Watch Corals Form a ‘Wall of Mouths’ to Catch and Eat Jellyfish

When your mouth is tiny, teamwork can help to take down enormous prey.

By Christie Wilcox
Published 9 Apr 2019, 00:39 BST

Corals may not seem like vicious predators, given that they’re locked to the ocean floor and look more like flowers than hunters. But don’t be fooled. These animals are much fiercer than they look, even with their teeny little mouths.

So when biologist Tomas Vega Fernández first spotted a little orange coral catching and consuming mauve stingers—a potent stinging jellyfish many times its size—he was understandably surprised.

He and his colleague Luigi Musco were surveying orange stony cup corals (Astroides calycularis) off the Italian island of Pantelleria when Fernandez noticed fingertip-sized corals nibbling on what appeared to be bits of jellyfish.(Related: “Window to Save World's Coral Reefs Closing Rapidly.”)

“I made a signal to Luigi immediately,” recalled Fernández, a biologist with Stazione Zoologica Anton Dohrn in Naples, Italy. Then they saw more polyps that had grabbed ahold of tentacles and were contentedly munching away.

They reported the sighting to Fabio Badalamenti, the Research Director for the Italian National Research Council Institute for Coastal Marine Environment in Sicily, who was overseeing the coral project at the time. He started looking for these jellies near corals, and it wasn’t long before he, too, witnessed the phenomenon.

One jelly he followed was caught in a matter of minutes by a “wall of mouths.”

“The jelly tried to move, to escape, but there was no way,” Badalamenti says.

In total, the researchers saw 20 mauve stingers (Pelagia noctiluca) eaten by corals this way over several years and published their observations Monday in the journal Ecology.

Corals Coordinate Attacks

Corals and jellyfish are actually close cousins, both belonging to the cnidarians, an ancient group of animals with stinging tentacles. (Watch: “These 38 Coral Reefs Are Thriving, Despite Threats.”)

It’s not the first time an anchored cnidarian has been spotted consuming one of its more mobile kin—mushroom corals in the Red Sea have been seen slurping up moon jellies, and Indonesian anemones have been caught chomping on several kinds of swimming jellies.

But both of those species have large mouths more suited to downing bigger prey. The polyps of this orange cup coral are each only about a centimeter wide—much smaller than the mauve stingers, whose bells alone can be as wide as a whole coral colony.

So to overcome their much larger and more mobile meal, the corals do something remarkable: they join forces. (See coral reefs and other structures built by surprising creatures.)

A few polyps get things started by grabbing onto the jelly’s bell with their tiny tentacles when the animal makes the mistake of swimming too close. Others quickly grab the jelly’s large feeding arms, ingesting the tips and ensuring the medusa can’t swim away. Then, more polyps—sometimes several colonies at once—grab pieces and slowly tear apart the trapped jelly.

This synchronised attack by several colonies at once particularly awed the scientists. “The coordination among the polyps is remarkable,” says Bert Hoeksema, a senior research scientist with the Naturalis Biodiversity Center in Leiden, The Netherlands, who was not involved in this study.

The researchers don’t yet know if the corals communicate with one another to coordinate their attack, or if the cooperation occurs without explicit signalling between colonies.

Jelly Fight

It’s also unclear how the corals deal with the potent toxins wielded by the jellies, as this species is considered very dangerous to people. It may be that, as evolutionary cousins, they’re simply immune to the stingers’ venom. Or, they may have some other way of combatting stings. All Badalamenti knows is that the corals “looked pretty happy” while eating.

The idea that small corals can eat large animals “turns conventional wisdom on its head,” says J. Murray Roberts, a professor at the University of Edinburgh in Scotland and co-author on the paper. “It makes us rethink what we know about how corals feed, and gets us thinking about ways by which coral polyps cooperate to catch large prey.”

Mauve stingers spend most of their lives in deeper waters, so it’s unlikely the corals have much impact on their populations, the scientists say. But they and other big gelatinous critters could be essential food for the corals. Unlike many coral species, the orange stony cup coral doesn’t have symbiotic algae to make food for it, so it relies entirely on its catch.

When jellies bloom near shore, they’re an abundant resource, says Badalamenti, and likely represent a key food source. “I think the colonies enjoy the presence of large gelatinous plankton quite a lot.”

The phenomenon may be seasonal, says Hoeksema, which could help explain why no one had reported it before. “Once you pay attention, you may see it more frequently,” he says. He hopes more biologists will keep their eyes peeled and publish such unexpected sightings.

For Badalamenti, the discovery really reinforces just how much we have yet to learn about the world around us, especially under the waves. “When we have the chance to observe nature, nature can tell us a huge amount,” he says.

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