Hagfishes

Hagfishes, or slime eels, are a small group of jawless marine vertebrates in the order Myxiniformes, within the class Myxini. They are among the most primitive living vertebrates. Adults lack jaws, true vertebrae, and paired fins; instead they have a cartilaginous skull, a flexible notochord, and a ventral mouth equipped with keratinous tooth-like structures on a rasping tongue. They have multiple gill openings along the sides of the body and a slender, eel-like form.

They lack scales and have a relatively simple visual system, relying instead on chemosensory and tactile cues. Hagfishes secrete copious slime from specialized glands when threatened; the slime can swell in water and clog the gills of potential predators, serving as a defense. Their skin and body are adapted for an eel-like lifestyle, and they possess a reduced or degenerate eye.

Ecology and behavior: Hagfishes are primarily scavengers and detritivores, feeding on dead or dying vertebrates and some invertebrates. They can invade carcasses through natural openings and extract tissue by tying themselves into knots for leverage. They are typically found in cold, marine environments worldwide, from shallow coastal waters to deep sea, with many species adapted to deep-water habitats.

Reproduction and life history: Reproductive biology is poorly understood. Eggs are large and yolk-rich, and the specifics of fertilization and development remain largely undocumented, varying among species.

Classification and relation to other fishes: Hagfishes are not true eels; they are jawless vertebrates in the class Myxini and order Myxiniformes. They represent one of the most primitive lineages of living vertebrates and play a key role in studies of vertebrate evolution.