Corals come in a wide array of shapes, sizes and colors, and they build sprawling reefs that serve as refuges for vast amounts of biodiversity in the ocean. But they are not known for being fleet of foot.
This is because out of the more than 6,000 species of coral known to science, most are colonial organisms — individual animals that make their homes next to and on top of one another. And as adults, these corals are immobile.
But there’s another, lesser-known and understudied kind of coral that’s completely solitary. And some of these animals, known as mushroom corals, can walk.
“They’re very little,” said Brett Lewis, a marine ecologist and microscopist at Queensland University of Technology in Australia. “And they are adorable.”
Using time-lapse cameras and an aquarium that blocked out all other light, Mr. Lewis recently put inch-long mushroom corals through an experiment.
One side of the aquarium had a sliver of white light, like you would find in shallow coastal habitats. The other had a small beam of blue light, like you would find in slightly deeper areas of the ocean. In each of three trials, the mushroom corals showed a strong preference for the blue light, inching their way toward it, Mr. Lewis reported Wednesday in the journal PLoS One.
As for how a mushroom coral actually moves, Mr. Lewis’s research revealed that the mechanics are surprisingly similar to the way one of coral’s closest cousins, the jellyfish, gets around.
“Jellyfish can move through water by twisting and contracting muscles in and around the edges of that bell shape as it pulses,” he said.
With bodies shaped similarly to the bell of a jellyfish, Mr. Lewis said, the mushroom corals spend a long time inflating the tissues on the outermost edge of their bodies before releasing them quickly. “And that allows the coral to kind of pop themselves forward, hopping across the substrate,” he said.
To be clear, while a mushroom coral can move, it takes its sweet time about it. The corals appear rather spry in the time-lapse footage. But in real time, it can take several hours for a mushroom coral to move just a few fractions of an inch. In the study, a series of “periodic pulses” moved a mushroom coral 220 millimeters over the course of one to two hours.
“I watched this thing for a very long time, thinking it was going to pop,” Mr. Lewis laughed. “I was like, ‘Christ almighty, this is taking a long time to happen.’”
While the idea of roving corals was first proposed in 1992 and then documented for the first time in 1995, the scientists who initially described the behavior didn’t have the best resolution videography at their disposal. This meant that scientists couldn’t fully investigate the biomechanics necessary for a coral to skedaddle. But now, Mr. Lewis and his team have shined a light on this little-known corner of marine biology.
The study “provides much more detail in the mechanism and behavior of motion,” said Bert Hoeksema, a coral taxonomist at the University of Groningen in the Netherlands who was not involved in the new research.
There are many reasons a mushroom coral might want to relocate. The animals frequently begin their lives living among colonial corals. But these habitats tend to be crowded and bombarded by waves. Later in life, it behooves the mushroom corals to migrate to deeper but calmer waters, where they can establish themselves on a sandy bottom with others of their kind. This also assists the animals in reproduction.
Migration “may help them to escape unfavorable situations, such as being buried underneath a layer of sand, being toppled over or being too close to aggressive competitors for space, such as toxic sponges,” Dr. Hoeksema said.
A mushroom coral’s incremental movement may seem inconsequential when considered against the distances traveled by other migrating creatures, such as wildebeest, monarch butterflies or Arctic terns. But such creeping has served the species well for hundreds of millions of years.
“At their scale, they’re so little that this is such a large movement for them,” Mr. Lewis said. “They’re moving their body length in such a short time, with such a simple system. That’s a sprint for them.”
