Among the many mysteries hidden beneath the waves, few creatures are stranger than the hagfish. Looking more like an eel than a fish, this ancient survivor has puzzled scientists for centuries. How could such a slow, apparently defenceless animal persist in oceans filled with swift and powerful predators? The answer, it turns out, is one of the most astonishing defensive strategies in the animal kingdom—a weapon made almost entirely of seawater.
For centuries, hagfish have been regarded as little more than deep-sea scavengers—primitive, jawless fishes that spend their lives cleaning up dead animals on the ocean floor. But underwater video footage has revealed a very different story. Far from being passive scavengers, hagfish are active predators equipped with one of the most extraordinary attack and defence systems in the animal kingdom.
Researchers working off the coast of New Zealand deployed baited, remote underwater video cameras to observe life on the seafloor. What they captured was remarkable. When sharks and other predatory fish tried to prey on a hagfish, the attack ended almost instantly.
Within less than half a second, the hagfish discharged jets of slime directly into the predator’s mouth. The expanding mucus clogged the predator’s gills, causing it to gag violently and release its prey. The attackers could be seen convulsing their gill covers in frantic attempts to clear the sticky mass, while the hagfish calmly moved away and continued with its own meal.
When hagfish get stuck in their own slime, they simply remove it by tying themselves in a knot (above), which they slide along their bodies to squeeze the mucus from their gills. The researchers’ study described this defence against both sharks and predatory fish, providing the first direct evidence that hagfish slime functions exactly as scientists had long suspected: by blocking the water flow across the gills of predators.
A defensive weapon unlike any other
Hagfish possess between about 90 and 200 slime glands running along each side of their bodies. When threatened, the glands at the site of the attack discharge, firing concentrated jets of mucus into the predator’s mouth.
The slime consists largely of seawater held together by microscopic protein threads and mucus molecules. Once released, it expands almost instantly into an enormous volume of sticky gel that is capable of overwhelming the delicate gill structures of fish.
Not just scavengers
Perhaps even more surprising was another discovery captured by the underwater cameras. Instead of gathering around the bait like scavengers, several slender hagfish ignored it completely and began searching the seafloor around nearby burrows. Using sensitive barbels around their mouths, they explored the sediment until one suddenly plunged headfirst into a burrow. After several minutes, the hagfish emerged, dragging out a red bandfish, the first direct observation of active hunting by any hagfish species.
The researchers believe the hagfish probably gripped its prey using its powerful tooth plates, before incapacitating it with slime inside the confined burrow. By tying its body into an overhand knot, the hagfish gained enough leverage to pull the fish from its refuge—a remarkable example of how these ancient animals compensate for their lack of jaws. While the role of slime in subduing the prey remains a hypothesis, the observations suggest hagfish use it for hunting as well as defence.
Survivors from another age
Hagfish occupy a unique place in vertebrate evolution. They belong to one of the oldest surviving lineages of fishes, with ancestors stretching back more than 300 million years. Their basic body design has changed little over that immense span of time.
The findings on how they use slime suggest that their evolutionary success may owe much to an unusual combination of abilities: a highly effective slime defence, powerful feeding structures, knotting behaviour that provides mechanical leverage, and a flexible lifestyle that includes both scavenging and active predation. Rather than being simple scavengers, hagfish appear to be versatile hunters, capable of exploiting a variety of food sources while possessing a defensive system unlike anything seen in other fishes.
For divers fortunate enough to encounter one, the hagfish is no longer simply one of the ocean’s strangest animals. It is one of its most ingenious survivors.
Ethologist Ila France Porcher, author of Yes, Fish Feel Pain, The True Nature of Sharks and six other books on wildlife behaviour, spent 15 years closely observing wild fish and sharks in Tahiti, resulting in several scientific papers. Her writings are based on decades of first-hand observations of wildlife and focus on individuality and intelligence, challenging traditional views of animal minds. Her work has been featured on Shark Week, in scientific discussions, conservation debates and international media for its unique blend of field observation, art and science.
