paracanthopterygian
- Related Topics:
- anglerfish
- toadfish
- Polymixiiformes
- ophidiiform
- Gadiformes
paracanthopterygian, (superorder Paracanthopterygii), any member of a large group of predatory, primarily marine fishes that forms one of about six major branches of the Teleostei, or bony fishes. Approximately 1,340 living species of paracanthopterygian fishes have been described. They range in length from just a few centimetres to roughly 2 metres (about 7 feet). Well-known forms include the anglerfish (order Lophiiformes) and the cod (order Gadiformes).
General features
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 (that is, at the tail end) vertebrae, and the placement of the pelvic fins. (In this group the pelvic fins are usually located in the midbody region or even farther toward the head.)
The Paracanthopterygii are divided into five orders. The Batrachoidiformes, or toadfishes, comprise about 78 species, whereas Gadiformes, the order that encompasses the codfishes, contains about 555 species. About 210 species are contained within the order Lophiiformes, which is made up of the anglerfishes. There are about 9 species of percopsiforms, or trout-perches, and about 385 species of ophidiiforms (that is, the pearlfishes and cusk-eels).
Most of the orders are primarily marine, with worldwide distribution; the percopsiforms, however, occur only in the freshwater environments of North America. Batrachoidiforms 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 (Merluccius, Urophycis), haddocks (Melanogrammus), pollocks (Pollachius), and whitings (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.
The largest of the Paracanthopterygii are the codfishes, which grow to about 2 metres (about 7 feet) in length and attain weights that may exceed 90 kg (about 200 pounds). Certain goosefishes (Lophiiformes) reach a length of about 2 metres and a body weight of 35 kg (about 75 pounds); other lophiiforms are as small as 2.5 cm (about 1 inch) long. Batrachoidiforms grow to about 30 cm (about 12 inches) in length, and the largest percopsiforms are about 15 cm (6 inches) long.
Natural history
Life cycle and reproduction
Eggs of the oyster toadfish (Opsanus tau) of the western Atlantic—one of the most carefully studied batrachoidiforms—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 begin life on their own. This fish gets its name from the fact that some have been found living in live oysters. Luminous organs known as photophores, 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. The fecundity of some codfish species is prodigious. The European ling (Molva molva) may deposit as many as 60 million eggs each season. The eggs and larvae of most species are found among the plankton (small aquatic organisms that drift with ocean currents). 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 ocean bottom; the buoyant eggs rise partway 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 the order Ophidiiformes (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 7 to 8 cm (about 3 inches), the vexillifer 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 cm (8 to 4 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 and cusk eels (family Ophidiidae). Ophidiids are both oviparous (egg-laying) and viviparous (live-bearing). The males of some viviparous species produce spermatophores (sperm cases).
The lophiiforms are primarily bottom fishes as adults, but many produce floating rafts of eggs. The eggs of the deep-sea anglerfishes (suborder Ceratioidei) have not been studied or observed, 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 3 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 also present in male anglerfishes but do not appear in members of suborder Ceratioidei.) The esca is commonly luminous; the female also has other light-producing organs. In 1922 a specimen of the anglerfish Ceratias holboelli was discovered with small specimens attached to its abdomen that 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 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 percopsiforms. They are known to spawn in the spring of the year in shallow water.