Slime fish

Here’s an unusual defensive weapon—slime.

Hagfish have four rows of teeth—two rows on each side of its mouth—that they use to rip into dead or dying animals. Here they are eating a shark, accompanied by an anemone and several brittle stars. Ryan Somma, CC BY-SA 2.0, (adjusted and cropped).

Hagfish are eel-like jaw-less fish that live on the seabed, searching for worms, small invertebrates, fish, and carrion to eat. On finding a carcass or dying animal, it tears open a hole, burrows its way in, and begins eating on the inside, while also absorbing nutrients through its soft, scale-less skin. In captivity, using skin absorption alone they’ve go eleven months without eating. Invertebrates, such as sea stars, corals, and jellies, can also eat by absorption, but hagfish are the only known vertebrate-like animal that can do it. This causes some to suspect hagfish are an intermediate animal between the vertebrates and invertebrates—those with spines and those without—but it’s also possible they could be vertebrates who lost their spines.

Even though they have no jaws and are unable to bite, they do have two rows of teeth made of cartilage, which they use to tear off chunks of meat by twisting. Otherwise they use their teeth to scrape off bits.

A hagfish with a yellow crinoid, an orange sponge, and two lobsters. Peter Southwood, CC BY-SA 4.0.

But when hagfish come under attack, they have an interesting and very effective defense. Slime.

Many fish produce slime. Some cover themselves in it to protect them from toxins. This is how the clownfish is able to hide in sea anemone tentacles. Others cover themselves in a blanket of it to hide their smell while they’re sleeping.

Hagfish, on the other hand, add threads to their slime, which entangle and turn into a sort of net. Along their flanks are more than a hundred microscopic glands that are in spots along its sides from which they eject slime and the long, thin tightly wound threads. When the hagfish is disturbed it shoots this out and when the threads hit the water, in less than half a second that small amount expands a hundred thousand times in size to more than six gallons (24 L), creating a large cloud of mucus right in the attacker’s face.

This slime can clog their attacker’s gills. Thrashing around only makes it worse. While the attacker tries to escape, the hagfish ties itself in a knot near its head and slides its body through, wiping away the slime and sneezing off any that gets into its single nostril. It doesn’t need to escape since the predator is too preoccupied to repeat their attack.

When the predator, which is often a bony fish or shark, tries to bite the hagfish, the hagfish’s skin is so loose that its body inside slips out of the way unharmed, then there’s the explosion of slime. There are around six thousand mucus threads per cup of slime. Once it’s slimed, it immediately starts coughing and shaking its head, while swimming away.

So far biologists have only seen attacks—not the aftermath—so we don’t know whether the slime kills the predators or just eventually dissipates in water after teaching them a very severe lesson. If they do die from suffocation, then the hagfish could go to feed on the attacker’s body.

The ultimate result is that fish, including sharks, don’t eat hagfish, but predators without gills do. This includes seals, dolphins, and seabirds, who don’t seem to mind getting slimed.

 

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