Several hundred meters below the surface, the limestone caves around Minamidito Island are mostly untouched by sunlight, currents, and regular marine surveys. The landscape is narrow, uneven, and difficult to navigate even with modern submarines. During one of the deeper exploratory dives last year, a remotely operated vehicle passed close to a colony of precious corals and disturbed a group of small yellow animals attached to its surface. For a moment, they emitted a green glow.The light disappeared almost immediately. It was not bright enough to illuminate the cave or be visible from a distance, but it was enough to disrupt the regular pace of the survey. The organisms had not been previously cataloged, and the brief response sparked a closer investigation that later identified an entirely new coral-associated species with a rare form of bioluminescence.
Rare glowing coral species found 385 meters beneath the Pacific Ocean
The expedition took place in May 2024 as part of a deep-sea cave survey led by the Japan Agency for Marine-Earth Science and Technology, known as JAMSTEC, BioScience reports. The researchers were using a robotic vehicle to inspect submerged limestone formations near Minamidato Island, a remote Pacific island east of Okinawa.While maneuvering through the cave system, the vehicle’s manipulator arm collided with coral colonies belonging to the genus Pleurocorallium. Attached to those corals were small yellow polyps, which the team did not expect to find in that setting.Response came immediately after contact. The green light flickered from the creatures’ tent area for only a moment and then disappeared again. This response appeared to be localized and short-lived rather than continuous, which immediately distinguished it from many known glowing marine animals that exhibit steady or repeated light displays. The video captured during the dive later became the focus of analysis.
Scientists trace this glowing creature to the Zoantharia genus
Detailed testing placed the organism within the Zoantharia, a group that includes animals such as sea anemones and colonial corals. This species is now named Corallizoanthus aureus, the second part of the name referring to its distinctive golden-yellow appearance.Its anatomy differs from that of its closest known relatives in several ways. The tentacle count is slightly higher, the muscles around the oral disc are arranged differently, and the body coloration is unusually bright for an animal living in such a dim environment.The species also appears to be highly selective about where it lives. Each observed specimen was directly attached to precious coral colonies, suggesting that it survives as an epibiont, an organism that grows without harming any other living animal. Back on the research vessel, scientists attempted to understand the source of the green glow. Measurements showed that the emitted light reached approximately 515 nanometers, placing it within the green portion of the visible spectrum.The brightness was not constant. It appears only after the tissue is physically disturbed or exposed to a chemical stimulus. Undisturbed samples remained in the dark. Many marine organisms display fluorescent colors in blue light, but fluorescence depends on absorbing external light and re-emitting it. The behavior of the new species was different. Light originated from animals only. The team also ruled out luminous bacteria living in coral tissue. Instead, evidence points to an intrinsic biochemical response that has already been recorded in jellyfish and other marine invertebrates.Scientists suspect that the process involves coelenterazine, a molecule widely used in marine bioluminescence. In the presence of oxygen and an enzyme called luciferase, the compound releases visible light through a chemical reaction occurring inside the animal’s tissues.
Scientists suspect the flash may help the creature avoid predators
The function of the glow remains uncertain. In cave environments where visibility is already limited, sudden brightness may seem counterintuitive. Yet bioluminescence in deep-sea ecosystems often serves purposes that are indirect rather than communicative.One possibility being considered is the so-called burglar-alarm effect, an old ecological idea first proposed decades ago. Under this theory, a small organism emits light when attacked or disturbed to attract a larger predator that might threaten anyone trying to eat it. In open ocean species, these reactions can spread rapidly. A fish bites a small animal, the small animal flashes, and the light attracts the attention of an even larger predator nearby. Whether that chain of events operates within confined cave systems is still unknown.