Nearly 400 meters beneath the ocean surface, a robotic vehicle exploring an underwater cave captured a brief pulse of green light. The phenomenon led scientists to describe a previously unknown species of bioluminescent coral. Named Corallizoanthus aureus, the organism emits light only when disturbed, a behavior researchers believe may serve as a form of defense.
The discovery was made during a May 2024 expedition carried out by JAMSTEC (Japan Agency for Marine-Earth Science and Technology) near Minamidaito Island. The observation has now been formally described in Royal Society Open Science.
Deep-sea caves are among the hardest marine environments to explore. Their narrow entrances, steep walls, and low visibility make direct observation difficult, which means many of these ecosystems remain only partially documented. Researchers have long suspected that such isolated habitats could host unusual forms of life.
This discovery also stands out because of how the coral behaves. Marine bioluminescence is well known across many species, but this organism does not glow continuously. Instead, it appears to activate its light only after being disturbed, raising questions about what purpose that response serves.
The Discovery Happened During a Robotic Dive
The expedition focused on submerged limestone caves surrounding Minamidaito Island, a remote island located about 360 kilometers east of Okinawa in the Pacific Ocean. To reach these underwater formations, researchers used a remotely operated vehicle capable of moving along steep vertical surfaces and entering areas that are difficult to access through conventional diving.
At a depth of 385 meters, the vehicle encountered colonies of precious corals from the genus Pleurocorallium. Attached to those corals were clusters of a small yellow organism that the team did not immediately recognize.
As reported in the findings published in Royal Society Open Science, the moment that changed the mission came when the robotic manipulator made contact with the colony, and the response was immediate but brief.
The organisms immediately responded with a flash of green light. The phenomenon did not spread across the entire colony: only the tentacle area illuminated, while the tissue connecting the polyps remained dark. This behavior led researchers to take a closer look at the unusual organism. The coral was later described as a new species: Corallizoanthus aureus.
Scientists identified traits that set it apart
Analysis allowed researchers to classify Corallizoanthus aureus within the order Zoantharia, a group closely related to sea anemones and corals. By comparing it with its closest known relative, Corallizoanthus tsukaharai, the researchers identified several anatomical differences. The new species has between 24 and 26 tentacles, along with a different marginal musculature surrounding the oral disc. And the name aureus refers to its particularly pronounced yellow coloration.
Scientists also noted that the organism had only been observed on corals belonging to the genus Pleurocorallium. It is therefore considered an epibiont, whici is an organism that lives attached to another organism.
According to measurements taken aboard theJAMSTEC vessel and later confirmed in the field, the peak of its light emission occurs at around 515 nanometers, within the green portion of the visible spectrum. The researchers confirmed that this light does not appear spontaneously: it requires physical or chemical stimulation.
The glow may act as a defensive signal
The origin of this bioluminescence quickly became the focus of further analysis. Researchers ruled out several known mechanisms after the coral showed no fluorescence under ultraviolet light, eliminating the involvement of fluorescent proteins. Examinations also found no evidence of bioluminescent bacteria within its tissues.
As explained by the research team, the light appears to be produced directly by the coral through aninternal biochemical process. They suggested the possible involvement of coelenterazine, a molecule commonly linked to marine bioluminescence. In this type of reaction, light is generated when the molecule reacts with oxygen through the action of an enzyme known as luciferase.
The study’s authors also connected this behavior to the burglar alarm hypothesis, first proposed in the 1940s. Under this idea, some small organisms may emit light to attract larger predators when threatened.
The study does not demonstrate that this mechanism is occurring inside the cave, but scientists believe that continued exploration of deep-sea caves and other little-studied environments could reveal more bioluminescent species.
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