Hagfish, Fish from 550 Million Years

Fish World | Hagfish, Fish from 550 Million Years | Hagfishes are marine craniates of the class Agnatha or Myxini, also known as Hyperotreti. Some researchers regard Myxini as not belonging to the subphylum vertebrates. That is, they are the only animals that a skull, but not a spine.
Despite their name, there is some debate about whether they are strictly fish (as there is for lampreys), since they belong to a much more primitive lineage than any other group that has been placed in the category of fish (Chondrichthyes and Osteichthyes). The first fossils dating from about 550 million years, or earlier at the Lower Cambrian. Their unusual feeding habits and slime-producing capabilities have led members of the scientific community and popular media to synchronize hagfish as the most "disgusting" of all sea creatures. Although hagfish are sometimes called "slime eels", they are not really eels.
Hagfishes average about half a meter (18 in), the largest known species is Eptatretus Goliath with a copy included in 127 cm, while Myxine Myxine pequenoi kuoi and seem no more than 18 cm away.
Hagfishes have elongated, eel-like bodies, and paddle-like tail. They have cartilaginous skulls (although the part about the brains in the first place is a fibrous sheath) and tooth-like structures composed of keratin. Colors depend on the species, ranging from pink to blue-gray, and black or white spots may be present. Eyes are simple eyespots, not compound eyes that can resolve images. Hagfish have no real fins and have six or eight barbels around the mouth and one 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 protracts and pulls. These teeth are used to grab and pull food toward the pharynx.
Hagfish are long and vermiform, and can radiate large amounts of phlegm or mucus (from which the typical species Myxine glutinosa was named) of unusual composition. When captured and held, for example by the tail, they separate the microfibrous mucus, which expands into a gelatinous and sticky stuff in combination with water, if they remain trapped, they can join a half-knot, which works its way out of the head to the tail of the animal, scraping the mucus when it comes to free and their captor, and the mucus. It is suggested that this peculiar behavior helps them to release themselves from the clutches of predatory fish or the interior of their "prey" and the "sliming" might act as a distraction to predators.
Recently reported that the mucus water sweeps in microfilaments, creating a slow-to-disposed visco-elastic material, instead of a simple gel, and it is proposed that the primary protective effect of mucus is related to impairment of the function of a predator fish gills. It is observed that most of the known predators of hagfish are varieties of birds or mammals, it is suggested that the lack of marine predators can be explained by a "gill-clogging hypothesis", whereby an object of the mucus is detrimental to gill function of marine animals which prey to attack the hagfish. If true, it can be regarded as a highly successful evolutionary strategy against predators.

Free-swimming hagfish also "slime" when agitated and will later clear the mucus out through the same travel-node behavior. The reported gill-clogging effect suggests that the travel-knot behavior is useful or even necessary to the hagfish's own gill function to recover after "sliming".

An adult hagfish can secrete enough slime to 20 liters (5 gallons) bucket of water into slime in a matter of minutes. Research is ongoing regarding the properties and potential applications of the components of hagfish slime filament protein.

Very little is known about hagfish reproduction. In some species, sex ratio has been reported as high as 100:1 in favor of women. Some hagfish species are thought to be hermaphroditic, with both an ovary and a testicle (there is only one gamete producing organ in both women and men). In some cases it is thought that the ovary remains non-functional until the individual reaches a certain age or meet a certain pressure on the environment. These two factors together suggest that the survival of hagfish is quite high.

Depending on the species, females lay 1 or 2, to 20-30, cool, yolky eggs. These tend to total due to having Velcro-like tufts at both ends. Hagfish are sometimes seen curled around small clutches of eggs. It is not certain whether this is the actual breeding behavior. Hagfish have no larval stage, unlike lampreys, that a long larval phase.

Hagfishes have a mesonephric kidney and are often neotenic their pronephric kidney. The kidney (s) are dispatched through mesonephric / archinephric channel. Unlike many other vertebrates, this channel is independent of the reproductive system. Unlike all other vertebrate animals, the proximal tubule of the nephron also connected with the coelom, provided lubrication. The single testis or an ovary is not a transport channel. Instead, the gametes are released into the coelom until they find their way to the rear end of the caudal region, where they find an opening in the digestive system.

While marine polychaete worms on or near the seabed are an important source of food, can and often hagfish feeding and removal of the bodies of dead and dying / injured marine animals much larger than themselves. They are known for their victims to devour from the inside. Hagfishes have the ability to absorb dissolved organic matter on the skin and gills, which adapt to a scavenging lifestyle, which hagfish to sporadic opportunities to maximize feeding. From an evolutionary perspective, hagfish represent a temporary state between the overall nutrient absorption capabilities of aquatic invertebrates and the more specialized digestive system of aquatic vertebrates.

Like leeches, they have a slow metabolism and can survive months between feedings, their eating, seems quite powerful. In captivity, hagfish observed that the overhand knot behavior use "in reverse" (tail-to-head) to help them obtain mechanical advantage from hunks of meat from carcasses of fish or cetaceans, eventually creating a opening to allow entry to the interior of the body cavity of larger carcasses. It is expected that a healthy larger sea creature would be able to outfight or outswim this kind of attack.

However, this energetic opportunism on the part of the hagfish a great nuisance to fishermen, as they can eat or spoil entire deep-drag netted catches before they can be pulled to the surface. Since hagfish are usually found in large clusters on and near the bottom, a single trawl catch contain dozens or even hundreds of hagfish as bycatch, and all the other struggling, captive sealife easy prey for them.
The digestive tract of the hagfish is unique among vertebrates, because the food in the gut is enclosed in a permeable membrane, analogous to the peritrophic matrix of insects

Hagfish are usually not eaten because of their repulsive appearance, and their viscosity and unpleasant habits. However, a given species, the inshore hagfish, found in the Pacific Northwest, valued as food on the Korean peninsula. The hagfish is alive and annoyed by the rattling of a stick pack. This produces the large amount of mucus that gives the hagfish his deputy, if inaccurate, name, "slime eel". This mucus is often used as protein in various forms of cooking in the region.