A new study, published in the Biodiversity Data Journal, provides a profound look at life up to nearly 10 kilometers below the ocean's surface in the Japan, Ryukyu, and Izu-Ogasawara trenches. The research catalogs at least 108 distinct organism groups (morphotaxa), including the deepest-ever observation of a fish and a baffling, unidentified animal that has left global taxonomic experts stumped.
Conducted during a two-month 2022 expedition aboard the vessel DSSV Pressure Drop, the mission was a collaboration involving scientists from the Minderoo-UWA Deep-Sea Research Centre and the Tokyo University of Marine Science and Technology and funded by Caladan Oceanic LLC and Inkfish.
Rather than rely on traditional trawls and physical sampling - which can damage fragile organisms and rarely capture behavior - the team used a dual approach: crewed submersible transects to study seafloor-associated animals and their habitats, and free-fall baited landers to target bait-attending fauna such as fishes and decapods.
This combination enabled us to build the most comprehensive visual baseline yet for abyssal and hadal megafauna in the Northwest Pacific to date.
- the team noted.
An animal science cannot name
Perhaps the most enigmatic encounter was a unique, slow-gliding organism, currently designated as Animalia incerta sedis, filmed twice at depths down to 9,137 meters. Despite extensive consultations with global taxonomic experts, the animal cannot be confidently assigned to any known phylum. While it shares some visual traits with nudibranchs or sea cucumbers, its identification remains a mystery.
Crinoid meadows and carnivorous sponges
Further, the crewed submersible transects allowed researchers to observe dense aggregations of deep-sea life in their natural benthic habitats and record rare behaviors. At the base of the Boso triple junction at 9,137 meters, the team traversed stunning "crinoid meadows" consisting of over 1,500 stalked crinoids anchored to rock terraces.
Additionally, the submersibles recorded carnivorous sponges belonging to the Cladorhizidae family in the Izu-Ogasawara Trench at depths between 9,568 and 9,744 meters, representing the deepest in-situ observation of carnivorous sponges to date.
A new depth record for fish
Building on previously published findings from the expedition, baited landers captured footage of a snailfish (Pseudoliparis sp.) feeding at a record depth of 8,336 meters - the deepest in-situ observation of a fish ever recorded. These landers also revealed the presence of the massive "supergiant" scavenging amphipod, Alicella gigantea, across all three surveyed trenches, as well as several other bait-attending fishes and invertebrates at great depth.
Patterns and pressures
The study revealed that while many organism groups are shared across the region, local patterns differ significantly, with the Japan Trench hosting the highest number of observed morphotaxa. These differences highlight how geological processes, depth, and nutrient inputs from surface waters shape life in subduction zones. The researchers also made an observation regarding human impact: "While it’s easy to think of deep-sea trenches as untouched wilderness, our findings also showed evidence of human-derived debris, likely transported by downslope processes."
A foundation for future exploration
The team emphasized the necessity of a non-destructive visual approach. "Historically, our understanding of abyssal and hadal ecosystems, including those associated with subduction features, relied largely on trawls and physical samples," they explained. "While these methods provide essential information, they can damage fragile organisms and rarely capture behaviour or ecological context."
By producing a comprehensive, illustrated guide of these habitats, the research team hopes to support future imagery-based biodiversity surveys.
This study was not simply about observing deep-sea organisms, but also aimed to establish a foundation for future research at these depths. More than anything, the hadal zone remains one of Earth’s least-explored and most intriguing frontiers.
- the team concluded.
Original source:
Jamieson AJ, Swanborn DJB, Bond T, Cundy MC, Fujiwara Y, Lindsay D, Stott MS, Kitazato H (2026) Faunal biodiversity of the lower abyssal and hadal zones of the Japan, Ryukyu and Izu-Ogasawara trenches (NW Pacific Ocean; 4534-9775 m). Biodiversity Data Journal 14: e182172. https://doi.org/10.3897/BDJ.14.e182172
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