Hagfish
Pacific hagfish resting on bottom280 m down off Oregon coast
Scientific classification
Kingdom: Animalia
Phylum: Chordata
Subphylum: Vertebrata
Class: Myxini
Order: Myxiniformes
Family: Myxinidae
Genera: Eptatretus, Myxine, Nemamyxine, Neomyxine, Notomyxine, Paramyxine, Quadratus
A hagfish is a marine chordate of the class Myxini, also known as Hyperotreti. 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 commonly defined fish (Chondrichthyes and Osteichthyes).
They are long, vermiform and can exude copious quantities of a sticky slime or mucus (from which the typical species Myxine glutinosa was named). When captured and held by the tail, they escape by secreting the fibrous slime, which turns into a thick and sticky gel when combined with water, and then cleaning off by tying 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. Some authorities conjecture that this singular behavior may assist them in extricating themselves from the jaws of predatory fish. However, the "sliming" also seems to act as a distractant to predators, and free-swimming hagfish are seen to "slime" when agitated and will later clear the mucus off by way of the same traveling-knot behavior.
Instead of vertically articulating jaws like Gnathostomata (vertebrates with jaws), they have a pair of horizontally moving structures with toothlike projections for pulling off food. There are typically short tentacle-like protrusions around the mouth.
Hagfish enter both living and dead fish, feeding on the insides (polychaete marine worms are also prey). They tend to be quite common in their range, sometimes becoming a nuisance to fishermen by devouring the catch before it can be pulled to the surface. Not unlike leeches, they have a sluggish metabolism and can go months between feedings.
Hagfish average about half a metre (18 inches) in length; Eptatretus carlhubbsi is the largest known, with a specimen recorded at 116 cm, while Myxine kuoi and Myxine pequenoi seem to reach no more than 18 cm. An adult hagfish can secrete enough slime to turn a large bucket of water into gel in a matter of minutes.
There 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 inbetween Prevertebrate and Gnathostome. Thus, their classification is as an extremely primitive Vertebrate.
Recent molecular biology analyses tend to classify hagfish as vertebrates (see references), their molecular evolutive distance from Vertebrata (sensu stricto) being short.
The circulatory system of the hagfish has both closed and open blood vessels, with a heart system that is the most primitive of all vertebrates, bearing some resemblance to that of some worms. This system comprises a "brachial heart", which functions as the main pump, and three types of accessory hearts: the "portal" heart(s) which carry blood from intestines to liver; the "cardinal" heart(s) which move blood from the head to the body, and the "caudal" heart(s) which pump blood from the trunk and kidneys to the body. None of these hearts are innervated, so their function is probably modulated, if at all, by hormones.
Individual hagfish are hermaphroditic, with both ovaries and testes, but the female gonads remain non-functional until the individual has reached a particular stage in the hagfish lifecycle. Hagfish do not have a larval stage, in contrast to lampreys, which have a long larval phase.
Hagfish are eaten in Japan and South Korea, and their skin is made into "eel leather" (used for so-called "eelskin" products ) in Korea.
In recent years hagfish have become of special interest for genetic analysis investigating the relationships among chordates. It has also recently been discovered that the mucus excreted by the hagfish is unique in that it includes strong, threadlike fibres similar to spider silk. Research continues into potential uses for this or a similar synthetic gel or of the included fibres. Some possibilities include new biodegradable polymers, space-filling gels, and as a means of stopping blood flow in accident victims and surgery patients.
This guide is licensed under the GNU Free Documentation License. It uses material from the Wikipedia.
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