Myxiniformes

Hagfish, the class Myxini (also known as Hyperotreti), are eel-shaped, slime-producing marine fish (occasionally called slime eels). They are the only known living animals that have a skull but no vertebral column, although hagfish do have rudimentary vertebrae. Along with lampreys, hagfish are jawless; they are the sister group to jawed vertebrates, and living hagfish remain similar to hagfish from around 300 million years ago. The classification of hagfish had been controversial. The issue was whether the hagfish was a degenerate type of vertebrate-fish that through evolution had lost its vertebrae (the original scheme) and was most closely related to lampreys, or whether hagfish represent a stage that precedes the evolution of the vertebral column (the alternative scheme) as is the case with lancelets. Recent DNA evidence has supported the original scheme. The original scheme groups hagfish and lampreys together as cyclostomes (or historically, Agnatha), as the oldest surviving class of vertebrates alongside gnathostomes (the now-ubiquitous jawed vertebrates). The alternative scheme proposed that jawed vertebrates are more closely related to lampreys than to hagfish (i.e., that vertebrates include lampreys but exclude hagfish), and introduces the category craniata to group vertebrates near hagfish. Hagfish are typically about 0.5 m (19.7 in) in length. The largest known species is Eptatretus goliath, with a specimen recorded at 127 cm (4 ft 2 in), while Myxine kuoi and Myxine pequenoi seem to reach no more than 18 cm (7.1 in) (some have been seen as small as 4 cm (1.6 in)). Hagfish have elongated, eel-like bodies, and paddle-like tails. The skin is naked and covers the body like a loosely fitting sock. They have cartilaginous skulls (although the part surrounding the brain is composed primarily of a fibrous sheath) and tooth-like structures composed of keratin. Colors depend on the species, ranging from pink to blue-grey, and black or white spots may be present. Eyes are simple eyespots, not lensed eyes that can resolve images. Hagfish have no true fins and have six or eight barbels around the mouth and a single nostril. Instead of vertically articulating jaws like Gnathostomata (vertebrates with jaws), they have a pair of horizontally moving structures with tooth-like projections for pulling off food. The mouth of the hagfish has two pairs of horny, comb-shaped teeth on a cartilaginous plate that protracts and retracts. These teeth are used to grasp food and draw it toward the pharynx. Its skin is only attached to the body along the center ridge of the back and at the slime glands, and is filled with close to a third of the body's blood volume, giving the impression of a blood-filled sack. It is assumed this is an adaptation to survive predator attacks. The Atlantic hagfish, representative of the subfamily Myxininae, and the Pacific hagfish, representative of the subfamily Eptatretinae, differ in that the latter has muscle fibers embedded in the skin. The resting position of the Pacific hagfish also tends to be coiled, while that of the Atlantic hagfish is stretched. Hagfish are long and vermiform, and can exude copious quantities of a milky and fibrous slime or mucus from some 100 glands or invaginations running along their flanks. The species Myxine glutinosa was named for this slime. When captured and held, e.g., by the tail, they secrete the microfibrous mucus, which expands into up to 20 litres (5¼ US gallons) of sticky, gelatinous material when combined with water; one litre of slime has about 40 milligrams of mucus and proteins. If they remain captured, they can tie themselves in an overhand knot, which works its way from the head to the tail of the animal, scraping off the slime as it goes and freeing them from their captor. This singular behavior may assist them in extricating themselves from the jaws of predatory fish or from the interior of their own prey, and the sliming might act as a distraction to predators. Recently, the slime was reported to entrain water in its keratin-like intermediate filaments, creating a slow-to-dissipate, viscoelastic substance, rather than a simple gel. It has been proven to impair the function of a predator fish's gills. In this case, the hagfish's mucus would clog the predator's gills, disabling their ability to respire. The predator would release the hagfish to avoid suffocation. Because of the mucus, few marine predators target the hagfish. Other predators of hagfish are varieties of birds or mammals.