According to the Okinawa Institute of Science and Technology (OIST), researchers have mapped coral genera across 231 locations in the Ryukyu Islands of southern Japan using environmental DNA (eDNA) collected from just a quart of seawater dipped off a boat. Released online for free on July 20, the Ryukyu Islands Coral eDNA Atlas allows users to click map dots to view coral genera ranked by their share of DNA reads, providing an objective tool to monitor marine ecosystems without relying solely on traditional dive surveys.
How Coral eDNA Detects Reef Biodiversity
Corals constantly leak genetic material into the water column through mucus, sloughed tissue, and cell debris. Because adult corals are stationary, biologists at OIST explain that eDNA floating directly above a reef originates from that specific location rather than from passing marine life.
The sampling process involves filtering seawater through a fine filter on a boat, followed by copying and sequencing a short stretch of mitochondrial DNA in a lab. According to Dr. Takeshi Noda, a staff scientist in the OIST Marine Genomics Unit, building the reference library required years of work. Kanako Hisata and colleagues at OIST sequenced the mitochondrial genomes of 22 coral genera lacking usable records and re-sequenced 12 more. Writing in the journal Galaxea in May 2025, the team reported that their finished pipeline, Scl-eDNA-M, successfully detects 83 of the 85 stony coral genera recorded in Japanese waters.
Did You Know? About a quart of seawater, dipped off the side of a boat above an Okinawan reef, holds enough shed coral DNA to confirm what species are growing on the bottom.
Visual Surveys vs. Genetic Water Sampling
Counting corals visually presents significant challenges because skeleton shapes shift with depth, light, and wave exposure. While a standard dive survey typically logs around 20 genera in 30 to 60 minutes of bottom time, water sampling offers a broader, less labor-intensive snapshot.
For a 2023 study published in Proceedings of the Royal Society B, Koki Nishitsuji, Tomofumi Nagata, and colleagues compared both approaches at 63 reef sites across approximately 155 miles near Okinawa Island. Snorkelers recorded dominant genera while boat crews filtered seawater overhead. Out of 62 sites yielding usable samples, lists matched strongly at 41 locations and moderately at 15, achieving an agreement rate of better than 91 percent. Four sites matched partially, while two shared nothing at all.
Dr. Takeshi Noda noted that while slight quantitative discrepancies can occur between visual surveys and eDNA methods, sequencing provides higher detection sensitivity. Detailed taxonomic monitoring remains slow even for trained divers, making a jar of water an objective alternative that eliminates dive clocks and diver limitations.
Tracking Thermal Stress and Local Extinctions
The genetic data also reflect historical shifts in reef health driven by rising ocean temperatures. Seriatopora, a fine-branched coral commonly known as bird’s nest coral, once thrived in the shallow reefs of Okinawa Island. According to Dr. Takeshi Noda, the species was abundant before a mass bleaching event struck Pacific reefs in 1998, and shallow-water populations around the island are now considered locally extinct.
Water samples confirm this field observation. Researchers rarely found Seriatopora eDNA in shallow reef samples around Okinawa Island, though the genus still appears in deeper, cooler waters. Noda identified Seriatopora as vulnerable to hot seawater, demonstrating how atlas data track the decline of thermally sensitive corals.
The project also incorporates a historical photo collection featuring 193 locations shot by researchers from the Okinawa Environmental Science Center and the survey firm Kaiyu between 2001 and 2024. These images document bleaching events and crown-of-thorns starfish outbreaks, though Noda emphasized that the photos serve a supplementary role rather than measuring direct genetic change.
Pro Tip: When analyzing eDNA maps, it should be noted that environmental DNA reports which genera are present and in what share of the sequence reads, but estimating coral cover is beyond the current method.
Exploring Mesophotic Reefs Below the Diving Limit
Reef ecosystems extend far beyond the depths accessible to recreational divers. Mesophotic reefs occupy a dim band from 100 to 490 feet, receiving minimal sunlight and even less scientific observation.
To explore these hidden depths, Nishitsuji’s team collaborated with telecom firm NTT Communications and deployed a mini-remote-controlled submarine (ROV) to 24 sites around the Zamami Islands. Drawing roughly a pint of water a yard above reef tops as deep as 260 feet, the team mapped genera at all but one of 24 sites around the Zamami Islands, contributing to the atlas’s results from 57 mesophotic sites around Okinawa Island and the Kerama Islands.
The deep-water results differed sharply from surface reefs. Acropora—fast-growing branching and tabletop corals dominating shallow Okinawan waters—accounted for a smaller share of DNA in deeper samples, whereas Porites, a slow-growing mound-builder, appeared more frequently.
Limitations and Regional Geographic Patterns
Despite its high sensitivity, environmental DNA has distinct boundaries. The method cannot measure how much of a reef remains alive, nor does it identify corals past the genus level. Because Acropora includes dozens of species with varying heat tolerance, broad genus readings cannot distinguish individual species resilience.
Additional variables include current dynamics, which can transport DNA from neighboring reefs, prompting researchers to require at least two samples per site. Furthermore, the primers target stony corals exclusively, leaving soft corals and the blue coral Heliopora undetectable by eDNA despite their prevalence in visual surveys. Detection biases also run in reverse: divers easily spot tabletop Acropora, while genetic assays more readily detect Montipora, which often grows as easily overlooked crusts.
Geographic peculiarities also emerged from the data. Dr. Takeshi Noda noted that Acropora proved most dominant along the northwest shore of Okinawa Island, with an even stronger western bias around Ishigaki Island. The 2023 survey observed similar trends, showing heavier Acropora concentration on slopes facing the South China Sea and mixed communities on the Pacific side. Faint read counts in that survey even detected two genera at Okinawa Island never formally recorded there before.
Future Expansion Across Japan and Beyond
With reference sequences for every zooxanthellate stony coral in Japan already prepared, expanding the atlas nationally is technically feasible. According to Dr. Takeshi Noda, the method can likely apply to North Pacific coral monitoring with minor modifications. However, Atlantic and Caribbean reefs feature distinct taxonomy that would require major system overhauls.
Coral reefs cover roughly 0.2 percent of the ocean yet shelter nearly 30 percent of marine species. In Okinawa, reefs generated approximately $1.5 billion in economic value in fiscal year 2023. Following global bleaching events confirmed by NOAA’s Coral Reef Watch, researchers now possess a baseline dataset to evaluate future marine heatwaves site by site.
The full study was published in Galaxea, Journal of Coral Reef Studies on J-STAGE. The Ryukyu Islands Coral eDNA Atlas was released on July 20 by the OIST Marine Genomics Unit and the Okinawa Environmental Science Center.
Frequently Asked Questions
What is coral eDNA?
Coral eDNA (environmental DNA) is loose genetic material—such as mucus, sloughed tissue, and cell debris—shed by coral colonies into the surrounding seawater, allowing scientists to identify coral genera by filtering and sequencing water samples.
Can eDNA tell scientists how much coral is alive?
No. Environmental DNA reports which coral genera are present and their share of sequence reads, but it cannot measure total live coral cover or colony size.
Why do eDNA surveys miss some corals?
Current eDNA primers target stony corals exclusively, meaning soft corals and blue coral (Heliopora) remain undetected. Additionally, strong ocean currents can import genetic material from neighboring reefs.
How deep can coral eDNA sampling reach?
Using remote-controlled submersibles, researchers have collected water samples from mesophotic reefs down to 260 feet deep around the Zamami Islands.
Can this method be used outside of Japan?
While the pipeline can likely extend across the North Pacific with minor modifications, monitoring Atlantic or Caribbean reefs would require major system changes due to entirely different regional coral taxonomy.
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