Introduction

The weird and wonderful hagfish has managed to capture the interest of scientists since the 1700s but we still know very little about this animal. The hagfish was scientifically described in 1753 by one of the disciples of Carl von Linné, a Swede named Per Kalm. In 1747, Per Kalm left the Swedish town of Uppsala. His ultimate destination was North America, but boat repairs forced him to stay a while in Grimstad, Norway where he found hagfish. Thinking it was a blind lamprey, Kalm wrote down his observations and later shared them with Linné. The description can be found in his “En resa till Norra America” (A Journey to Northern America) from 1753. Linné discussed the strange “lamprey” with Norwegian Bishop and naturalist Johan Ernst Gunnerus and they both classified it as a worm. In 1763, Gunnerus wrote about the creature and he then referred to it as a Sleep-Marken. During last decade of the 18th century the animal got its own scientific name – Myxine glutinosa – and was placed in the roundmouth family. Today we know that Myxine glutinosa, commonly known as the Atlantic Hagfish,is only one of many species in the large hagfish family Myxinidae. There are currently 67 described species of hagfish and the number might very well increase as the bottoms of our oceans become more thoroughly explored.  

Quick hagfish facts

Geographical range: temperate regions in both hemispheres
Environment: marine waters
Reproductive guild: nonguarders
Largest species: Eptatretus goliath (1275 mm / roughly 50 inches)
Smallest species: Myxine pequenoi (183 mm / roughly 7 ¼ inches) and Myxine kuoi (187 mm / roughly 7 ¼ inches

Taxonomy

Kingdom:     Animalia
Phylum:       Chordata
Class:           Myxini
Order:          Myxiniformes
Family:         Myxinidae

The Hagfishes are found in the family Myxinidae which contains 72 described species divided into 6 different genera.

List of genera in the family Myxinidae

Eptatretus
Myxine
Nemamyxine
Neomyxine
Notomyxine
Paramyxine
Quadratus

List of species in the family Myxinidae

Eptatretus bischoffii
Eptatretus burgeri                                     Inshore hagfish
Eptatretus caribbeaus
Eptatretus carlhubbsi                                Giant hagfish
Eptatretus chinensis
Eptatretus cirrhatus
Eptatretus deani                                       Black hagfish
Eptatretus eos
Eptatretus fernholmi
Eptatretus fritzi                                        Guadalupe hagfish
Eptatretus goliath                                     Goliath hagfish
Eptatretus grouseri
Eptatretus hexatrema                                Sixgill hagfish
Eptatretus indrambaryai
Eptatretus lakeside
Eptatretus laurahubbsae
Eptatretus longipinnis
Eptatretus mcconnaugheyi                       Shorthead hagfish
Eptatretus mccoskeri
Eptatretus mendozai
Eptatretus menezesi
Eptatretus minor
Eptatretus multidens
Eptatretus nanii
Eptatretus octatrema                                Eightgill hagfish
Eptatretus okinoseanus
Eptatretus polytrema                                Fourteen-gill hagfish
Eptatretus profundus                               Fivegill hagfish
Eptatretus sinus                                        Cortez hagfish
Eptatretus springeri                                  Gulf hagfish
Eptatretus stoutii                                      Pacific hagfish
Eptatretus strahani
Eptatretus  wayuu
Eptatretus wisneri
Myxine affinis                                          Patagonian hagfish
Myxine australis
Myxine capensis                                       Cape hagfish
Myxine circifrons                                     Whiteface hagfish
Myxine debueni
Myxine  dorsum
Myxine fernholmi
Myxine formosana
Myxine garmani
Myxine glutinosa                                      Atlantic Hagfish
Myxine hubbsi
Myxine hubbsoides
Myxine ios                                                White-headed hagfish
Myxine jespersenae                                  Jespersen's hagfish
Myxine knappi
Myxine kuoi
Myxine limosa
Myxine mccoskeri
Myxine mcmillanae
Myxine paucidens
Myxine pequenoi
Myxine robinsorum
Myxine sotoi
Nemamyxine elongata
Nemamyxine kreffti
Neomyxine biniplicata
Notomyxine tridentiger
Paramyxine atami                                     Brown hagfish
Paramyxine cheni
Paramyxine fernholmi
Paramyxine moki
Paramyxine sheni
Paramyxine walkeri
Paramyxine wisneri
Quadratus ancon
Quadratus nelsoni
Quadratus taiwanae
Quadratus yangi

Etymology

The hagfishes belong to the family Myxinidae, in the order Myxiniformes, in the class Myxini. All these names are derived from the Greek word “myxo” which means slime. This is of course a reference to the amazing slime-producing ability exhibited by these fishes.

Geographical distribution and habitat

Hagfish are found in temperate seas in both hemispheres, but hagfish has not been found in the Red Sea. A majority of the species live in rather cold environments where the water is at least 20 meters (66 feet) deep. When hagfish live in warmer parts of the world, they are normally only found in really deep waters. Hagfish can survive at remarkable depths and have been found 1700 metres (5600 feet) down into the ocean.

The hagfish needs the salinity in its habitat to be stable, because these fishes have virtually no osmoregulation and are therefore highly vulnerable to salinity changes. The hagfish is the only known vertebrate with body fluids isosomotic with seawater. This means that the body fluids of the hagfish have the same total osmotic pressure or osmolality as seawater.

The typical hagfish habitat is a muddy bottom and it will often burrow itself into the soft bottom and keep only its head above the mud. They can form dense congregations consisting of up to 15 000 specimens. In many regions, hagfishes are one of the most abundant groups of demersal fishes. (A demersal fish is a fish that lives at or near the sea floor.) Little is known about the life of the hagfishes, but a few studies have been made that indicate that hagfishes do not migrate more than 100 km (62 miles).

Description and characteristics

The body of the hagfish resembles that of an eel, but the hagfish is no true eel. The hagfish lacks jaw, scales, paired fins and dorsal fin. A part of the caudal fin extends onto the dorsal and ventral surfaces.

The hagfish does have a skeleton, but this is made up by of cartilage instead of bones and is limited to a notochord and the cartilaginous rays of the caudal fin. Cartilage is also used to strengthen the tentacles, soft palate, nasopharyngeal duct and “tongue” of the fish. The labyrinth organ and the olfactory organ are both sheltered by capsules consisting of cartilage. The hagfish has neither branchial arches nor any braincase proper; the brain is simply enclosed by a fibrous sheath.

The hagfish is unique among the non-extinct members of the phylum Chordata since it has a partial cranium but no vertebrae. It differs from all other vertebrates by having no cardiac innervation, no radial muscles, no eye lens and no extrinsic eye muscles. The eyes are poorly developed and located under the skin, which makes the fish nearly blind.

The hagfish has a laterally biting mouth and a “rasping tongue”, i.e. a cartilaginous plate equipped with two pairs of comb-shaped horny teeth. The cartilaginous plate is protractable and retractable. One single tooth can be found in the upper part of the mouth.

Four pairs of tentacles surround the mouth and the opening for the nasopharyngeal duct. One of the anatomical features that characterises the hagfish is the duct which leads from the esophagus to the exterior on the left side only and opens behind the rearmost gill opening. This duct is called the oesophagocutaneous duct.

A hagfish can have 1 to 16 external gill openings. When the hagfish breathes, it draws in water through its nasopharyngeal duct instead of using its mouth. The nasopharyngeal duct opens at the anterior tip of the head and proceeds first to the median olfactory organ and then to the pharynx and gill pouches of the fish.

The hagfish is famous for its ability to exude slime and has mucous pores arranged in two ventrolateral lines. Each line consists of roughly 70-200 glands containing both mucous cells and thread cells.

Young hagfishes have both ovaries and testes, but as they age they will become male or female. They are however capable of changing sex from one season to another.

Sliming and knotting

The hagfish is famous for its ability to emit mucus and even its scientific name is derived from the Greek word for slime. One single hagfish can fill a milk jug with slime in no time. The slime is probably a way for the hagfish to fend of predators, since the slime can be used to form a protective slime-cocoon. If a fish tries to eat the cocoon, the slime can clog its gills and make it suffocate. If you try to chew hagfish slime, it will expand.

Hagfish slime is strong and hard to remove, because unlike the other known forms of slime found in nature hagfish slime is reinforced with special fibres. Inside the slime, each fibre is no thicker than a thousandth of a millimetre, but the fibres are very strong and can reach a length of half a metre (1 foot 8 inches). We still do not know how the fibres are put together, but scientists are trying to find out since that knowledge might make it possible to create really strong synthetic materials.  

When the predator is gone and the hagfish needs to get rid of the slime, it simply ties itself into a knot and wipes the slime away by passing the knot down along the body. It will also sneeze to remove the slime from its nostrils. Tying yourself into a knot is not only useful when you need to rid yourself of slime; it can also be used to escape if you have been captured by a predator and to pull on food. 

Ecology

Eat

Earlier, the hagfish was believed to be a scavenger only, but it is actually a night-active hunter as well. The hagfish is famous for its habit of burrowing into dead or dying animals and devour them from the inside. Newer studies have however showed that the hagfish diet consists primarily of marine worms and other invertebrates, not large carcases. A majority of the typical hagfish diet is made up by worms from the Class Polychaeta (bristleworms). This doesn’t mean that hagfish will turn down an opportunity to feast on dead or dying animals; food is scarce deep down in the ocean and the hagfishes are certainly not finicky feeders. Dead and dying fish and other animals are an important source of nutrients for them and the hagfishes play an important role in the ecosystem of the sea by breaking down decaying animals that has sunk down to the sea floor. Hagfish will also happily eat fish caught in fishing nets, sometimes reducing the catch significantly for the fishermen.  

The hagfish has a fairly primitive look and since it was viewed as a form of parasite earlier, scientists suggested that its primitiveness was caused by this parasitic lifestyle. Today, most scientists instead think that the hagfish is so perfectly adapted to its environment that no changes have been necessary during the most recent several hundred million years.

The hagfish is almost blind, but it has not problem detecting food since it is equipped with highly developed senses of touch and smell. No less than four pairs of sensing tentacles are located around its mouth.  

The mouth of the hagfish is perfectly adapted to tearing flesh away from dead and dying animals and is also great when it comes to catching marine invertebrates such as bristleworms. Inside the mouth you will find a single tooth and a tongue equipped with pairs of rasps. Every time the hagfish pulls back its tongue into the mouth, the rasps will pinch together. The hagfish attach its mouth to a fish or other animal and gradually work its way through the animal.  

The metabolism of the hagfish is remarkably slow and up to seven months can pass between meals.

…and be eaten

The hagfish is preyed upon by some marine mammals and large sea living invertebrates. Most animals do however stay away from the hagfish since they risk being suffocated by the slime.

Hagfish and evolution

The hagfishes are believed to have been around for at least 500 million years without changing much and are therefore of great interest for scientists that wish to understand more about how different life forms have evolved on our planet. The hagfish is for instance equipped with an anatomical feature that is believed to be the beginning of a vertebral column. The hagfish has no cerebrum or cerebellum, no jaws and no stomach, and its skeleton is made up by cartilage instead of bones.

Earlier, many scientists assumed that many of the unique anatomical and physiological features found on hagfishes were caused by a slow degeneration brought on by the parasitic lifestyle of these fishes. Today we know that hagfishes are predators and scavengers rather than parasites and there is now a broad consensus over the idea that hagfishes are primitive creatures that probably are quite similar to the common ancestor of all animals with a cranium.

The first animals that evolved a simple skull were probably quite similar to the hagfishes of today. According to this theory, the vertebrates have been evolving separately from the hagfishes during the last 500 million years or so. This does not mean that all anatomical and physiological features found on the hagfishes of today were present in hagfishes 500 million years ago; some biological and anatomical features might very well be the result of development or degeneration caused by their life-style and the environment in which they live. The hard thing is to figure out which features that were there when the vertebrates headed for their own separate path in evolutionary history and which ones that weren’t.  

Hagfishes are very different compared to all the other animals with craniums. They have multiple veinous hearts and no cardiac innervation. They do have acoustico-lateral nerve fibres, but no sensory-line neuromasts. The oxygen affinity of their blood cells is much lower and their pituitary gland is not as complex as in other animals with craniums. While all the other craniates have a body fluid content below 10%, it exceeds 10% in hagfish.  

We still know very little about the history and phylogeny of hagfishes, partly because only one fossilized hagfish has been found. (All the members of a family hail from the same ancestral species and people interested in phylogeny want to learn when the branching took place.) The fossilized hagfish belongs to the now extinct species Myxinikela siroka and was found in Francis Creek Shale in the north-eastern part of Illinois, USA. The fossil dates back to the late Carboniferous era which makes it roughly 330 million years old. It had been protected inside a so called ironstone, which is a concretion of iron carbonate. This fossil is amazingly similar to the living hagfishes of today and it is therefore reasonable to assume that hagfishes have changed very little during the last 300 million years. A second fossil has been found in the same layer of rock as the first one, but this fossil does not have the same distinctive tentacles as all other hagfish species. It is therefore uncertain if it should be considered a hagfish fossil or not.

The hagfish and the scientist

Why is the hagfish important for scientists?

The hagfish have provided important information for scientists engaged in the study of human health problems such as diabetes and cancer. They are also used to study areas such as digestion, kidney function and brain development. By looking at a primitive animal that haven’t changed much in several hundred million years, we can develop a deeper understanding of the complex organs found in our own bodies.

Hagfishes have the most primitive form of insulin known to science

The hagfish has for instance been used by researchers studying the islets of Langerhans, a special type of pancreatic cells responsible for producing insulin. Hagfishes are of special interest since their insulin is the most primitive form of insulin known to science. In hagfish, insulin is produced in a small organ named the islet organ which is located on the bile duct wall at the section where the bile duct opens into the bowels. By learning more about this primitive form of insulin, researchers hope to understand more about how the insulin production works in the human body and why it sometimes malfunctions.

Hagfishes might serve as canaries for cancerogenous compounds

As a by-product of the early diabetes research with hagfish, scientists discovered that even primitive species can develop cancer. During their experiments with hagfish insulin, scientists working at the Uppsala University in Sweden noticed unusual knots in the livers of their hagfishes. These knots turned out to be tumours, including cancer tumours. Further research showed that 1-8 % of the hagfishes had cancer tumours in the liver or in the inslet organ. As the scientists continued to dig deeper into the mystery, they made another even more interesting discovery: all the hagfishes with cancer had been caught in the Gullmar fjord (Gullmarsfjorden) in southern Sweden, while the cancer-free specimens had been caught out in the open sea. Then, in the early 1970s, the cancer rate for hagfishes caught in the Gullmar fjord suddenly dropped dramatically and after a while the tumours disappeared altogether. According to one of the scientists involved in the study, Professor Emeritus Sture Falkmer, the sudden disappearance probably had something to do with how Sweden banned the use of chlorinated pesticides and chemicals such as DDT and PCBs at this time, and the fact that sewage treatment plants were built in the Gullmar fjord.

Since the hagfish feed on carcases, it might be able to worn us about cancerogenous compounds in the environment. It is a well known fact that many dangerous pollutants congregates along the food chain, and a scavenger like the hagfish will therefore ingest large amounts of pollutants. Today, primary liver cancer is rare in humans living in Western Europe, but fairly common in Africa and China where the environmental laws tend to be less strict or poorly enforced. 

Hagfishes help us understand the evolution of the eye

For many years, scientists have tried to understand how a primitive eye only capable of distinguishing light from dark could evolve into the extremely complicated eyes that humans – and many other animals – are equipped with today. Even Charles Darwin was puzzled by the intricate mechanisms found in the human eye and how this remarkable body part could have developed from a simple light/darkness sensor. According to Professor Shaun Collin at the University of Queensland and his colleagues Professor Trevor Lamb at the Australian National University and Professor Ed Pugh at the University of Pennsylvania, the hagfish might help us shed some light on this age-old mystery. Together, these scientists have identified animals equipped with eyes that bridge the evolutionary gap between the two types of eyes described above and the hagfish is one of them.
According to Professor Collin, a clear progression from a very primitive eye to a highly complex eye seems to have taken place in no more than 30 million years. This might sound like an awful long period of time, but in evolutionary terms it is little more than an afternoon.  Professor Collin has studied the hagfish and the primitive eye-like structure that these fishes have positioned beneath an opaque eye-patch on both sides of the head. Earlier, most scientists thought that the hagfish had degenerated from a precursor akin to the lamprey eel and that the eyes had deteriorated in the process. According to Professor Collin, it is more plausible that the hagfishes are the remnants of an earlier sister group and not a degenerated form of a lamprey-like antecedent. The eye of the hagfish might therefore tell us a lot about how the modern complex eye gradually evolved through the course of evolution.

Hagfish reproduction

We still know very little about how hagfish reproduce, but we do know that each fish contains both ovaries and testes and that the parents do not guard their offspring. One and a half century ago, the Royal Danish Academy of Sciences and Letters (Kongelige Danske Videnskabernes Selskab) announced an award to anyone who could solve the mystery about how the hagfish reproduces. As of 2008, no one has received the award.

As mentioned above, a hagfish will contain both ovaries and testes, but these fishes are functionally non-hermaphroditic. Young fishes are hermaphroditic, but as they age they will become either male or female. The hagfish may however switch sex from season to season if it finds itself in a group where another sex would be more favourable.

When studied, hagfish females have produce up to 30 yolky one inch long eggs with tough shells. If this low number is true for all hagfish, it means that it might take a long time for a hagfish population to recover when harmed, e.g. by over-fishing or pollution.

A one inch long egg with a large yolk requires a lot of energy to produce, but will on the other hand give the offspring a better chance of survival from the start. The really high number of hagfishes found in some areas suggests that hagfish have a low mortality rate, since it would be difficult to form such large populations if a high mortality rate was combined with this low number of eggs per female.

Unlike many other fishes, hagfishes do not have any larval stage; they undergo direct development without any metamorphosis. When the eggs hatch, the emerging fry are already miniature versions of their parents.   

Commercial importance and conservational concern

In most parts of the world hagfish is viewed as a useless by-catch, but there are a few countries in South East Asia where hagfish is appreciated on the dinner table. Nearly 5 million pounds (2 268 000 kg) of hagfish meat is for instance consumed in South Korea each year.

The tough and soft skin of the hagfish is also a popular commodity and is used to make wallets, purses, handbags, boots and similar items. The skin is normally market under the name “eelskin” or “eel skin”, not hagfish skin. 

In Asia, over fishing has led to a significant decrease in the local hagfish population and Asian fisheries have therefore begun to show an interest in the hagfish populations in the Atlantic. While this might be beneficial for local economies along the Atlantic coasts, it could also pose a threat to the Atlantic hagfish population. Care and caution must be exercised unless we wish to see the Atlantic hagfish population go the same way as the Asian one. As mentioned above, studies indicate that the hagfish female only produces a low number of eggs each breeding period and hagfish are therefore extra sensitive to over-fishing.

Today, the hagfish has become of interest for scientists all over the world due to several reasons. Hagfish are believed to have existed for at least 500 million years and might provide us with important clues about our early ancestors and the evolution of life on earth. Secondly, the strange slime that hagfishes produce so freely have turned out to have some remarkable qualities which need to be studied further. Hagfish slime is fibre-reinforced and about as strong as spider silk, without having the immensely complex structure of spider silk. Will we see bullet proof vest constructed out of hagfish slime instead of Kevlar in the future? Last but not least, hagfish are studied by scientists who are trying to develop a deeper understanding of health problems such as diabetes and cancer and learn more about how we can prevent and treat them.

Recent addition to the hagfish family

As late as 2006, a new member was added to the hagfish family Myxinidae when the Goliath hagfish was scientifically described by Mincarone & Stewart and given the name Eptatretus goliath. Eptatretus goliath was described from a specimen caught at a dept of 811 metres (2,661 feet) at the head of the Hauraki Canyon off the northeast North Island in New Zealand. The species was named Goliath since it was a true giant in the world of hagfish; the specimen measured an astonishing 127,5 cm (approximately 50 inches). This makes Eptatretus goliath the largest known species of hagfish.    

Just like the other members of the genus Eptatretus the Goliath hagfish is seven-gilled, but it distinguish itself from the rest by having three-cusp multicusps in anterior and posterior rows, 11–13 unicusps in anterior rows, nine unicusps in posterior rows, total cusps 54, 14–15 prebranchial pores, 57–58 trunk pores, 13–14 tail pores, 92 total pores, and a prominent ventral finfold.

References:

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http://www.tolweb.org/Hyperotreti

“Friends of Oceanography Public Lecture Series Explores the Strange, Wondrous, and Disgusting Hagfish”, University of Rhode Island, 2002-03-25. Retrieved on 2008-03-28. 
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http://www.physorg.com/news115919015.html

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http://www.ucmp.berkeley.edu/annelida/polyintro.html

“A New Species of Giant Seven-gilled Hagfish (Myxinidae: Eptatretus) From New Zealand”, BioOne Online Journals, Volume 2006, Issue 2 (May 2006). Retrieved on 2008-03-28. 
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