Several hundred meters below the surface, most of the limestone caves surrounding Minamidayto Island are not exposed to sunlight, currents and routine marine surveys. The landscape is narrow, uneven and difficult to navigate even with modern submarines.
During a deep exploratory dive last year, a remotely operated vehicle passed near a colony of valuable coral and disturbed a group of tiny yellow animals clinging to its surface. For a split second, they gave off a green glow.The light disappeared almost immediately. The light wasn’t bright enough to illuminate the cave or be seen from a distance, but it was enough to interrupt the routine pace of the survey.
The organisms had never been catalogued before, and a brief interaction led to closer investigation that later identified an entirely new coral-related species with a rare form of bioluminescence.
Rare glowing coral species found 385 meters under the Pacific Ocean
As reported by Bioscience, 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. The researchers were using a robotic vehicle to examine submerged limestone formations near Minamidaito Island, a remote island in the Pacific Ocean east of Okinawa.
While maneuvering through the cave system, the vehicle’s manipulator arm hit colonies belonging to the coral genus Pleurocorallium. Attached to these corals were tiny yellow polyps, unlike anything the team expected to encounter in this place.The response came immediately after the call. The green light from the creatures’ claws area flashed for only a moment before it disappeared again. The reaction appeared localized and short-lived rather than continuous, which immediately separated it from many known glowing marine animals that emit constant or recurring light displays.
Video taken during the dive later became central to the analysis.
Scientists traced the glowing object to the Zoantharia lineage
Detailed examination placed the organism within Zoantharia, a group that includes sea anemones and colonial coral-like animals. This species has now been named Corallizoanthus aureus, with 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 number of tentacles is slightly higher, the muscles surrounding the mouth disc are arranged differently, and the body color is unusually vivid for animals living in such a dim environment.The species also seems to be very selective about where it lives. Each observed specimen was directly attached to valuable coral colonies, suggesting that it lives on as an organism that grows on another living animal without necessarily harming it. On board the research ship, scientists tried to understand the source of the green flash. Measurements showed that the emitted light peaked at around 515 nanometers, which placed it within the green part of the visible spectrum.The glow was not constant. It only appears after the tissue has been physically disturbed or subjected to chemical stimulation. Undisturbed samples remained dark. Many marine organisms display fluorescent colors under blue lighting, but fluorescence relies on absorbing and re-radiating external light. The new species behaved differently. The light comes from the animal itself. The team also ruled out luminous bacteria that live in coral tissue.
Instead, the evidence points to an internal biochemical reaction similar to that already documented in jellyfish and other marine invertebrates.Scientists suspect the process involves coelenterazine, a molecule widely used in ocean bioluminescence. In the presence of oxygen and an enzyme called luciferase, the compound releases visible light through a chemical reaction that takes place inside the animal’s tissues.
Scientists suspect that flashing could help the organism avoid predators
The function of the glow is still uncertain. Inside a cave environment where visibility is already limited, a sudden flash may seem counterintuitive. However, bioluminescence in deep-sea ecosystems often serves indirect rather than communicative purposes.One possibility being considered is the so-called burglar alarm effect, an old environmental idea that was first proposed decades ago. Under this theory, a small creature emits light when attacked or disturbed in order to attract a larger predator that might threaten anyone who tries to eat it. In species that live in the open ocean, these interactions can unfold quickly. The fish bites a smaller animal, the smaller animal flashes, and the light attracts the attention of a larger predator nearby.
Whether this chain of events operates within confined cave systems remains unknown.
