been classified together in the clade Otocephala. Relationships among the five ostariophysan orders have been in flux, and some authorities have demonstrated the close connection between the knifefishes (Gymnotiformes) and the catfishes (Siluriformes) and classify the two orders as one. Species limits and relationships among genera and families of all five ostariophysan orders have been studied intensively by a vast array of ichthyologists, making the ostariophysans one of the most well-known teleost groups.

Ralph W. Yerger Lynne R. Parenti

Additional Reading
James W. Atz, Dean Bibliography of Fishes 1968 (1971), the first volume of a comprehensive, computerized bibliographic series; George Albert Boulenger, Catalogue of the Fresh-Water Fishes of Africa, 4 vol. (1909–16, reprinted 4 vol. in 2, 1964), an important, illustrated, systematic account of Old World tropical groups; Philip J. Darlington, Jr., Zoogeography: The Geographical Distribution of Animals (1957, reprinted 1982), an excellent account of the distribution of freshwater fishes; Carl H. Eigenmann and George S. Myers, The American Characidae, 5 pt. (1917–29), a classic treatise, only one-third completed upon the death of the author; M.M. Ellis, “The Gymnotid Eels of Tropical America,” Mem. Carneg. Mus., 6:109–204 (1914), a comprehensive, systematic, and morphological study; William K. Gregory and G. Miles Conrad, “The Phylogeny of the Characin Fishes,” Zoologica, 23:319–360 (1938), an old, somewhat equivocal, but important contribution on classification; Harry Grundfest, “Electric Fishes,” Scient. Am., 203:115–124 (1960), a semipopular but authoritative article; William T. Innes, Exotic Aquarium Fishes, 20th ed. (1979), a well-illustrated, informative handbook of popular aquarium fishes; John G. Lundberg and Jonathan N. Baskin, “The Caudal Skeleton of the Catfishes, Order Siluriformes,” Am. Mus. Novit. 2398 (1969), a description of anatomy, evolution, and relationships; W. Pfeiffer, “Alarm Substances,” Experientia, 19:113–123 (1963), an excellent review article; C.T. Regan, “The Classification of the Teleostean Fishes of the Order Ostariophysi,” Ann. Mag. Nat. Hist., Series 8, 8:13–32, 553–577 (1911), an old, but historically useful treatise on morphology and classification; John H. Todd, “The Chemical Languages of Fishes,” Scient. Am., 244:99–108 (1971), a description of contemporary experiments on communication; Stanley H. Weitzman, “The Osteology of Brycon meeki, a Generalized Characid Fish, with an Osteological Definition of the Family,” Stanford Ichthyol. Bull., 8:1–77 (1962), an important review of characid classification and osteology; Jasper S. Lee, Commercial Catfish Farming, 3rd ed. (1991).

* * *

Universalium. 2010.

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