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A primitive group of fish, hagfish (order Hyperotreti, family Myxinidae) resemble eels in their external appearance. These fish lack a backbone, jaws, true fins, and scales. Their body is tubelike and often covered in a slimy substance that is secreted from abundant glands in the skin. The body is often a pale fleshy pink, but is occasionally brown-gray above and pink below. They may reach up to 2 ft (60 cm) in length, but most measure about 1.5 ft (40 cm); females are often larger than males.

Hagfish are bottom-dwelling fish of soft, muddy substrates, living at a depth range of 65.5-1,968 ft (20-600 m). They feed on bottom-dwelling crustacea and polychaete worms, but may also scavenge on dead fish. The mouth is a simple slit surrounded by a ring of fleshy barbels that have a sensory role; other barbels are located around the nostrils and probably fulfill a similar role. The tongue is serrated and consists of two plates of gristle with many horny teeth that are continuously replaced as they wear out with use.

Hagfish have an unusual but simple system of obtaining oxygen from the surrounding water. Unlike most fish that have a complex arrangement of gills, hagfish have a simplified set of gills in a series of paired pouches that open to the pharynx and the exterior. Water enters the body through the snout, is compressed in the body and expelled through these breathing pouches. Under normal swimming conditions this does not present a problem, but when the fish is feeding, the flow of water is seriously reduced, if not completely cut off. This has led to the suggestion that hagfish may be able to tolerate temporary periods of oxygen deficiency and are later able to expel all metabolic body wastes when normal breathing resumes.

Fifteen species of hagfish have been recognized to date, all of which are marine-dwelling with the majority living in temperate oceans; one species has been recorded from the tropical waters off Panama. Hagfish are completely blind, but have a very keen sense of smell, which is used to locate food. They are active hunters (although poor swimmers), and also attack sick and dead fish, latching onto their prey and absorbing the flesh of the other species through continued rasping motions of their tongues. While hagfish play an important role in eliminating weak or ill fish, they can sometimes be a nuisance to commercial fisheries, as they are known to attack captured fish on long line fisheries.

Once they have reached sexual maturity—usually by the time they grow to 10-11 in (25-28 cm)—hagfish may breed throughout the year. Some species are hermaphrodites (each fish has both male and female reproductive organs), but others are either male or female. Hagfish lay relatively large yolky eggs in a horny shell with hooked filaments at each end. The young are free-living and resemble the adults when they hatch.

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