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Hagfish filmed choking sharks with slime, and actively hunting fish

Hagfish have been swimming in the oceans for 300 million years, and there are 77 species spread all over the world.

Not Exactly Rocket Science
By Ed Yong
Oct 27, 2011 1:00 PMJul 7, 2023 3:39 PM


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The hagfish looks like an easy meal. Its sinuous, eel-like body has no obvious defences, but any predator that moves in for a bite is in for a nasty surprise. The hagfish releases a quick-setting slime that clogs up the predator’s gills, causing it to gag, choke and flee. Scientists have known about this repulsive defence for decades, but Vincent Zintzen has finally filmed it in the wild. His videos also prove that hagfish, generally thought to be scavengers of the abyss, are also active hunters that can drag tiny fish from their burrows.

Hagfish are sometimes classed as fish although that’s in dispute, for they lack both backbones and jaws. Instead, their mouths contain a wide plate of cartilage, armed with two rows of horny teeth. It uses these to rasp away at carcasses that sink from above. Watch a dying whale settle on the ocean floor, and it will soon be covered in writhing hagfishes.

They are disgusting feeders. They burrow deep into corpses and eat their way out, and can even absorb nutrients through their skin. And if they’re threatened or provoked, they produce slime – lots of slime, oozing from the hundreds of pores that line their bodies. The slime consists of large mucus proteins called mucins, linked together by longer protein threads. When it mixes with seawater, it massively expands, becoming almost a thousand times more dilute than other animal mucus.

A single hagfish can clog a bucket of water within minutes, and in 2006, Jeanette Lim showed that the slime can equally clog the gills of predators. But until now, no one had ever seen the animals use this defence against an actual predator. They have mostly been filmed at whale carcasses with remote vehicles; their predators had a glut of whale meat at hand, and may have been put off by the noisy, bright vehicles. Instead, Zintzen filmed hagfish in more natural conditions, using a network of baited cameras. “Our units are not moving, producing minimum noise and using lights emitting only in the blue to avoid deterring the fauna,” he says.

You can see the results below. The hagfish in the videos are attacked by sharks, conger eels, wreckfishes and more. In less than half a second, the predator’s mouth and gills are filled with slime. It leaves, gagging and convulsing, slime hanging in long wisps from its head. Even voracious seal sharks turn tail. The cameras didn’t follow the fleeing predators, so Zintzen doesn’t know if they eventually died or if the slime dissolved away. Either way, the hagfish, uninjured and oblivious, just carried on feeding. Its defence is so effective that it can totally ignore the fact that a shark just tried to bite it.

The slime could also give the hagfish a competitive edge among other scavengers. If many hagfish were feeding off Zintzen’s bait, it soon became draped in slime. The mucus puts off fish competitors, allowing the hagfish to monopolise their morsels.

Zintzen also filmed some hagfish hunting, a behaviour that had been suspected but never witnessed. While they’re typically regarded as scavengers, some scientists have suggested that their numbers are so great that they couldn’t possibly be sustained by corpses alone. On top of that, some people have found the flesh of prawns, worms and fish among the stomach contents of hagfish.

Zintzen filmed slender hagfish chasing after red bandfish, ensconced in sandy burrows. The hagfish completely ignored the bait that Zintzen was offering. Instead, they seemed to search the ocean floor for hidden burrows, using the whisker-like barbels on their faces. Once they found an entrance, they rapidly burrowed inside, emerging several minutes later.

“When I first reviewed this video, I thought: those hagfish are not very clever. They have the bait right above their head and they keep on searching the sediment for it.” Then, Zintzen noticed one particular hagfish that had stuck the front third of its body inside a hole. It twisted its body into a knot, using it for leverage to push against the sediment. Twenty seconds later, it withdrew from the burrow with a red bandfish, dead and motionless, in its mouth. “I then only understood what was actually happening: they were hunting!”

Hagfish are well known for their ability to tie themselves in knots, which can travel down the length of their bodies. This could help to clear their own bodies of slime (they can choke on their own mucus) or free themselves from the grip of a predator. Here, the knot seemed to give the hagfish leverage for pulling the bandfish from its burrow. Zintzen thinks that the hagfish may even have used its mucus as an offensive weapon, to choke the bandfish inside its burrow.

Hagfish have been swimming in the oceans for 300 million years, and there are 77 species spread all over the world. While the jawed fishes have undoubtedly taken over the seas, the hagfishes have clung on. They have a defence that makes them all but untouchable. Their only predators are either very large fish whose gills are too big to clog, or mammals, which don’t have gills and whose stomachs can easily digest or expel the slime. They also have a versatile style of feeding that includes scavenging, opportunistic feeding and active hunting.

“They are fantastic animals, but most probably, you need to be a scientist (and a somewhat strange one) to state this and love them,” writes Zintzen.

Reference: Zintzen, Roberts, Anderson, Stewart, Struthers & Harvey. 2011. Hagfish predatory behaviour and slime defence mechanism. Scientific Reports. http://dx.doi.org/10.1038/srep00131

Images and videos by Zintzen et al.

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