Fish
Hagfish
Myxini · also called hagfish, slime eel
The hagfish is a living relic — a boneless, jawless, eel-shaped animal from an early branch of the vertebrate tree, older than any fish with jaws. It scavenges dead whales from the inside, ties itself in knots, and, when something tries to eat it, turns the surrounding water into slime.
Overview
Hagfish — about 80 species — live on cold and temperate seabeds worldwide, from coastal mud to the deep sea. They are nocturnal, spend much of their time burrowed in sediment, and locate food by smell and touch with barbels around the mouth; their eyes are reduced to light-sensing patches. They are jawless, feeding instead with two plates of rasping, tooth-like structures that pinch together.
Not quite a fish
A hagfish is a jawless craniate — it has a skull but only a cartilage-reinforced notochord, not a true backbone. With the lampreys, it is one of the only living jawless vertebrates and represents the earliest branch of the vertebrate lineage, from before jaws, bone, and paired fins. It also has four hearts, no stomach, and can absorb nutrients through its skin.
The slime
Along its body a hagfish has about 100 pores, each fed by a gland packed with coiled protein threads and mucus. Threatened, it fires these into the water; the packets burst, the threads tangle, and the mucus swells, converting up to 20 litres of seawater into thick slime in under a second. A predator that bites gets its mouth and gills clogged and lets go, and sharks quickly learn to leave hagfish alone. The threads are studied as a template for strong sustainable fibres.
The knot
A hagfish routinely ties itself into an overhand knot and slides it along its body — to scrape off its own slime before it suffocates, to brace and gain leverage while tearing at a carcass, and to lever itself out of a predator’s grip. Being boneless makes it possible.
The clean-up crew
Hagfish are key deep-sea scavengers, burrowing into sunken fish and whale carcasses and eating them from within, recycling large dead animals on the ocean floor.
Status
Most hagfish species have not been individually assessed and are collectively Least Concern, but several are heavily fished — for meat, especially in Korea, and for the “eel-skin” leather trade — and those populations are declining, as hagfish are slow-growing and produce few eggs.
Hagfish: common questions
Is a hagfish a fish?
It depends on how strictly you define "fish." A hagfish is a vertebrate, or very nearly one — it has a skull but only a rudimentary backbone (a notochord with cartilage elements rather than true vertebrae). It is a jawless craniate, and along with the lampreys it represents the earliest branch of the vertebrate family tree, split off before jaws, paired fins, and bone evolved. So it is "a fish" in the loose sense of an aquatic gilled vertebrate, and it is often studied as a window into what the very first vertebrates were like, but it is not a fish in the same sense as a trout.
How does the slime work, and how much is there?
Along its flanks a hagfish has roughly a hundred slime pores, each connected to a gland that stores tightly coiled protein threads (thousands of them, each up to 15 cm long when unwound) packaged with mucus into tiny cells. When the hagfish is threatened it fires these into the water, where the packets burst, the threads unravel and tangle, and the mucus swells with seawater. The result is that a single hagfish can convert a bucket — around 20 litres — of seawater into thick, cohesive slime in under a second. It is one of the fastest and most dramatic defensive secretions known, and the threads are being studied as a model for strong, sustainable synthetic fibres.
Why does a hagfish tie itself in knots?
The knot is a multipurpose tool. After sliming, the hagfish forms an overhand knot near its tail and passes it forward along its body, scraping the slime off so it does not suffocate in its own secretion. When feeding, it anchors its teeth in a carcass and forms a knot to brace against, using the leverage to tear off a chunk. And if a predator grabs it, it can knot up and push against the predator's mouth to lever itself free. Being boneless and extremely flexible makes all of this possible.
What do hagfish eat, and why do they matter?
Hagfish are the ocean's clean-up crew for large dead animals. When a big fish or a whale sinks to the seabed, hagfish arrive, burrow in through the mouth, gills, or anus, and consume it from the inside out, absorbing some nutrients directly through their skin and gills as they do so. They also eat marine worms and small live prey in the mud. By rapidly recycling carrion they keep the deep-sea floor functioning, and a "whale fall" can support hagfish and other scavengers for months.