Hagfish, of the class Myxini (also known as Hyperotreti), are eel-shaped, slime-producing marine fish (occasionally called slime eels). They are the only known living animals that have a skull but no vertebral column, although hagfish do have rudimentary vertebrae. Along with lampreys, hagfish are jawless; they are the sister group to jawed vertebrates, and living hagfish remain similar to hagfish from around 300 million years ago.
Its skin is only attached to the body along the center ridge of the back and at the slime glands, and is filled with close to a third of the body's blood volume, giving the impression of a blood-filled sack. It is assumed this is an adaptation to survive predator attacks. The Atlantic hagfish, representative of the subfamily Myxininae, and the Pacific hagfish, representative of the subfamily Eptatretinae, differ in that the latter has muscle fibers embedded in the skin. The resting position of the Pacific hagfish also tends to be coiled, while that of the Atlantic hagfish is stretched.
Recently, the slime was reported to entrain water in its keratin-like intermediate filaments, creating a slow-to-dissipate, viscoelastic substance, rather than a simple gel. It has been proven to impair the function of a predator fish's gills. In this case, the hagfish's mucus would clog the predator's gills, disabling their ability to respire. The predator would release the hagfish to avoid suffocation. Because of the mucus, few marine predators target the hagfish. Other predators of hagfish are varieties of birds or mammals.
The origins of the vertebrate nervous system is of considerable interest to evolutionary biologists, and cyclostomes (hagfish and lampreys) are an important group for answering this question. The complexity of the hagfish brain has been an issue of debate since the late 19th century, with some morphologists believing that they do not possess a cerebellum, while others believe that it is continuous with the midbrain. It is now believed that the hagfish neuroanatomy is similar to that of lampreys. A common feature of both cyclostomes is the absence of myelin in neurons.
Depending on species, females lay from one 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. If this constitutes actual breeding behavior is uncertain.
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 sea life make easy prey for them.
In most of the world, Hagfish are not often eaten. In Korea, the hagfish is a valued food, where it is generally skinned, coated in spicy sauce, and grilled over charcoal or stir-fried. It is especially popular in the southern port cities of the peninsula, such as Busan. The inshore hagfish, found in the Northwest Pacific, is eaten in Japan. As hagfish slime binds vast amounts of liquid even at low temperatures, it was proposed as an energy-saving alternative for the production of tofu that does not require heating.