Description:<br />
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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.
They are part of the subphylum Vertebrata so,
taxonomically speaking, they are vertebrates.
They do not have vertebrae so, anatomically, they're
not vertebrates.
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.
Species
About 64 species are known, in 5 genera. A number of the
species have only been recently discovered, living at depths
of several hundred metres. Some of the species are listed
here:
Nemamyxine elongata Richardson, 1958
Nemamyxine kreffti McMillan and Wisner, 1982
Genus
Neomyxine:
Neomyxine biniplicata (Richardson and
Jowett, 1951)
Genus
Notomyxine:
Notomyxine tridentiger (Garman, 1899)
Genus
Paramyxine:
Paramyxine atami Dean, 1904
Paramyxine cheni Shen and Tao, 1975
Paramyxine fernholmi Kuo, Huang and Mok, 1994
Paramyxine sheni Kuo, Huang and Mok, 1994
Paramyxine wisneri Kuo, Huang and Mok, 1994
Genus
Quadratus:
Quadratus ancon Mok, Saavedra-Diaz and Acero P.,
2001
Quadratus nelsoni (Kuo, Huang and Mok, 1994)
Quadratus taiwanae (Shen and Tao, 1975)
Quadratus yangi
References
J.M.
Jřrgensen, J.P. Lomholt, R.E. Weber and H. Malte (eds.)
(1997).
The biology of hagfishes. London:
Chapman & Hall.
Delarbre et al
(2002). "Complete Mitochondrial DNA of the Hagfish,
Eptatretus burgeri: The Comparative Analysis of
Mitochondrial DNA Sequences Strongly Supports the
Cyclostome Monophyly". Molecular Phylogenetics and
Evolution22 (2): 184-192.
Bondareva and
Schmidt (2003). "Early Vertebrate Evolution of the
TATA-Binding Protein, TBP". Molecular Biology and
Evolution20 (11): 1932-1939.
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