There are about 80 species of hagfish, some known only from very few specimens, and doubtless
more remain to be discovered. The most studied species is Myxine glutinosa, the Atlantic Hagfish
which grows up to 95 cm in length but the Goliath Hagfish (Eptatretus goliath) reaches 127.5 cm.
Eptatretus species occur in the Pacific and are seven-gilled hagfishes. Other genera include:
Paramyxine, Notomyxine, Neomyxine and Nemamyxine. Hagfish are eel-like but lack paired fins,
having only the caudal (tail) fin. They generally have 3 or 4 pairs of anterior tentacles, depending
on species, the top and backmost nasal tentacles, then the oral (preoral) tentaclesand finally the
labial tentacles nearest the mouth. These tentacles have both a mechanosensory and a
chemosensory function. The single pair of eyes are reduced and situated beneath the translucent
skin on the back of the head behind the oral region and infront of the gill-bearing branchial region,
visible as pigmented spots.
Hagfish are agnathans: jawless fishes. They lack true jaws, in fact they lack bone altogether.
Instead they have a toothplate (dental plate) bearing teeth made of keratin (the same protein in
the dead outer layers of mammalian skin, hair and horns). Hagfish are classed as vertebrates, but
are not vertebrates in the strict sense for they lack vertebrae. However, they are chordates since
they possess a notochord or stiffened rod running along their backs, just beneath the spinal cord.
The hagfish notochord is a pressurised tube, consisting of a core cylinder of pressurised
vacuolated cells surrounded by a thin but dense fibrous collagenous sheath. A basement
membrane (a thin membrane of glycoproteins) forms a tube around the core, immediately inside
the fibrous sheath. In mammals the notochord of the embryo is replaced by vertebrae and persists
in the centres of the intervertebral discs until about age 4 years old in humans, when it is replaced
by cartilaginous cells.
Hagfish swim by eel-like undulations (at speeds under 2 knots). The notochord is a stiff elastic rod
which stores elastic energy during swimming: when the muscles contract to bend the body, the
notochord springs back to facilitate the undulation. Despite having no vertebrae, hagfish are the
closest such chordate to the vertebrates, according to phylogenetic trees and lie right on the
border between invertebrate and vertebrate.
Despite their outwardly simple wormlike form, hagfish have some remarkable equipment. We begin
by looking at their circulatory system.
The main heart is the branchial heart, so called because it supplies the gills behind which it is
situated. This pump consists of three chambers arranged in series. First, deoxygenated blood from
head region (via the right anterior cardinal vein), from the liver (via the portal vein) and from the tail
region (via the caudal vein) enter the large sinus venosus, which is divided into anterior and
posterior sub-chambers by a constriction. This empties into the single atrium which then empties
into the single ventricle. The ventricle then empties into the aorta which supplies the gills. These
pumps do not empty completely when they contract, but they are sufficiently efficient to empty more
than half the blood they contain.
The circulatory system of hagfishes is a semi-open system, meaning that blood is not confined to
definite vessels (lined by endothelium) but flows into sinuses where it bathes the tissues directly.
Such sinuses occur around the gills and under the skin. Even in humans, which have a closed
circulation, there are sinuses in some organs, such as the liver, but in the hagfish they are much
more extensive. This gives hagfishes a low blood pressure but also allows them to accommodate a
very large blood volume in the expansive sinuses, in fact they have the highest blood volume to
body mass of any chordate, with 17 ml of blood per 100 g of mass, which would be equivalent to a
70 kg human holding about 12 litres of blood rather than the usual 5. The sinuses hold about 30%
of this blood which is able to absorb oxygen across the skin (which has no scales in hagfish). In
well oxygenated waters hagfish can absorb enough oxygen across their skin to survive even if the
nostril is blocked.
Myxine glutinosa is a pinkish-grey colour when at rest, turning a bright pink when active and
turning deep purple under extreme exertion. This is due to changes in the colour of the blood as it
loses oxygen. The sinuses under the skin (subdermal sinuses) give hagfish a loose fitting skin,
likened to a sock.
Accessory hearts are used to maintain the flow of blood through these extensive sinuses, albeit at
low pressure. A portal heart pumps blood coming from the gut, delivered in the posterior cardinal
vein and containing the products of metabolism, to the liver for processing. Two cardinal hearts
propel blood from the extensive head sinuses, though these pumps are apparently operated by
extrinsic muscles (those operating the velum, described below), lacking muscles of their own and
hence have been referred to as 'propulsors' rather than 'pumps'. Two caudal hearts help propel
blood from the sinuses in the tail.
Hagfish have 5 to 15 pairs of gills, depending on the species (though individuals of a species may
have anomalous numbers of gills). In Myxine glutinosa these gills open via a single pair of lateral
pores, behind the last pair of gills, but in other species each gill may have its own opening. The
gills are situated in the pharyngeal (throat) region, referred to as the branchial region. Water
enters the single large nostril (nasohypophasial aperture) at the anterior end of the animal (part
of the prebranchial region which extends from the tip of the snout to the first pair of gills). Where
the nasal passage, running from the nostril, meets the pharynx or throat, there is a remarkable
mechanical structure called the velum which acts as a respiratory pump. (Lampreys also have a
The velum is an elongated structure hanging down from the roof of the pharynx (resembling an
upside-down T in cross-section) with two rollable elongated wings, one on either side. The velum
folds furl and unfurl, propelling water back through the pharynx and over the gills. The downward
unfurling of the scrolls propels water down and back through the pharynx and over the gills
(Johansen and Hol, 1960). From the pharynx an afferent duct branches off to supply each gill
which is enclosed in its own gill pouch, forming the 'gill body'. Although the velum is the main pump,
the gill pouches have muscular walls which undergo peristalsis to assist the flow over the gills. The
gills differ from those in other fish, even the closely related lampreys, in consisting of a series of
multiple folds (folds bearing smaller folds to the sixth order, greatly increasing their surface area).
Two valves or sphincters in the afferent duct leading to each gill pouch (one sphincter at each
end) contract and close when the gill pouches contract, ensuring that the water flows over the gills
and out via the efferent duct.
Above: the respiratory tract of hagfishes. The unfurling of the velum scrolls propels water back
down the pharynx (oesophagus) and down the afferent ducts leading to each gill pouch.
Contraction of the gill pouch then assists the flow of water over the gill folds and out through the
efferent duct. In some hagfish each gill pouch opens to the outside via its own lateral gill pore,
whilst in others, such as Myxine, the efferent ducts unite into a pair of common ducts, each of
which opens via a single lateral gill pore behind the last gill at the back of the branchial region. The
efferent gill duct also contains a single sphincter at its base near the gill pouch and peristalsis also
occurs in the efferent duct. Rings of cartilage around the gill pores prevent them from closing when
the water is expelled. Any excess water or detritus carried in through the nostril exit via the
oesophago-cutaneous duct which connects the pharynx to the outside behind the last gill sac on
the left side only. It is confluent with the left (rearmost) gill pore which therefore appears enlarged.
Hagfish often burrow into soft sediments, forming U-shaped burrows with the prebranchial region
protruding to draw in water for respiration through the nostril.
Hagfish feed largely upon small bottom-dwelling (benthic) invertebrates, but will also hunt small fish
and feed parasitically on larger fish (it is presumed that these fish are already weakened and
vulnerable) or by scavenging dead carcasses. Whale carcasses have been seen crawling with
large hagfish. The hagfish lacks true jaws (true jaws operate like a clamp or vice) but has a
toothplate (dental plate) capable of piercing the scaly skin of other fish. The toothplate bears a
number of teeth, arranged in two rows on either side, which are not true teeth since they are
composed of keratin protein (the same protein in hair, horns and antlers) rather than bone-like
dentine. The toothplate slides along a plate of cartilage by the action of pulley-like muscles, rather
as if it was on a reversible 'conveyor-belt'. When retracted (by contraction of the retractor muscle)
the toothplate folds up vertically with the teeth facing inwards but when it is protracted or extended
(by contraction of the protractor muscle) it folds out flat, exposing the rasping teeth. By rapid
alternate retraction-protraction (a repeating grasp-retract-release-extend cycle) the teeth can
rapidly rasp away a fishes skin and when they retract the teeth pull flesh into the mouth.
A single keratinous palatine tooth hanging down from the roof of the buccal cavity also assists in
pulling food inwards. The hagfish may also knot its body and slide the knot forwards to the mouth in
order to get leverage to prize off chunks of flesh. Mucus and head movements force food into the
intestine where it is digested. Hagfish may completely bury themselves in a rotting carcass and it is
likely that they also absorb some nutrients across their skin into teh blood sinuses.
The head is supported by a skull of cartilage and fibrous connective tissue which form a continuous
structure. The brain is encased in a fibrous sheath and further partially enclosed in cartilage. The
main cartilages in Myxine are:
1. Dorsal longitudinal palatine: this extends forwards from the front of the braincase to form the roof
of the buccal cavity and ends in the cornual process which supports the cartilage of the tentacles.
2. Subnasal bar: situated above the cornual process of the dorsal longitudinal palatine and beneath
rings of cartilage in the wall of the nasohypophyseal canal. These rings keep the respiratory nasal
passage open (rather like the cartilage rings in the human trachea or windpipe).
3. Auditory capsules (x 2): these support the sensory structures of the ear and form a single
semicircular canal for the detection of the angular acceleration of the head (lampreys have a pair of
orthogonal (at right angles to one another) semicircular canals and most vertebrates have three).
4. Nasal capsule
5. Palatine commissure: situated at the end of the palatine bar, this cartilage supports the palatine
6 Tongue (lingual) skeleton: divided into anterior, middle and posterior segments. The anterior
segment consists of a medial and two lateral bars. The middle segment supports the first and third
branchial arch (which offer support for branchial region). The second gill arch is borne on the first
and extends to the posterior of the velar plate.
7. Velar plate: cartilaginous rods which supports the velum and provide attachment for the muscles
of the velum which cause it to furl and unfurl.
There are no bones or ribs in the rest of the body and no cartilage (the only cartilage outside the
skull mentioned in the literature are the rings which keep the gill pores open). A notochord runs
along the back of the hagfish. A notochord is a stiffened elastic rod which aids swimming by storing
elastic energy when the swim muscles bend the tail to help the tail move back in the opposite
direction, greatly improving the efficiency of swimming. In Myxine glutinosa the notochord is a
pressurised tube, the core of which consists of large vacuolated cells, maintaining the cells in an
infalted and turgid state to maintain stiffness. These cells are bolted to one another by
desmosomes (essentially protein rivets that bolt adjacent cells together strongly). A thin, but dense
collagenous sheath enclosed this cellular core. The spinal nerve cord is directly above the
notochord and supported by it.
In mammals, inclusing humans, the notochord develops as an embryonic structure and becomes
replaced by the bony vertebrae, but persists in the centre of the intervertebral discs until about the
age of 4 years when the notochord cells begin to be replaced by cartilage cells. In Amphioxus, the
notochord consists of transverse plates of striated muscle enclosed in a cartilaginous sheath.
The functional (mesonephric) kidney extends nearly the length of the body cavity and has 30 or
more renal corpuscles along its length. The renal corpuscles filter blood into a Bowman's capsule
which connects via a very short (non-ciliated) tubule to the excretory duct (archinephric duct) which
ends in a urinary sinus which opens to the outside via the excretory pore situated on the urogenital
papilla. A small pair of pronephric kidneys (embryonic kidneys) is retained. Each of these has one
renal corpuscle with three glomeruli (knots of capillaries where filtration of the blood occurs in a
functional kidney). These kidneys have ciliated tubules which open into the pericardium (the
fluid-filled coelomic cavity around the branchial heart) but they do not connect to an excretory duct
and so have an unknown function, but are not thought to function as normal kidneys in blood
filtration. (It has been speculated that they may have a lymphatic function or manufacture blood
The kidneys filter the blood, but there is no effective osmoregulation: hagfish are osmoconformers,
meaning that they swell in dilute seawater and shrink in hypersaline water as water moves freely
across their skin.
Hagfish are renowned for their ability to produce massive volumes of strong sheet-like slime very
quickly when agitated! A hagfish in a bucket of water can gel the water solid by slime secretion. The
slime is released from slime glands, arranged in two ventrolateral rows along the length of the
animal from behind the front of the branchial region to the tip of the tail. Each gland opens via a
visible slime pore. These glands contain gland thread cells (GTCs) which contain protein threads
up to 15 cm long which give the slime its strength and fibrous nature, and gland mucus cells
(GMCs) which secrete mucus which swells on contact with water and helps stretch out the threads
These cells secrete their contents in a holocrine manner, meaning that when the gland contracts
and empties its contents through the slime pores, the cells lose their membranes and their contents
are expelled. The mucus is released inside disc-shaped vesicles (7 micrometres or 7 millionths of a
metre in diameter), bounded by phospholipid bilayer membranes possibly containing water
conducting protein channels (aquaporins) which allow water to rapidly enter the vesicle, swelling
and bursting it. This seems to be under the control of calcium ions which may be transported into
the vesicle from sea water by calcium transporter proteins in the vesicle membrane. It takes only
about 0.1 s for the released vesicles to swell and rupture.
The slime is used in defense: if a shark bites a hagfish then its mouth will become immediately filled
with slime, the strong fibres of which clog the gills of sharks, causing them to immediately disengage
and attempt to shake the slime free! Hagfish use their ability to tie themselves into sliding knots to
sweep their bodies clean of slime and other dirt.
Article created: 11 Mar 2018.