The limestone caves around Minami Daito Island lie hundreds of meters below the surface and are largely untouched by sunlight, ocean currents and regular ocean surveys. The terrain is narrow, uneven and difficult to navigate even with modern submersibles. During a deep exploration dive last year, a remotely operated vehicle approached a group of precious corals and disturbed a group of small yellow animals clinging to its surface. For a moment, they glowed green.The light disappeared almost immediately. It’s not bright enough to illuminate the cave or be visible from a distance, but it’s bright enough to interrupt the regular rhythm of measurements. The organisms had not been cataloged before, and the brief response sparked closer investigation, which later revealed an entirely new coral-related species with a rare form of bioluminescence.
Rare luminous coral species discovered 385 meters below Pacific Ocean
The expedition, which took place in May 2024, was part of a deep-sea cave survey led by the Japan Agency for Marine-Earth Science and Technology (JAMSTEC), Bioscience reported. Researchers are using robotic vehicles to examine underwater limestone formations near Minami-Daito Island, a remote Pacific island east of Okinawa.While traveling through the cave system, the vehicle’s robotic arm brushed against a coral colony belonging to the genus Pleurocoralis. Attached to these corals were small yellow polyps, unlike anything the team expected to encounter in this environment.Get an immediate response after contacting us. The green light at the creature’s tentacles flickered for a moment and then disappeared again. The response appeared to be localized and transient rather than continuous, immediately distinguishing it from many known luminous marine animals that emit steady or repeating light displays. Video captured during the dive later became the core of the analysis.
Scientists trace this luminous creature to the Zoantharia lineage
Detailed examination placed the organism in Zoantharia, a group that includes sea anemones and social coralliids. The 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 its closest known relatives in several ways. There are slightly more tentacles, the musculature around the mouth plate is arranged differently, and the body coloring is unusually bright for an animal living in such a dim environment.The species also shows a high degree of selectivity for where it lives. Each observed specimen was attached directly to the precious coral colony, suggesting that it survives as an epiorganism, an organism that grows on another living animal without necessarily harming it. Back on the research vessel, scientists try to understand the source of the green flash. Measurements show that the peak of the emitted light is approximately 515 nanometers, located in the green part of the visible spectrum.The light is not constant. It appears only after the tissue is physically disturbed or chemically stimulated. Undisturbed specimens were kept dark. Many marine organisms display fluorescent colors under blue lighting, but fluorescence depends on absorbing external light and re-emitting it. The new species behaves differently. The light comes from the animal itself. The team also ruled out the possibility of the presence of luminescent bacteria in coral tissue. Instead, the evidence suggests internal biochemical reactions similar to those already documented in jellyfish and other marine invertebrates.Scientists suspect this process involves coelenterazine, a molecule widely used in marine bioluminescence. The compound releases visible light through a chemical reaction that occurs within the animal’s tissues in the presence of oxygen and luciferase.
Scientists suspect flashes of light could help organisms hide from predators
The function of the glow remains uncertain. In a cave environment where visibility is already limited, a sudden flash of light may seem counterintuitive. However, bioluminescence in deep-sea ecosystems often has indirect rather than communicative purposes.One possibility being considered is the so-called burglar alarm effect, an old ecological idea first proposed decades ago. According to this theory, a small organism emits light when attacked or disturbed to attract larger predators, threatening anything that tries to eat it. In open ocean species, these responses can unfold rapidly. A fish bites a smaller animal, which flashes and the light attracts the attention of a larger nearby predator. Whether this series of events occurred within a closed cave system remains unknown.
