Hagfish, of the class Myxini /mɪkˈsaɪnaɪ/ (also known as Hyperotreti) and order Myxiniformes /mɪkˈsɪnɪfɔːrmiːz/, are eel-shaped jawless fish (occasionally called slime eels).

[14] If they remain captured, they can tie themselves in an overhand knot, and work their way from the head to the tail of the animal, scraping off the slime and freeing themselves from their captor.

[15] Recently, the slime was reported to entrain water in its keratin-like intermediate filaments excreted by gland thread cells, creating a slow-to-dissipate, viscoelastic substance, rather than a simple gel.

[17][18] The reported gill-clogging effect suggests that the travelling-knot behavior is useful or even necessary to restore the hagfish's own gill function after sliming.

Hagfish thread keratin (EsTKα and EsTKγ; Q90501 and Q90502), the protein that make up its slime filaments, is under investigation as an alternative to spider silk for use in applications such as body armor.

[20] With combined draw-processing (stretching) and chemical crosslinking, recombinant slime keratin turns into a very strong fiber with an elastic modulus reaching 20 GPa.

This pharyngocutaneous duct is used to clear large particles from the pharynx, a function also partly taking place through the nasopharyngeal canal.

[24][25] The unidirectional water flow passing the gills is produced by rolling and unrolling velar folds located inside a chamber developed from the nasohypophyseal tract, and is operated by a complex set of muscles inserting into cartilages of the neurocranium, assisted by peristaltic contractions of the gill pouches and their ducts.

[29] The origins of the vertebrate nervous system are of considerable interest to evolutionary biologists, and cyclostomes (hagfish and lampreys) are an important group for answering this question.

[40] The hagfish circulatory system has been of considerable interest to evolutionary biologists and present day readers of physiology.

The hagfish circulatory system also includes multiple accessory pumps throughout the body, which are considered auxiliary "hearts".

[44] In Cole's monograph, he described sections of the skeleton that he termed "pseudo-cartilage", referring to its distinct properties compared to jawed chordates.

The nasal capsule is considerably expanded in hagfish, comprising a fibrous sheath lined with cartilage rings.

Obtaining embryos and observing reproductive behavior are difficult due to the deep-sea habitat of many hagfish species.

[47] It is unclear how hagfish go about laying eggs, although researchers have proposed three hypotheses based on observations of the low percentage of males and small testis.

Unlike many other vertebrates, this duct is separate from the reproductive tract, and the proximal tubule of the nephron is also connected with the coelom, providing lubrication.

[50] Not much was known about hagfish embryology until recently, when husbandry advances enabled considerable insight into the group's evolutionary development.

[55] Hagfish have the ability to absorb dissolved organic matter across the skin and gill, which may be an adaptation to a scavenging lifestyle, allowing them to maximize sporadic opportunities for feeding.

Analysis of the stomach content of several species has revealed a large variety of prey, including polychaetes, shrimp, hermit crabs, cephalopods, brittle stars, bony fishes, sharks, birds, and whale flesh.

[59] In captivity, hagfish are observed to use the overhand-knot behavior in reverse (tail-to-head) to assist them in gaining mechanical advantage to pull out chunks of flesh from carrion fish or cetaceans, eventually making an opening to permit entry to the interior of the body cavity of larger carcasses.

This energetic opportunism on the part of the hagfish can be a great nuisance to fishermen, as they can devour or spoil entire deep drag-netted catches before they can be pulled to the surface.

The digestive tract of the hagfish is unique among chordates because the food in the gut is enclosed in a permeable membrane, analogous to the peritrophic matrix of insects.

The fossil hagfish Myxinikela siroka, from the Late Carboniferous of the United States, is the oldest-known member of the group.

[64] The validity of the taxon "Craniata" was further examined by Delarbre et al. (2002) using mtDNA sequence data, concluding the Myxini are more closely related to the Hyperoartia than to the Gnathostomata—i.e., that modern jawless fishes form a clade called the Cyclostomata.

The group Cyclostomata is characterized by two significant characteristics; keratinous tooth plates and movement of postotic myomeres to the orbitals.

[33] The following hagfish and lamprey phylogeny is an adaptation based on the 2019 work of Miyashita et al.[69] †Haikouella †Haikouichthys †Myllokunmingia †Metaspriggina Gnathostomata (jawed fish) †Cornovichthys †Achanarella †Ciderius †Birkeniida †Lasanius †Euphanerops †Jamoytius †Pipiscius †Conodonta (conodonts) †Myxinikela †Tethymyxine tapirostrum Rubicundus eos Rubicundus lopheliae Myxine glutinosa Neomyxine biniplicata Eptatretus stoutii Eptatretus burgeri "Paramyxine" spp.

†Gilpichthys †Hardistiella †Mayomyzon †Myxineidus †Priscomyzon †Mesomyzon Geotria australis Mordacia mordax Petromyzon marinus Lampetra fluviatilis Lethenteron camtschaticum In most of the world, hagfish are not often eaten.

But in Korea, the hagfish is a valued food, where it is generally skinned, coated in spicy sauce, and grilled over charcoal or stir-fried.

[citation needed] Due to their value in Korean cuisine, most hagfish caught for food elsewhere in the world is fished with intent of being exported to South Korea.

As hagfish slime binds vast amounts of liquid even at low temperatures, it was proposed as an energy-saving alternative for the production of tofu that does not require heating.