Hagfish

Hagfish are eel-shaped, slime-producing marine fish that belong to the class Myxini (also known as Hyperotreti) and order Myxiniformes (occasionally called slime eels). Although hagfish have rudimentary vertebrae, they are the only known living animals with a skull but no vertebral column. Hagfish, like lampreys, have no jaws; they are the sister group to jawed vertebrates, and modern hagfish look quite similar to hagfish from 300 million years ago.

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The taxonomy of hagfish had long been a point of contention. The question was whether the hagfish was a degenerate type of vertebrate-fish that had lost its vertebrae through evolution and was most closely related to lampreys (the original scheme), or whether hagfish represented a stage that preceded the evolution of the vertebral column (the alternative scheme), as lancelets do. DNA evidence has just been discovered that backs up the initial plan.

Lampreys and hagfishes are included together as cyclostomes (or previously, Agnatha) as the oldest surviving class of vertebrates with gnathostomes in the original arrangement (the now-ubiquitous jawed vertebrates). The alternative theory stated that jawed vertebrates are more closely related to lampreys than to hagfish (i.e., vertebrates include lampreys but not hagfish), and the category Craniata was created to classify vertebrates that are close to hagfish.

Scientific Classification

Kingdom: Animalia

Phylum: Chordata

Subphylum: Vertebrata

Superclass: Cyclostomata

Class: Myxini

Order: Myxiniformes

Family: Myxinidae

Hagfish scientific name is Myxini

Hagfish Species 

There are roughly 70-82 species, with the figure shifting depending on the source due to new species being discovered on a regular basis. The Atlantic hagfish, scientifically known as Myxine glutinosa, is the most studied species. Other species include the Goliath (Eptatretus goliath), Pacific or California hagfish (Eptatretus stoutii), and black hagfish (Eptatretus stoutii/Eptatretus deani).

Physical Characteristics of Hagfish Body

Hagfish are roughly 50 cm (19.7 in) long on average. Eptatretus goliath is the largest known species, with a specimen measuring 127 cm (4 ft 2 in), whereas Myxine kuoi and Myxine pequenoi appear to reach no more than 18 cm (7.1 in). Some have been as little as 4 cm in size (1.6 in).

Hagfish have paddle-like tails and elongated eel-like bodies. The skin is exposed and drapes across the body like a sock. They have a dull pink colour and a worm-like appearance. They have cartilaginous skulls (albeit the region enclosing the brain is mostly made of a fibrous sheath) and keratin-based tooth-like features. Colours range from pink to blue-grey, and black or white markings may be present depending on the species. Eyes are not lensed eyes that can discern images, but rather simple eyespots. Hagfish having no genuine fins and a single nose, as well as six or eight barbels around the mouth. They feature a pair of horizontally moving structures with tooth-like projections for taking off food instead of vertically articulating jaws like Gnathostomata (vertebrates with jaws). Two pairs of horny, comb-shaped teeth protract and retract on a cartilaginous plate in the hagfish mouth. Food is grasped with these teeth and drawn toward the pharynx.

Its skin is only joined to the body along the back's central ridge and at the slime glands, and it is filled with nearly a third of the body's blood volume, giving the appearance of a blood-filled sack. This is thought to be an adaptation to fend off predators. The Atlantic hagfish, which belongs to the Myxininae subfamily, differs from the Pacific hagfish, which belongs to the Eptatretinae subfamily, in that the latter possess muscle fibres implanted in its skin. The Pacific hagfish prefers to repose in a coiled position, but the Atlantic hagfish prefers to stretch.

Slime 

Hagfish are long and vermiform, with about 100 glands or invaginations running along their flanks that can discharge vast amounts of a milky and fibrous slime or mucus. Hagfish may make a large amount of slime, which combines with seawater as a defence mechanism when they are in danger. The slime secreted by hagfish has very fine filaments, making it more durable and sticky than other creatures' slime. The slime's filaments are formed of proteins and help it to be flexible. They can instantly release a considerable amount of slime to flee if they are caught by a predator. If they are still being held captive, they can bind themselves in an overhand knot and work their way from the animal's head to its tail, scraping off the slime and liberating themselves. According to rheological studies, the viscosity of hagfish slime increases in elongational flow, which favours gill obstruction in suction-feeding fish, while it reduces in shear, which makes scraping off the slime by the migrating knot easier.

The slime has recently been shown to entrap water in its keratin-like intermediate filaments, resulting in a viscoelastic, slow-to-dissipate substance rather than a simple gel. The function of a predator fish's gills has been shown to be harmed by it. The mucus from the hagfish would plug the predator's gills, preventing them from breathing. To escape asphyxia, the predator would release the hagfish. Few marine predators hunt the hagfish because of its mucus. Hagfish are also preyed upon by a variety of birds and mammals.

When agitated, free-swimming hagfish slime and then clean the mucus utilising the same travelling-knot behaviour. The reported gill clogging impact shows that the travelling-knot behaviour is beneficial, if not essential, in restoring the hagfish's own gill function following sliming.

Hagfish thread keratin, the protein that makes up the slime filaments, is being researched as a possible replacement for spider silk in applications like body armour. When these alpha-keratin proteins in hagfish slime are stretched, they change from a -helical shape to a stiffer sheet structure. Recombinant slime keratin becomes a highly strong fibre with the rigidity of 20 GPa after combining draw-processing (stretching) with chemical crosslinking.

Respiration 

A hagfish breathes by inhaling water by its pharynx, passing through the velar chamber, and exhaling it through the internal gill pouches, which can range from five to sixteen pairs depending on the species. The gill pouches open independently in Myxine, but canals extending rearward from each opening under the skin have consolidated to produce a shared aperture on the ventral side known as the branchial opening. The pharyngocutaneous duct (oesophageocutaneous duct) connects the oesophagus to the left branchial aperture, which is thus larger than the right one. 

The pharyngocutaneous duct is utilised to eliminate big particles from the pharynx, a role that is also partially performed by the nasopharyngeal canal. The coalescence of the gill apertures is less complete in other species, and each pouch in Bdellostoma opens individually to the exterior, like in lampreys. 

Rolling and unrolling velar folds located inside a chamber developed from the nasohypophyseal tract produce the unidirectional water flow passing the gills, which is controlled by a complex set of muscles inserting into cartilages of the neurocranium, assisted by peristaltic contractions of the gill pouches and their ducts. Hagfish also have a well-developed dermal capillary network that supplies the skin with oxygen when the animal is buried in anoxic mud, as well as a high tolerance for both hypoxia and anoxia, with a well developed anaerobic metabolism. The skin has also been suggested to be capable of cutaneous respiration.

Eye

The hagfish eye lacks a lens, extraocular muscles, and the three motor cranial nerves (III, IV, and VI) seen in more complex vertebrates, which is important for understanding how more sophisticated eyes evolved. Extant hagfish lack a parietal eye as well. When present, Hagfish eyespots can sense light, but none of them can resolve precise images as far as we know. The eyes of Myxine and Neomyxine fish are partially hidden by the trunk musculature. Paleontological data, on the other hand, implies that the hagfish eye is degenerative rather than pleisiomorphic, as fossils from the Carboniferous show hagfish-like vertebrates with sophisticated eyes. This would suggest that ancestrally Myxini possessed complex eyes.

Reproduction 

The reproduction of hagfish is a mystery. Due to the deep-sea environment of many hagfish species, obtaining embryos and watching reproductive behaviour is difficult. Females outweigh males in the wild, with the exact sex ratio varying depending on the species. Females of E. burgeri, for example, outnumber males almost 1:1, whereas females of M. glutinosa are substantially more frequent. Before development, several hagfish species are sexually undifferentiated and have gonadal tissue in both the ovaries and testis. It has been proposed that girls develop faster than males, which could explain the disparity in sex ratios. The testis of a hagfish is quite tiny.

Females lay one to thirty tough, yolky eggs, depending on the species. Because they have Velcro-like tufts on both ends, they like to clump together. Although academics have presented three ideas based on observations of the low percentage of males and small testis, it is unclear how hagfish deposit eggs. Female hagfish lay eggs in small fissures in rock formations, the eggs are laid in a burrow beneath the sand, and the eggs are held in a small region by the slime generated by the hagfish. It's worth noting that none of these hypotheses has been backed up by direct evidence. In contrast to lampreys, hagfish do not have a larval stage.

Hagfish have a mesonephric kidney and their pronephric kidney is frequently neotenic. The mesonephric/archinephric duct drains the kidney(s). This duct is distinct from the reproductive tract in many animals, and the proximal tubule of the nephron is also connected to the coelom, providing lubrication. There is no transmission duct in a single testicle or ovary. Instead, the gametes are released into the coelom until they reach the caudal region's posterior end, where they enter the digestive system through an aperture.

The embryo of a hagfish can develop for up to 11 months before hatching, which is shorter than that of other jawless vertebrates. Until recently, little was known about hagfish embryology, when breakthroughs in husbandry allowed for significant insight into the group's evolutionary progress. New research on the evolution of neural crest cells backs up the theory that these cells are shared by all vertebrates and are regulated by a common set of genes. Hagfish have Gonadotropins, which are hormones secreted by the pituitary glands and sent to the gonads to encourage development. This indicates that hagfish have an early version of the Hypothalamic–pituitary-gonadal axis, which was previously thought to be exclusive to Gnathostomes.

Feeding 

While polychaete marine worms on or near the seafloor constitute a key food source, hagfish may devour and even eviscerate the corpses of much larger sea animals. They have a reputation for devouring their prey from the inside out. Hagfish can absorb dissolved organic substances through their skin and gills, which may be an adaptation to a scavenging lifestyle that allows them to take advantage of occasional feeding chances. From an evolutionary standpoint, hagfish represent a transitional condition between aquatic invertebrates' broad nutrition absorption pathways and aquatic vertebrates' more specialised digestive systems.

They have a slow metabolism and can go months without eating, similar to leeches; yet, their feeding behaviour appears to be extremely active. Polychaetes, shrimp, hermit crabs, cephalopods, brittlestars, bony fishes, sharks, birds, and whale flesh have all been discovered in the stomach contents of different species.

Hagfish have been recorded in captivity using the reverse overhand-knot behaviour (tail-to-head) to aid them in gaining mechanical advantage to take off chunks of flesh from carrion fish or cetaceans, finally producing an entrance to allow admission to the inside of the body cavity of larger carcasses. A healthy larger marine creature would most certainly be able to outfight or outswim such an attack.

The hagfish's zealous opportunism can be a major headache for fishermen, as they can eat or ruin entire deep drag-netted catches before they can be hauled to the surface. A single trawler's catch could contain several dozen or even hundreds of hagfish as bycatch, and all the other struggling, trapped marine life makes easy food for them.

Because the food in the gut of the hagfish is encased in a permeable membrane, similar to the peritrophic matrix of insects, the digestive tract of the hagfish is unusual among chordates. Hagfish have also been seen pursuing the red bandfish, Cepola haastii, in its burrow, potentially suffocating the fish with their slime before grabbing it with their teeth plates and hauling it out.

Commercial Use 

As Food 

Hagfish is not commonly eaten in most parts of the world. Hagfish is a prized delicacy in Korea, where it is usually skinned, covered in a spicy sauce, and roasted over charcoal or stir-fried. It is especially popular in the peninsula's southern port cities, such as Busan. In Japan, the inshore hagfish, which can be found in the Northwest Pacific, is consumed. Because hagfish slime can bind large amounts of liquid at low temperatures, it has been recommended as an energy-saving option for making tofu without heating.

In Textiles 

The threads of hagfish slime can be used as a super-strong fabric. Chapman University's Douglas Fudge has done a study in this area.

Skins 

The threads of hagfish slime can be used as a super-strong fabric. Chapman University's Douglas Fudge has done a study in this area.

Interesting Facts About Hagfish 

1. Although they have primitive vertebrae, these fish are the only vertebrates with a skull but no spinal column.

2. Their skin allows them to absorb nutrition.

3. When they tie themselves in an overhand knot with their slime, they are able to avoid capture.

4. Their slime clogs the gills of aquatic predators, causing them to suffocate and release the hagfish.

5. The hagfish's flexible skin allows for whole-body flexibility, allowing it to tie itself into knots without harming itself.

6. No sight: Hagfish don't have compound eyes; instead, they have light-detecting eyespots. Even these patches are covered by skin in certain hagfish. This isn't to say they won't be able to navigate their way around the ocean. Hagfish have a highly developed sense of hearing and smell, which allows them to readily travel and find food. Around the hagfish mouths, they feature many barbels and detecting tentacles.

In conclusion, hagfish are interesting creatures that are also called slime eels because of their appearance but they are actually jawless fish. In this article, we have come across hagfish Introduction, Scientific classification, Physical characteristics, Respiration, feeding, commercial use and some of the interesting facts about hagfish. They do, in fact, play a crucial part in sustaining the ocean's health. Unfortunately, several hagfish species are on the verge of extinction, necessitating concerted efforts on our part to ensure their survival.