any member of the superorder Paracanthopterygii, a predatory, primarily marine group that forms one of about six major branches of the Teleostei (teleost), or bony fishes, the dominant modern aquatic vertebrates. Approximately 1,160 living species of paracanthopterygian fishes have been described. They range in length from just a few centimetres to roughly two metres (more than six feet). Well-known forms include the anglerfish and the cod.

In general body form there is considerable diversity, but ichthyologists have classed the Paracanthopterygii as a discrete group, largely on the basis of a distinctive musculature of the jaws, the structure of the caudal (i.e., at the tail end) vertebrae, and the placement of the pelvic fins (they are usually in the midbody region or even farther toward the head).

The Paracanthopterygii comprises six orders: Batrachoidiformes, or toadfishes (toadfish), about 45 species; Gadiformes, or codfishes (cod), about 800 species; Gobiesociformes, or clingfishes, about 100 species; Lophiiformes, or anglerfishes (anglerfish), about 210 species; Percopsiformes, or trout-perches (trout-perch), about eight species; and Polymixiiformes, or beardfishes (beardfish), three species. Most of the orders are primarily marine, with worldwide distribution; the percopsiforms, however, occur only in fresh waters of North America. Batrachoidiforms and gobiesociforms occur mainly in tropical and temperate shallow water along continental coasts and to a limited extent in fresh water. Gadiforms are represented by both shallow-water and deep-sea types. The most widely known gadiforms are the commercially important species and the only economically important paracanthopterygians: the true cods (Gadus), hakes (hake) (Merluccius, Urophycis), haddocks (haddock) (Melanogrammus), pollocks (pollock) (Pollachius), and whitings (whiting) (Merlangius). All are abundant in waters of the continental shelf of the North Atlantic, where they have been commercially fished for centuries from both Europe and North America. Lophiiforms live in shallow waters of tropical reefs as well as in the ocean depths. Polymixiiforms occur at moderate depths in most warm seas, generally near continents.

The largest of the Paracanthopterygii are the codfishes, which grow to about two metres in length and attain weights that may exceed 90 kilograms (about 200 pounds). Certain goosefishes (goosefish) (Lophiiformes) reach a length of about two metres and a body weight of 35 kilograms (about 75 pounds); other lophiiforms are as small as 2¹/₂ centimetres (about one inch) long. Batrachoidiforms grow to about 30 centimetres (one foot) in length, gobiesociforms to about eight centimetres (three inches). The largest percopsiforms are about 15 centimetres (six inches) long. Polymixiiforms reach no more than 30 centimetres in length.

Eggs of the oyster toadfish (Opsanus tau) of the western Atlantic—one of the most carefully studied batrachiforms—are laid in dark recesses of all sorts, including sunken tin cans and shoes. The male guards the eggs and young for about three weeks, after which the young fishes begin life on their own. The fish gets its name from the fact that some have been found living in living oysters. Luminous organs known as photophores (photophore), numbering several hundred and set in long horizontal rows, are believed to be sexual attractants in the midshipman (Porichthys)—so named because the organs resemble rows of bright buttons on a naval uniform. The northern midshipman (P. notatus), a common species on the eastern Pacific coast, spawns in shallow water, attaching its eggs to a rocky surface. The male guards the eggs. Like other batrachoidiforms, the midshipman lives and grows on the ocean bottom.

Most species of codfishes (which comprise some 70 species of Gadiformes) migrate over long distances. They gather in late winter and early spring to spawn, each species going to a particular area. The periodic movements are closely related to seasonal variations in water temperature. Fecundity of some codfish species is prodigious. The European ling (Molva molva) may deposit as many as 60,000,000 eggs each season. The eggs and larvae of most species are found in the plankton (i.e., the aquatic organisms, collectively, suspended in the sea). Weeks or months elapse before the eggs hatch. Young codfishes are commonly found in very shallow water, but they move into deeper water as they become older. The eggs of grenadiers (family Macrouridae), a bottom-feeding group of cods, are believed to be laid near the bottom; the buoyant eggs rise part way to the surface. The larvae are known mainly from below 100 fathoms (about 180 metres, or 600 feet); older larvae occur at greater depths. In the Mediterranean pearlfish (Carapus acus), a member of another codlike group (family Carapidae), clumps of eggs, released by the female in late summer, appear at the surface and hatch into a specialized larva, the vexillifer, which lives amid the plankton. After attaining a length of about seven to eight centimetres (about three inches), it transforms to another larval stage, the tenuis, descends to the bottom, and becomes a parasite in a sea cucumber (Holothuria tubulosa or Stichopus regalis). The tenuis, apparently dependent upon its host for survival, undergoes a further transformation to the juvenile stage; in the process, its length decreases from 20 to 10 centimetres (eight to four inches). The Mediterranean pearlfish is believed to pass most of its life in the host. Very little is known of the general biology of reproductive habits of the brotulas (brotula) and cusk eels (cusk eel) (family Ophidiidae), also of the cod group. They are both oviparous (egg-laying) and viviparous (live-bearing). The males of some viviparous species produce spermatophores (sperm cases). The European eelpout (Zoarces viviparus) of the cod family Zoarcidae bears living young about five centimetres in length and numbering as many as 400. Fertilization is internal, and embryonic development occurs in the ovary of the female. Other eelpouts are believed to be live-bearing, but the ocean pout (Macrozoarces americanus) of the western Atlantic lays eggs that are guarded by one or both parents.

The lophiiforms are primarily bottom fishes as adults, but many produce floating rafts of eggs. The eggs of the deep-sea anglerfishes (suborder Ceratioidei) are unknown; but it is believed that they float to the surface; the larvae occur in surface waters, gradually descending to deeper waters as they grow older. The females of the deep-sea anglers are from three to 13 times as large as the males. Females have an illicium, or “fishing pole,” which is a modified spine of the dorsal, or back, fin that has moved forward onto the top of the head. At the tip of the illicium is a fleshy enlargement, the esca, used to lure prey within range of capture. (The illicium and esca are generally present also in male anglerfish other than in the Ceratioidei.) Commonly the esca is luminous; the female also has other light-producing organs. In 1922 a specimen of the anglerfish Ceratias holboelli was discovered; small specimens attached to its abdomen were thought to be its young. A few years later similar finds led to the discovery that the smaller fish were really mature males living parasitically on the female. Further investigation showed that the males, soon after their transformation from the larval state, bite onto an older, larger female, after which the female and male tissues unite; the separate circulatory systems join; and the male becomes a permanent appendage of the female.

Little is known of the reproductive habits of the gobiesociforms, percopsiforms, or polymixiiforms. Gobiesociforms are known to lay eggs in shallow water, attaching them to rocks or plants; and percopsiforms are known to spawn in the spring of the year in shallow water.

All batrachoidiforms are bottom dwellers. True toadfishes (Batrachoidinae, about 25 species) occur in shallow or moderate depths along continental coasts; some ascend rivers. The oyster toadfish lives under rocks or amid debris, awaiting prey of almost any type, which is taken with a sudden snap. Venomous toadfishes (Thalassophryninae, about nine species) are restricted to the coasts and rivers of Central and South America. Because of their sluggish habits, these fishes are sometimes stepped on by man and can inflict painful wounds. Midshipmen (Porichthyinae, about 12 species), which are restricted to tropical and temperate coasts of the Americas, are unusual in being shallow-water fishes with photophores, a feature generally found in deepwater forms. Most midshipmen occur in depths of less than 50 fathoms (one fathom = six feet), and all are found in water shallower than 200 fathoms.

Gadiform fishes of the family Gadidae (about 70 species) are all marine species, except for the burbot (Lota lota); some, however, ascend rivers with the tides. Bottom dwellers, they occur on the continental shelves from shallow water to about 200 fathoms and, although distributed throughout the oceans, are most numerous in the eastern North Atlantic. Deep-sea cods (Moridae, about 70 species) are cold-water bottom fishes, living at greater depths along the continental slopes. Grenadiers (Macrouridae, about 300 species), typically bottom fishes, live along the continental slopes at depths of 100 to 1,000 fathoms. Few species are cosmopolitan in distribution, but the group as a whole is widely distributed in tropical and temperate latitudes. A few species of gadiforms (Muraenolepididae, four species) are confined to Antarctic seas, and, like the cods, they are bottom fishes, living at moderate depths. The pearlfishes (pearlfish) (Carapidae, about 27 species) are marine, mainly tropical, shallow-water, eellike fishes adapted to living inside the body of various invertebrates. They have been collected from a variety of hosts, including tunicates, oysters, and sea cucumbers. Brotulas (brotula) and cusk eels (Ophidiidae, about 250 species) are mainly bottom dwellers. Some are shallow-water species with nocturnal habits, but the group as a whole is the dominant teleostean family at depths greater than 2,000 fathoms. Some have been taken at depths of about 4,000 fathoms, the greatest depth at which any form of fish life is known. Eelpouts (Zoarcidae, about 80 species) are bottom fishes, commonly occurring from shallow water to depths of 1,000 fathoms. Species are most abundant in the higher latitudes of both hemispheres, especially in the north. Eelpouts are common in shallow water of Arctic and Antarctic seas.

Gobiesociforms (about 100 species) are mostly marine fishes, typically inhabiting the intertidal zone. Some species (Diademichthys) hide among the spines of sea urchins. In tropical America, four species (Gobiesox) are known from swift-flowing freshwater streams.

Of the lophiiforms, the ceratioids, or deep-sea anglerfishes, are the only abyssal (deep-sea) forms. They occur primarily at depths of 1,000 to 3,000 fathoms. Unlike other lophiiforms, they are midwater fishes, are uniformly black, and have no pelvic fins. They are apparently feeble swimmers, depending primarily on their light organs to attract prey. Some (Melanocetus, Linophryne) are known to swallow fishes several times their own length, accommodating them in a highly distensible stomach. Crustaceans and other invertebrates are also eaten. Many lophiiforms—so-called frogfishes (Antennariidae, about 60 species)—are shallow-water forms, commonly inhabiting coral reefs. Frogfishes typically have highly varied colour patterns, and some species are able to change colours. In habit they are sedentary but can use their fins to walk on the bottom and to climb over obstacles. The tropical sargassum fish (Histrio histrio), so called because it lives amid floating brown algae of the genus Sargassum, clings to the branches of algae with prehensile (i.e., adapted for seizing, or wrapping around) pectoral fins as it searches for prey, which is sucked into the mouth by the powerful jaws and expandable cheeks.

The lophiiform group known as goosefishes (goosefish) (Lophiidae, about 12 species) seldom occur in shallow water, preferring instead the moderate depths (10 to 500 fathoms) along the continental slopes of tropical and temperate region. The batfishes (batfish) (Ogcocephalidae, about 60 species), are mainly deepwater lophiiforms, but some (Ogcocephalus, Halieutichthys) are regularly found in water only a few feet deep. Like frogfishes, they walk on the bottom, using their pectoral fins. Batfishes are awkward swimmers and, when disturbed, tend to bury themselves in the bottom rather than swim away.

Percopsiforms (about eight species) live under conditions of dim light. Cave fishes, with eyes reduced to nonfunctional rudiments, have elaborate systems of sense organs in the skin of the head, body, and tail; they live in total darkness. Because of the secretive habits of percopsiforms, little is known about species other than the trout-perch (Percopsis omiscomaycus), which is widely distributed in central North America and is abundant in some of the Great Lakes, where it occurs in clear water to a depth of about 35 fathoms. Polymixiiforms, numbering only three marine species, are generally found at depths of 150 to 350 fathoms.

Batrachoidiforms generally have two dorsal fins; a small anterior fin, usually with two spines; and a long posterior fin. In venomous species, the hollow fin spines form an efficient apparatus for the injection of venom. A similar spine is found on each cheek (operculum).

Among the gadiforms, the dorsal and anal fins of some deep-sea cods are distinctively arranged as three dorsals and two anals. This arrangement also occurs in some codfishes (Gadidae). The macrourids are characterized by a long, tapering tail. A tubular light organ containing luminescent bacteria is sometimes present along the ventral midline of both sexes. All but a few species of macrourids have a well-developed swim bladder; (swim bladder) in the males of some species and in some codfishes, the swim bladder is equipped with drumming muscles, indicating that sound can be produced. In the bregmacerotids and muraenolepidids there are two dorsal fins, with the anterior fin represented by a single ray. Brotulas may have the pelvic fins either present or absent, but cusk eels have them anterior in position, under the lower jaw. They are kept in continuous probing motion, as the fish swims just off the bottom, and aid in detecting food. Zoarcids (eelpouts) are elongated, eel-like fishes. Their pelvic fins are either rudimentary or entirely absent.

Gobiesociforms, with a depressed head, wide mouth, and tapering body, resemble toadfishes, but they are distinctive in having a prominent sucking disk on the ventral surface. The paired pelvic fins, thoracic in position, form part of the disk, various fleshy pads and folds forming the remainder. The disk allows clingfishes to hold fast to rocky bottoms amid the often turbulent wave action of their shallow-water environment.

Some lophiiforms are unique among teleostean fishes in having only two gills. The ogcocephalids are somewhat flattened anglers, in this respect resembling lophiids rather than the ballon-like antennariids; they are distinctive in having the illicium, when not in use, concealed in a tube (illicial cavity) between the eyes and over the mouth. Like most anglerfishes they lack typical scales but are distinctively equipped with bony tubercles (projections) and spines imbedded in the skin.

The polymixiiforms are singular in having a pair of fleshy barbels, or “whiskers,” under the jaw. Each barbel is supported by three small bones.

Fossil batrachoidiforms include only material from lower Pliocene marine deposits (about 5,000,000–7,000,000 years old) of North Africa. These fossils are similar to a living species, Batrachoides didactylus.

Fossil gadiforms are relatively numerous and are known primarily from Tertiary marine deposits (about 2,500,000–65,000,000 years old) of the Northern Hemisphere. A Paleocene fossil (54,000,000–65,000,000 years old) has been identified as a codlike fish; some Eocene fossils (38,000,000–54,000,000 years old) have been identified for the families Bregmacerotidae and Gadidae; and Oligocene–Miocene fossils (7,000,000–38,000,000 years old) for the families Bregmacerotidae, Gadidae, Macrouridae, and Ophidiidae. In addition, many fossil ear stones (otoliths) and scales, beginning with specimens from the Cretaceous (65,000,000–136,000,000 years ago), are similar to the Gadiformes. Fossil gobiesociforms are unknown. Fossil lophiiforms include two species from Eocene marine deposits of Europe and one species from Pliocene marine deposits of North Africa; one Eocene species has been identified as a goosefish (Lophiidae), the other as a frogfish (Antennariidae).

Fossil percopsiforms include three genera from Tertiary freshwater deposits of North America and one (Sphenocephalus) from Cretaceous marine deposits of Europe. Of the North American genera, two (Amphiplaga, Erismatopterus from the middle Eocene) have been identified as trout-perches (Percopsidae), and one (Tricophanes, Oligocene–Miocene) as a pirate perch (Aphredoderidae). The relationships of Sphenocephalus are obscure. Fossil polymixiiforms include a diversified group of about six genera known primarily from Cretaceous marine deposits of Europe and the Middle East; a few others are known from the Tertiary.

The interrelationships of the groups listed here as paracanthopterygians are not yet well established, and the classification given here is provisional. There is considerable agreement that trout-perches (Percopsiformes) and cods (suborder Gadoidei) are closely related, and this agreement may be considered the basis of the group Paracanthopterygii. What other fishes should be included in the Paracanthopterygii is a question receiving continued study. Some ichthyologists have held that clingfishes (Gobiesociformes) are related to dragonets (Callionymidae); that eelpouts (Zoarcidae) are related to blennies (Bathymasteridae, Blenniidae, etc.); that brotulas, cusk eels (cusk eel), and pearlfishes (suborder Ophidioidei) are related to the river blackfish of Australia (the perchlike Gadopsis); that beardfishes (Polymixiiformes) are related to squirrelfishes and their relatives (Beryciformes); and that toadfishes (Batrachoidiformes) and anglers (Lophiiformes) also have their relationships within the great mass of perchlike fishes (Acanthopterygii). In addition, killifishes (Cyprinodontidae) and related live-bearers (Poeciliidae) are believed by some writers to be related to trout-perches (Percopsiformes) and by others to silversides, flying fishes, and their relatives (Atheriniformes). Thus, future study may result in the transfer of some groups from the Paracanthopterygii to the Acanthopterygii and vice versa.