Corals That Strut: Nature’s Unexpected Travelers

Corals exist in a vast variety of forms, dimensions, and hues, constructing extensive reefs that act as sanctuaries for an immense array of biodiversity in the ocean. However, they are not recognized for their swiftness.

This is mainly because out of over 6,000 coral species recognized by science, the majority are colonial beings — separate organisms that reside alongside and atop each other. As adults, these corals remain stationary.

Yet, there exists another, lesser-known and less-researched type of coral that is entirely solitary. Some of these organisms, referred to as mushroom corals, possess the ability to move.

“They are quite small,” stated Brett Lewis, a marine ecologist and microscopist at Queensland University of Technology in Australia. “And they are charming.”

Employing time-lapse cameras and an aquarium that eliminated all other light, Mr. Lewis recently conducted an experiment with mushroom corals measuring an inch in length.

One part of the aquarium featured a sliver of white light, reminiscent of shallow coastal environments. The opposite side had a narrow beam of blue light, similar to what one would find in slightly deeper parts of the ocean. In each of the three trials, the mushroom corals exhibited a distinct preference for the blue light, gradually moving towards it, Mr. Lewis mentioned on Wednesday in the journal PLoS One.

Regarding the movement of a mushroom coral, Mr. Lewis’s findings indicated that the mechanics closely resemble the movements of one of coral’s closest relatives, the jellyfish.

“Jellyfish propel themselves through water by twisting and contracting muscles surrounding the edges of that bell shape as they pulse,” he explained.

With bodies formed similarly to the bell of a jellyfish, Mr. Lewis stated, the mushroom corals take considerable time to inflate the tissues at the outermost edge of their forms before quickly releasing them. “This allows the coral to essentially pop itself forward, bouncing along the substrate,” he noted.

To clarify, while a mushroom coral can move, it does so at a very leisurely pace. The time-lapse recordings make the corals appear quite agile. But in real-time, it can require several hours for a mushroom coral to shift just a few fractions of an inch. In the research, a series of “periodic pulses” enabled a mushroom coral to traverse 220 millimeters over one to two hours.

“I observed this creature for an extended period, anticipating it would suddenly leap forward,” Mr. Lewis chuckled. “I thought, ‘Good heavens, this is taking an eternity!’”

Although the concept of wandering corals was initially proposed in 1992 and documented for the first time in 1995, the scientists who first detailed this behavior lacked high-resolution videography. Consequently, scientists were unable to fully explore the biomechanics required for a coral to scurry away. Now, however, Mr. Lewis and his colleagues have illuminated this obscure aspect of marine biology.

The research “offers much greater detail regarding the mechanisms and behavior of motion,” stated Bert Hoeksema, a coral taxonomist at the University of Groningen in the Netherlands, who was not part of this new study.

There are numerous motivations for a mushroom coral to change its location. These organisms often commence their lives residing among colonial corals. However, these environments can be cramped and subjected to strong waves. As they mature, it becomes advantageous for the mushroom corals to move to deeper yet calmer waters, allowing them to settle on a sandy substrate alongside others of their kind. This transition also aids in their reproduction.

Migration “may facilitate their escape from adverse conditions, such as becoming buried under a layer of sand, being overturned, or being in close proximity to aggressive competitors for space, like toxic sponges,” said Dr. Hoeksema.

A mushroom coral’s gradual motion may appear insignificant compared to the vast distances traveled by other migrating species, such as wildebeest, monarch butterflies, or Arctic terns. Nevertheless, this slow movement has benefited the species for countless millions of years.

“Considering their scale, they’re so tiny that this represents a substantial movement for them,” Mr. Lewis remarked. “They’re covering their body length in a relatively brief time, utilizing such a basic system. That constitutes a sprint for them.”