cross-posted from: https://lemmygrad.ml/post/4414923

Hagfish, of the class Myxini and order Myxiniformes, are eel-shaped jawless fish. Hagfish are the only known living animals that have a skull but no vertebral column, although they do have rudimentary vertebrae. Eel-like in shape, hagfishes are scaleless, soft-skinned creatures with paired thick barbels on the end of the snout. Depending on the species, they grow to about 40 to 100 cm (16 to 40 inches) long. Primitive vertebrates, hagfishes have a tail fin (but no paired fins) and no jaws or bones. Preferring deep, soft mud habitat, hagfish are found at depths of 53 to 3,788 ft (16 to 1,155 m) (Love and Passarelli 2020) but are more common in depths less than 1,200 ft (366 m) (Love 1996). California fishermen usually catch hagfish in depths less than 1,800 ft (549 m). They prey on small invertebrates living in the mud; they also scavenge dead and dying fish. Hagfish are noted for their unusual way of feeding — they slither into dead or dying fishes and eat them from the inside out, using their "rasping tongue" to carry food into their funnel-shaped mouths. Hagfish are a popular food item for sea lions, seals, dolphins, porpoises, octopus…and people. Hagfish can be 25 to 50% of some predator's diets. Hagfish aren't as attractive as their name implies. One interesting thing about Hagfish, when threatened, can exude copious quantities of a milky and fibrous slime or mucus, from specialized slime glands. When released in seawater, the slime expands to 10,000 times its original size in 0.4 seconds. This slime that hagfish excrete has very thin fibers that make it more durable and retentive than the slime excreted by other animals. The fibers are made of proteins and also make the slime flexible. If they are caught by a predator, they can quickly release a large amount of slime to escape. If they remain captured, they can tie themselves in an overhand knot, and work their way from the head to the tail of the animal, scraping off the slime and freeing themselves from their captor. Rheological investigations showed that hagfish slime viscosity increases in elongational flow which favors gill clogging of suction feeding fish, while its viscosity decreases in shear which facilitates scraping off the slime by the travelling-knot.