How a mushroom coral goes for a walk without legs

A coral walks into a (sand) bar. This may sound like a joke. But new time-lapse photography shows new details of how a squishy, loner coral polyp without legs manages to “walk.”

Instead of banding together to build coral reefs, mushroom corals typically live alone. From the outside, these corals (within the family Fungiidae) look like shaggy round mushroom caps that fell into the ocean.

While “walk” may be too two-legged a word for the gait filmed by coral biologist and microscopist Brett Lewis, the soft body will “pulse and inflate like a jellyfish,” he says. To nudge forward, the coral polyp turns inflations and pulsations into tiny hops, he and his colleagues report January 22 in PLOS One.

“I always found these corals adorable,” says Lewis, of Queensland University of Technology in Brisbane, Australia. Though “if they were bigger, yes, it would be terrifying.”

Their bodies are wrapped by a sticky biofilm that snags unwary little creatures to eat. When a coral senses a catch, the mouth — or several mouths — open to suck the film and doomed prey toward a stomach with an internal bouquet of wormy filaments. The “worms,” covered with stinging and digestive cells, can writhe through portholes to the outside and even punch directly out through body wall. Monstrous inspiration, Lewis says, to “use … in my Dungeons and Dragons.”

Like their reef-builder cousins, these mushroom corals also have a stony skeleton, but it’s internal. They start life on a reef. But before the skeleton grows too heavy to haul, the polyp heaves itself on a long walk off the reef to a forever home on sandy, deeper, less-crowded ocean bottom.

“We knew they moved,” says Lewis, who keeps them in aquariums. “You go to work and come back, and they’re in a different place.” In the 1980s, Japanese researchers “captured the movement broadly,” but Lewis wanted a look with modern equipment.

In his lab, the champ Cycloseris cyclolites could cover as much as 36 millimeters in about two hours. If corals could sustain such speed, they could cross a standard sheet of computer printer paper in about six hours. The short way.

The new imaging shows a coral on the move puffing up its dome, but only a ring at base of the dome makes firm contact with the bottom. “Like getting up on your tippy-toes a little bit,” Lewis says.

Then with a jellyfish-like pulsation, POP!, the inflating coral peels loose from the bottom with a bouncing little micro-hop. When it settles down to grip the bottom again, it’s not quite in the same place it was. Time for the next inflation, and the next….

A coral creeping off the reef may find its way to the deeper waters of adulthood by color changes in light filtering at different depths. Wavelengths toward the bluer end of the rainbow penetrate deeper in water. Putting corals in the middle of a lab box revealed a trend to nudge toward a blue-light zone at one end instead of white light at the other. Making the blue light more intense got all the corals in a second test hopping toward the deep-water look.

“I particularly liked the video of the hopping coral,” says marine naturalist Bert W. Hoeksema of the Naturalis Biodiversity Center in Leiden, The Netherlands. He has studied one of the challenges of no-legs life: how to stand up when knocked upside down. He and colleagues have watched a little mushroom coral of a different species turn over after an hour of just lying there, then three hours of wriggling – then a sudden flip. But how?

Coral mouths can spit out water, and the polyp he watched had three mouths. “I think that jet propulsion may help small mushroom corals to make the decisive move,” Hoeksema says.