Hagfish

Hagfish are jawless, eel-shaped marine animals, sometimes called slime eels, belonging to the class Myxini and order Myxiniformes. They are the only living creatures with a skull but no backbone, although they do possess basic vertebrae. These fish act as both predators and scavengers in the ocean, and their main defense is releasing large amounts of slime from skin glands. Genetic studies show that hagfish and lampreys—the only other living jawless fish—are more closely related to each other than to jawed vertebrates, placing them together in the superclass Cyclostomi. The earliest known hagfish relatives appear in the fossil record from the Late Carboniferous, about 310 million years ago, while modern forms first show up in the mid-Cretaceous, around 100 million years ago.

Most hagfish are around 50 cm long. The largest species, *Eptatretus goliath*, reached 127 cm, while the smallest, *Myxine kuoi* and *Myxine pequenoi*, top out at 18 cm, and some individuals have been seen as small as 4 cm. Their bodies are elongated and eel-like, with paddle-shaped tails. The skin hangs loosely like a sock, attached only along the back’s center ridge and at the slime glands, and holds nearly a third of the animal’s blood volume, making it look like a blood-filled sack—likely an adaptation against predators. Colors range from pink to blue-gray, sometimes with black or white spots, and they appear worm-like. They have cartilaginous skulls (with the brain area mostly a fibrous sheath) and tooth-like keratin structures. Eyes are simple spots that cannot form images. Instead of true fins, they have six or eight barbels around the mouth and a single nostril. Rather than vertically moving jaws, they possess a pair of horizontal structures with tooth-like projections for tearing food. The mouth holds two pairs of horny, comb-shaped teeth on a cartilaginous plate that can extend and retract to grasp and pull food toward the pharynx. Atlantic hagfish (subfamily Myxininae) and Pacific hagfish (subfamily Eptatretinae) differ: Pacific hagfish have muscle fibers in their skin and tend to rest coiled, while Atlantic hagfish rest stretched out.

When threatened, hagfish release a milky, fibrous slime from specialized glands. In seawater, this slime expands 10,000 times in size within 0.4 seconds. Its very thin protein fibers make it more durable and flexible than other animals’ slime. If caught by a predator, a hagfish can quickly expel slime to escape. If still held, it ties itself into an overhand knot and works the knot from head to tail, scraping off the slime and freeing itself. The slime’s viscosity increases in elongational flow, clogging the gills of suction-feeding fish, but decreases in shear, making it easy to scrape off with the traveling knot. The slime’s keratin-like intermediate filaments, produced by gland thread cells, create a slow-to-dissipate, viscoelastic substance rather than a simple gel. This clogs predator gills, forcing the predator to release the hagfish to avoid suffocation. Few marine predators target hagfish because of this slime; some birds and mammals do prey on them. Even when free-swimming, hagfish slime when agitated and later clear it using the same knot behavior, which may also restore their own gill function. The proteins in hagfish slime filaments (EsTKα and EsTKγ) are being studied as a potential alternative to spider silk for body armor, as they transform from an α-helical to a stiffer β-sheet structure when stretched. With draw-processing and chemical crosslinking, recombinant slime keratin becomes a strong fiber with an elastic modulus of 20 GPa. In 2017, a road accident on U.S. Highway 101 spilled 7,500 pounds of hagfish, producing enough slime to cover the road and a nearby car.

Hagfish breathe by taking water through the pharynx, past the velar chamber, and into internal gill pouches, which number from five to 16 pairs depending on the species. In most species, the gill pouches open individually, but in *Myxine*, the openings have merged, with canals running backward under the skin to form a single branchial opening on the underside. The esophagus connects to the left branchial opening (which is larger than the right) through a pharyngocutaneous duct that lacks respiratory tissue.