A ‘Glass Castle’ Discovered at the Bottom of the Sea Has Never-Before-Seen Inhabitants
A 2025 ocean expedition in two of Japan’s least-studied deep-sea zones turned up dozens of species previously unknown to science — including two worms that evolved to make their home inside the skeleton of a glass sponge.
The findings, spread across two newly published studies, represent one of the most dramatic expansions of known ocean biodiversity in recent memory.
They also suggest we’ve barely scratched the surface of what lives in the deep.
The Nippon Foundation-Nekton Ocean Census expedition launched in June 2025, conducted in partnership with the Japan Agency for Marine-Earth Science and Technology (JAMSTEC).
Scientists traveled aboard the JAMSTEC research vessel Yokosuka and made their dives using the Shinkai 6500, a crewed submersible.
The submersible returned more than 528 specimens from two understudied deep-sea regions: the Nankai Trough and the Shichiyo Seamount Chain.
After the expedition, taxonomists from around the world gathered at JAMSTEC headquarters for a Species Discovery Workshop. They identified 38 new species and flagged 28 additional candidates for further study.
Two confirmed new species drew particular attention: a pair of polychaete worms found living inside the body of a hexactinellid sponge, sometimes called a “glass sponge” or “glass castle.”
What is a ‘Glass Castle?’
Glass sponges build intricate, mesh-like skeletons out of silica, the same material used to make glass.
According to the National Oceanic and Atmospheric Administration (NOAA): “Some species of glass sponges produce extremely large spicules that fuse together in beautiful patterns to form a ‘glass house’—a complex skeleton that often remains intact even after the sponge itself dies.”
That durability matters. “The skeleton of the glass sponge, together with various chemicals, provides defense against many predators,” the NOAA adds.
So a glass sponge skeleton is a ready-made shelter on the ocean floor. The two newly discovered worm species — named Dalhousiella yabukii and Leocratides watanabeae — have evolved to take advantage of it.
Both live inside the body of their glass sponge host. The host sponge itself may be a new, yet-to-be-named species, according to the study.
A study detailing the worm discoveries, led by Dr. Naoto Jimi, was published in the Zoological Journal of the Linnean Society on March 9.
A Five-Fold Jump in Known Species at the Nankai Trough
The worm finding grabbed headlines, but the second study from the expedition may carry even bigger implications for ocean science.
Led by JAMSTEC researcher Dr. Chong Chen and published in Ecosphere on Nov. 18, it represents the most comprehensive biological survey of the Nankai Trough ever undertaken.
The Nankai Trough is one of Japan’s most geologically active deep-sea regions, but before this expedition, only 14 animal species were known to inhabit the area’s cold seeps.
The new census documented 80 species. That’s a five-fold increase from a single expedition.
The species found included 33 molluscs, 23 annelids, 11 arthropods, five ribbon worms, four echinoderms, three cnidarians, and one bryozoan.
In the Shichiyo Seamount Chain, scientists discovered coral gardens, sponge-dense habitats, and five new squat lobster species alongside the two new worm species.
The Shichiyo Seamount Chain is now recognized as a region of special interest for Japanese marine science.
Why This Changes What We Know About the Ocean Floor
The scale of what a single expedition uncovered — 38 confirmed new species, 28 candidates, and a jump from 14 to 80 known species in one region — offers a concrete measure of how much of the ocean remains unstudied.
Each species represents a distinct organism with its own evolutionary history, ecological niche, and potential relevance to fields from biomaterials to marine pharmacology.
“The discoveries made in the Nankai Trough and the Shichiyo Seamount Chain remind us how little of our ocean has truly been explored,” Mitsuyuki Unno, Executive Director of The Nippon Foundation, said in a news release.
“Each new species discovery is a step toward understanding, valuing, and ultimately safeguarding our shared ocean,” he added.
The Nippon Foundation-Nekton Ocean Census is an ongoing initiative, and the 28 candidate species from this expedition still await formal description.
A single voyage with one submersible expanded the known species count in one region by more than 400%. The 28 remaining candidates still need formal classification. The deep ocean has a lot left to reveal.
Production of this article included the use of AI. It was reviewed and edited by a team of content specialists.
