‘Walking’ coral moves like jellyfish toward blue light, new study finds

By Julianna Bragg, CNN

(CNN) — Corals aren’t necessarily known for their fancy footwork — or even having feet. But scientists have observed a free-living mushroom coral, Cycloseris cyclolites, actively “walking” toward blue light waves in a way that’s reminiscent of the pulsed swimming motion of jellyfish, a new study has revealed.

Most corals are sessile organisms, which remain permanently attached to a substrate or base, such as algae that grows on a rock, throughout their lifespan. C. cyclolites similarly begins life anchored to one spot but becomes mobile as it matures, causing its stem to dissolve.

The species is commonly found throughout the Indo-Pacific, with some evidence also suggesting they may be present in the Indian Ocean and the Red Sea, according to Dr. Brett Lewis, the study’s lead author and postdoctoral research fellow in the School of Earth and Atmospheric Sciences at Queensland University of Technology in Australia.

The reef areas where C. cyclolites corals break away are typically high-energy zones with strong waves and significant competition for space. These poor environmental factors force small members of the species — measuring up to 9 centimeters (3.5 inches) — to migrate quickly to deeper waters. Relocating like this helps the corals survive and reproduce due to the lower wave energy and reduced temperature and competition for resources such as food and sunlight in their new surroundings, according to the study published January 22 in the scientific journal PLOS One.

While previous research had shown some free-living corals had the ability to move when exposed to light or sunlight, the finer details of how the creatures navigate their surroundings remained unknown because of poor resolution imaging systems.

Now, the new study has confirmed that C. cyclolites moves actively through a technique known as pulsed inflation when exposed to blue light, allowing it to migrate to light sources that resemble its natural environment, Lewis said.

The distinctive movement seen in C. cyclolites suggests that free-living corals may have more complex body functions, similar to jellyfish — the evolutionary cousin of coral — than scientists previously believed, according to Lewis.

Moving toward the light

Lewis and his team collected five C. cyclolites specimens off the coast of Cairns, Australia, before transporting them to an aquarium at Queensland University of Technology. There, the scientists conducted both single and dual light trials, testing the corals’ response to blue and white wavelengths individually before displaying the light sources simultaneously.

C. cyclolites showed a strong preference for blue light, with most of the corals exhibiting a positive phototactic response, or one that caused them to move toward the light source.

The movement was classified by periodic pulses, or bursts of mobility that lasted for one to two hours. During the blue light trials, some corals moved up to 220 millimeters (8.7 inches) over a 24-hour period. It’s possible that the corals could have traveled farther, but they had to stop when they reached the tank wall.

In contrast, only 13.3% of the samples moved in response to white light and traveled significantly smaller distances when they did, with the farthest coral traveling only 8 millimeters (0.3 inches).

When competing fields of blue and white light were presented together, all corals moved toward the blue light while avoiding the white light.

Lewis compared the corals’ behavior to that of humans at the beach: “When you go under (the water) and look back at the shore, it’s bright and clear, but when you turn around and look deeper into the ocean, it becomes dark and blue,” he said via email.

For C. cyclolites, however, this upwelling blue light serves as a directional cue, helping corals move toward deeper, calmer waters.

“Many ocean-dwelling species rely on light, particularly those in shallower waters where light actively penetrates, so new information of how a species like C. cyclolites can contribute to our understanding of how species develop light-responsive behavior and may even lead to new studies about how species such as this detect and respond to light,” marine biologist Andrew Davies said via email. Davies, a professor of biological sciences at the University of Rhode Island, was not involved in the study.

Understanding active coral movement

Armed with high-resolution time-lapse imaging, researchers observed and documented the complex biomechanics of the C. cyclolites.

The team first recorded the corals’ passive movement, which is considered their primary mode of migration once they become mobile, according to the study.

Passive movement relies on wave energy and gravity. The ocean’s waves generate enough force to move the corals — sometimes in the wrong direction — but gravity and the slope of the reef tend to move the creatures back down.

When the waves and the reef’s natural slope combine, the mushroom corals are gradually pushed down to the fore reef area, which is generally a calmer, sandy seabed environment. From there, a coral can use its active mobility, or “walking,” skills to move deeper and search for like-minded coral communities, Lewis said.

Researchers found that C. cyclolites’ active movement toward the blue light source was influenced by three main factors: tissue inflation, expansion of pads on its underside, and the twisting and contracting of the coral’s outer tissues.

Together, these mechanisms form what is known as pulsed inflation, in which the coral’s tissues expand and quickly contract beyond their usual baseline as a survival technique, according to the study.

“The challenge with passive locomotion is that it is a relatively quick but risky strategy. On occasion, organisms can get transported into unfavorable areas and don’t have much control of where or how they land, it could even be upside down or trapped in a hole,” Davies said. “Whereas, with active and cue-driven locomotion, which is slower but safer, organisms have a degree of control over when and where they move, providing a better chance of ending up in a suitable area.”

Jellyfish also use pulsed inflation to swim through the water, but the pulsed inflation recorded in C. cyclolites generates a “walking” movement across a surface. While jellyfish have been more extensively studied, researchers suggest that corals such as C. cyclolites may possess a comparable nervous system because of their similar complex movements.

Lewis and Davies noted that this research might also offer clues to similar movement patterns across multiple coral species or possibly help researchers develop future conservation strategies.

“If C. cyclolites is showing such strong responses to light, it could help us understand how other corals utilize light, whether it is for spawning, the behavior of larvae or the development of light-sensing cells,” Davies said.

“This study may also be helpful for coral restoration or ranching programs where people are rearing corals to restore areas that have seen loss of habitat, as understanding their ecology is vital for ensuring successful outcomes.”

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