A deep-sea biodiversity survey at Japan’s Nankai Trough has expanded the known animal life associated with its cold seep habitats by roughly five times, revealing a far richer ecosystem than previous records suggested. The work sits within a broader Ocean Census mission supported by the Nippon Foundation and Nekton in partnership with JAMSTEC, and it is being presented as evidence that major deep-sea regions near Japan remain biologically under-documented despite long-standing scientific capability in the country.

 

Cold Seeps as Biodiversity Hotspots in a Geologically Active Zone

 

The Nankai Trough lies several hundred kilometres southwest of Tokyo and is one of Japan’s most tectonically active marine regions. Its cold seep ecosystems are fuelled by methane-rich fluids emerging from the seabed, creating specialised communities that depend on chemosynthetic processes rather than surface-driven food supply alone. A JAMSTEC-led study described as the most comprehensive biological survey of these seep habitats to date reports a jump from 14 previously recorded animal species to 80 seep-associated species, transforming the baseline understanding of what lives there.

 

What Was Documented and How Diverse the Community Is

 

The survey reports a broad mix of animal groups across the seep habitats, with molluscs and annelids forming a large share of recorded diversity alongside arthropods and a spread of other taxa including ribbon worms, echinoderms and cnidarians. The results also include range extensions, first national records and previously unreported species associations, indicating that the region is not only diverse but also ecologically complex, with relationships and distributions that were not visible under earlier sampling effort.

 

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How the Expedition Generated the Evidence

 

The findings stem from a June 2025 expedition aboard JAMSTEC’s research vessel Yokosuka using the Shinkai 6500 manned submersible for deep dives across both the Nankai Trough and the Shichiyo Seamount Chain. Researchers collected more than 528 specimens and processed them for imaging, cataloguing and preservation, enabling both morphological identification and molecular analysis. A dedicated species discovery workshop held at JAMSTEC later in 2025 brought together taxonomists to confirm newly discovered species and align the pipeline of formal scientific description.

 

Seamount Ecosystems Add a Second Front of Discovery

 

Beyond the trench system, the mission also explored the Shichiyo Seamount Chain southeast of Tokyo, a set of submerged volcanic peaks that had been biologically understudied prior to this work. Submersible dives revealed coral gardens and sponge-dense habitats, and separate studies describe new symbiotic relationships and newly identified species, including polychaete worms living inside glass sponges. The broader seamount work also reported new squat lobster species and new national records across multiple groups, reinforcing the view that these isolated deep-water features support distinct communities that are still being mapped.

 

Why the Results Matter Beyond Taxonomy

 

The combined work reframes both the Nankai Trough and the Shichiyo Seamount Chain as high-priority regions for deep-sea science because they offer unusually strong returns in discovery per unit of exploration effort. The deeper significance is that better baseline biodiversity knowledge is foundational for any future decisions related to conservation, industrial activity, and environmental risk in deep waters, and these findings show that even well-studied maritime nations can still uncover major biodiversity gaps in nearby offshore regions when survey intensity increases.