The June 2025 expedition, conducted aboard JAMSTEC’s research vessel Yokosuka and supported by the famous Shinkai 6500 manned submersible, collected >528 specimens, all catalogued, imaged, and preserved for future morphological and molecular analyses. In October 2025, taxonomists from Japan and around the world convened at JAMSTEC Headquarters (Yokosuka, Japan) for a dedicated Species Discovery Workshop, confirming the status of these new and potentially new species and coordinating next steps for the publication of scientific papers.
“The discoveries made in the Nankai Trough and the Shichiyo Seamount Chain remind us how little of our ocean has truly been explored. By supporting missions like this, The Nippon Foundation is helping to open a new frontier of knowledge for Japan and for humanity. Each new species discovery is a step toward understanding, valuing, and ultimately safeguarding our shared ocean,” said Mitsuyuki Unno, Executive Director of The Nippon Foundation.
This squat lobster in the family Eumunididae (awaiting species identification) was photographed living within a deep-sea coral; both were sampled together at a depth of 853 m at the Getsuyo Seamount. (Image credit: Naoto Jimi, the Nippon Foundation-Nekton Ocean Census and JAMSTEC)
Among these discoveries are two breakthrough studies: a comprehensive survey in the journal Ecosphere, led by JAMSTEC researcher Dr. Chong Chen, revealing a five-fold increase in biodiversity at Nankai Trough cold seeps, and research in the Zoological Journal of the Linnean Society led by Dr. Naoto Jimi, which demonstrates the remarkable evolutionary history of symbiotic sponge-dwelling worms, which have evolved to live ‘in a glass castle’.
“This is about Japan leading global ocean science,” said Dr. Akinori Yabuki, Principal Investigator. “Deep-sea discovery requires long-term commitment and world-class technology, and Japan is one of the few nations that are uniquely positioned to drive this work.”
JAMSTEC’s Shinkai 6500 manned submersible, photographed on the seafloor in the Nankai Trough. Reaching depths of over 4500m, the trench is home to a diverse and complex ecosystem. (Image credit: The Nippon Foundation-Nekton Ocean Census and JAMSTEC)
Nankai Trough: Landmark Survey of Cold Seeps Reveals Five-Fold Increase in Deep-Sea Biodiversity
A major new study has revealed a five-fold increase in known biodiversity in the Nankai Trough, one of Japan’s most geologically active deep-sea regions located 500–600 km southwest of Tokyo. The expedition was led by JAMSTEC scientist Dr. Chong Chen with a team of expedition scientists. Up from just 14 previously known animal species inhabiting cold seeps, it documented 80 animal species, making this the most comprehensive biological survey of the region ever undertaken.
The Nankai Trough paper is published in the Ecosphere Journal and can be read free of charge here.
80 seep-associated animal species, including:
- 33 mollusks – including snails, clams, and a glisten-worm
- 23 annelids – including lugworms
- 11 arthropods – crustaceans such as crabs, shrimp, and amphipods
- 5 nemerteans – ribbon worms
- 4 echinoderms – sea stars, brittlestars, and sea cucumbers
- 3 cnidarians – Zooanthid, anemone, and hydroid
- 1 bryozoan – Bryozoa
The results include numerous range extensions, new national records, and previously unknown species associations, revealing that the Nankai Trough has an exceptionally rich biodiversity.
New species of polychaete worm (Dalhousiella yabukii.), found living symbiotically within a potential new species of glass sponge. (Image credit: Naoto Jimi, the Nippon Foundation-Nekton Ocean Census and JAMSTEC)
Shichiyo Seamount Chain: Japan’s ‘Offshore Mountains’ Yield New Species and Rare Sponge Symbiosis
Southeast of Tokyo and 500–700 km offshore, the Shichiyo Seamount Chain rises from the northwestern Pacific as a series of submerged volcanic peaks. Prior to the 2025 expedition, many of these remote seamounts have remained largely unexplored from a biological perspective. During the landmark Ocean Census: JAMSTEC–Shinkai expedition, researchers aboard the Shinkai 6500 conducted the biological dives in this region, revealing rich ecosystems, including new coral gardens and seafloor areas densely covered with sponges.
Life in a ‘Glass Castle’
During the dive, a large glass sponge observed at Shichiyo Seamount was found to play an important biological role for certain polychaete worms. This study formally describes and names two new species of polychaete worms—Dalhousiella yabukii and Leocratides watanabeae—which were found living symbiotically within the same glass sponge. Glass sponges construct an intricate, mesh-like skeleton out of silica, the same material used to make glass. Living within this structure is akin to residing in a “glass castle,” as the rigid, translucent framework provides a protected cavity for symbiotic animals.
Although these worms share a single “glass castle,” analysis suggests their symbiotic lifecycle evolved independently. A paper detailing these findings has been published in the Zoological Journal of the Linnean Society and can be accessed at the following link: https://doi.org/10.1093/zoolinnean/zlag028
Beyond these symbiotic worms, the Shichiyo seamount dives also revealed:
- Five new species of squat lobsters, including deep-sea species belonging to the genus Munidopsis.
- Newly observed octocorals, nemerteans, amphipods, gastropods, and kinorhynchs.
- Several species previously thought to be rare or absent from Japanese waters.
Together, these findings position the Shichiyo Seamount Chain as a region of special interest for Japanese marine science, with rich biodiversity hidden across terrain that had remained unvisited by researchers until now.
