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Wikipedia:

“Hagfish are long and vermiform, and can exude copious quantities of a milky and fibrous slime or mucus from some 100 glands or invaginations running along their flanks.[10] The species Myxine glutinosa was named for this slime. When captured and held, e.g., by the tail, they secrete the microfibrous slime, which expands into up to 20 litres (5¼ US gallons) of sticky, gelatinous material when combined with water.[11] If they remain captured, they can tie 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 and freeing them from their captor. This singular behavior may assist them in extricating themselves from the jaws of predatory fish or from the interior of their own “prey”, and the “sliming” might act as a distraction to predators.

Recently, the slime was reported to entrain water in its microfilaments, 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 there are few marine predators that target the hagfish. Other predators of hagfish are varieties of birds or mammals.[12]

Free-swimming hagfish also “slime” when agitated, and later clear the mucus utilizing the same travelling-knot behavior.[13][14] The reported gill-clogging effect suggests that the travelling-knot behavior is useful or even necessary to restore the hagfish’s own gill function after “sliming”.

Hagfish slime is under investigation as an alternative to spider silk for use in applications such as body armor.[15]”