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For the punk rock band, see Hagfish (band).
Hagfish are marine craniates of the class Myxini, also known as Hyperotreti. Myxini is the only class in the clade Craniata that does not also belong to the subphylum Vertebrata.[1] That is, they are the only animals that have a skull but not a vertebral column. 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 is placed in the category of fish (Chondrichthyes and Osteichthyes). Their unusual feeding habits and slime-producing capabilities have led members of the scientific and popular media to dub the hagfish as the most "disgusting" of all sea creatures.[2][3][4] Although hagfish are sometimes called "slime eels," they are not eels at all.[5]
[edit] Physical characteristics[edit] Body featuresHagfish average about half a meter (18 in) long; The largest known species is Eptatretus goliath with a specimen recorded at 127 cm, while Myxine kuoi and Myxine pequenoi seem to reach no more than 18 cm. Hagfish have elongated, eel-like bodies (so flexible they sometimes tie themselves in knots). They have four hearts, two brains, and a paddle-like tail.[6] They have cartilaginous skulls (although the part surrounding the brain is composed primarily of a fibrous sheath) and tooth-like structures composed of keratin. Colours depend on the species, ranging from pink to blue-grey, and black or white spots may be present. Eyes are simple eyespots, not compound 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. [edit] Slime (and behavior) Hagfish are long and vermiform, and can exude copious quantities of a slime or mucus (from which the typical species Myxine glutinosa was named) of unusual composition. When captured and held e.g. by the tail, they secrete the microfibrous slime, which expands into a gelatinous and sticky goo when combined with water; 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, as well as the slime. It has been conjectured that this singular behavior assists them in extricating themselves from the jaws of predatory fish or from the interior of their own "prey", and that the "sliming" might act as a distraction to predators. Recently, though, it has been reported that the slime entrains water in its microfilaments, creating a slow-to-dissipate viscoelastic substance, rather than a simple gel, and it has been proposed that the primary protective effect of the slime is related to impairment of the function of a predator fish's gills.[7] Reportedly, most (all?) of the known predators of hagfish are birds or mammals, which could lend weight to the "gill-clogging hypothesis" as a highly successful evolutionary strategy tuned specifically to predatory fish.[8] Free-swimming hagfish also "slime" when agitated and will later clear the mucus off by way of the same traveling-knot behavior.[9][10] The reported gill-clogging effect suggests that the traveling-knot behavior is useful or even necessary to restore the hagfish's own gill function after "sliming". An adult hagfish can secrete enough slime to turn a 20 litre bucket of water into slime in a matter of minutes.[11] Research is ongoing regarding the properties and possible applications of the components of hagfish slime filament protein. [edit] EyeIn December 2003, an article was published by the University of Queensland claiming the hagfish's eye, which lacks both lens and extrinsic musculature, as being significant to the evolution of more complex eyes.[12] Hagfish eyespots when present can detect light, but as far as is known none can resolve detailed images. In Myxine and Neomyxine, the eyes are partly covered by the trunk musculature.[13] [edit] Reproduction and the Urogenital System .gif){kind=small} Very little is known about hagfish reproduction. In some species, sex ratio has been reported to be as high as 100:1 in favor of females. Some hagfish species are thought to be hermaphroditic, having both an ovary and a testicle (there is only one gamete production organ in both females and males). It some cases it is though that the ovary remains non-functional until the individual has reached a particular age or encounters a particular environmental stress. These two factors in combination suggest that the survival rate of hagfish is quite high. Depending on species, females lay from one or two, to 20 to 30 tough, yolky eggs. These tend to aggregate due to having Velcro-like tufts at either end. Hagfish are sometimes seen curled around small clutches of eggs. It is not certain if this constitutes actual brooding behavior. Hagfish do not have a larval stage, in contrast to lampreys, which have a long larval phase. Hagfish have a mesonephric kidney and are often neotenic of their pronephric kidney. The kidney(ies) are drained via mesonephric/archinephric duct. Unlike many other vertebrates this duct is separate from the reproductive tract. Unlike all other vertebrates, the proximal tubule of the nephron is also connected with the coelom, provided lubrication. The single testicle or ovary has no transportation duct. Instead, the gametes are released into the coelom until they find their way to the posterior end of the caudal region, whereby they find and opening in the digestive system. [edit] FeedingWhile polychaete marine worms on or near the sea floor are a major source of nutrition, hagfish can feed upon and often even enter and eviscerate the bodies of dead and dying/injured sea creatures much larger than them. They are known to devour their victims from the inside[14] Like leeches, they have a sluggish metabolism and can survive months between feedings.[15][16] But their feeding behavior appears, by contrast, quite vigorous. 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 hunks of flesh from carrion fish or cetaceans, eventually making an opening to permit entry to the interior of the body cavity of larger carcasses. It is to be expected that a healthy larger sea creature would be able to outfight or outswim this sort of assault. However, 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. Since hagfish are typically found in large clusters on and near the bottom, a single trawler's catch could contain several dozen or even hundreds of hagfish as bycatch, and all the other struggling, captive sealife makes easy prey for them. The digestive tract of the hagfish is unique among the vertebrates because the food in the gut is enclosed in a permeable membrane, analogous to the peritrophic matrix of insects.[17] [edit] Gastronomy Hagfish are usually not eaten owing to their repugnant looks, as well as their viscosity and unpleasant habits. However, a particular species, the inshore hagfish, found in the Northwest Pacific,[18] is valued as food in the Korean Peninsula. The inshore hagfish, known as kkomjangeo (꼼장어) or meokjango (먹장어) in Korean and Nuta-unagi in Japanese, is the only member of the hagfish family having a seasonal reproductive cycle. [edit] Genetic analysisIn recent years hagfish have become of special interest for genetic analysis investigating the relationships among chordates. [edit] ClassificationThere has been long discussion in scientific literature about the hagfish being non-vertebrate. Given their classification as Agnatha, Hagfish are seen as an elementary vertebrate in between Prevertebrate and Gnathostome. Recent molecular biology analyses tend to classify hagfish as invertebrates (see references) within subphylum Craniata, because of their short molecular evolutionary distance from Vertebrata (sensu stricto). A single fossil of hagfish shows that there has been little evolutionary change in the last 300 million years.[19] [edit] GeneraAbout 60 species are known, in 5 genera. A number of the species have only been recently discovered, living at depths of several hundred metres.
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