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Guest blog post by Dr. Denise Swanborn
The Minderoo-UWA Deep-Sea Research Centre at the University of Western Australia specialises in exploring life in the most extreme parts of the ocean – the abyssal (about 3,000-6,000 meters deep) and hadal (anything deeper than 6,000 meters) zones.
Reaching these depths is not easy. It takes specialised vessels, full-ocean depth-rated equipment, and enormous coordination and planning. Months – sometimes years – of preparation all lead up to a few weeks at sea.
In 2022, during a two-month expedition funded by Caladan Oceanic and Inkfish aboard the vessel DSSV Pressure Drop, together with scientists from the Tokyo University of Marine Science and Technology, we descended into some of the deepest places around Japan: The Japan Trench, Ryukyu Trench and Izu-Ogasawara Trench – the latter reaching nearly 10 km below the ocean’s surface.
Results are now available in “Faunal biodiversity of the lower abyssal and hadal zones of the Japan, Ryukyu and Izu-Ogasawara trenches (NW Pacific Ocean; 4534-9775 m)“, published in the Biodiversity Data Journal. We logged about 460 h of seafloor video footage, documenting at least 108 morphotaxa (distinct visual organism groups) between 4,534 and 9,775 m depth.
The bigger picture
We used two methods to document deep-sea life – crewed submersible transects and free-fall baited landers. This combination enabled us to build the most comprehensive visual baseline yet for abyssal and hadal megafauna in the Northwest Pacific to date.
The submersible ‘Limiting factor’ before the expedition launch. Photo credit to Minderoo-UWA Deep-Sea Research Centre, Inkfish and Caladan Oceanic. 
The ‘Limiting Factor’ submersible descending. Photo credit to Minderoo-UWA Deep-Sea Research Centre, Inkfish and Caladan Oceanic. 
The submersible observed from the lander. Photo credit to Minderoo-UWA Deep-Sea Research Centre, Inkfish, Caladan Oceanic.
Subduction trenches, such as the Japan Trench, Ryukyu Trench, and Izu-Ogasawara Trench, are the planet’s deepest geological features. They are formed by the process of subduction, in which one tectonic plate is forced beneath another. Although this movement is slow on human timescales – cms per year – over millions of years, it carves long, narrow depressions in the seafloor that reach extreme depths, reshaping entire ocean basins.
These subduction zones are also known for major seismic events, such as earthquakes, which can trigger submarine landslides, remobilising sediment and rapidly transporting material into the trenches.
The trenches we studied are situated along some of the most tectonically active margins on Earth. Combined with the effects of depth, high hydrostatic pressure, near-freezing temperatures, and lack of sunlight, life in subduction trenches is shaped by a remarkable combination of forces. Understanding biodiversity in these systems, therefore, also means consideration of the unique environmental context that has shaped the ecosystems observed today.
Why this study matters
Historically, our understanding of abyssal and hadal ecosystems, including those associated with subduction features, relied largely on trawls and physical samples. While these methods provide essential information, they can damage fragile organisms and rarely capture behaviour or ecological context.
Although observing them in their own habitat is still rare, image-based identification is central to biodiversity science at extreme depths. But without illustrated guides, consistent identification is difficult, often leaving researchers like ourselves with limited reference material.
So, this study was not simply about observing deep-sea organisms, but also aimed to establish a foundation for future research at these depths. With the help of taxonomic experts worldwide, we wanted to create baseline biodiversity data for the region at these depths – information that can inform imagery-based surveys and targeted specimen collection on subsequent expedition.
What did we see?
The submersible surveys enabled us to traverse different habitats and observe animals in their natural environment. Just as importantly, the submersible dives allowed us to record behaviour – something that remains poorly documented at these depths.
Bassozetus sp. at a 6,600 m depth. Credit to Jamieson et al., 2026, Minderoo-UWA Deep-Sea Research Centre, Inkfish and Caladan Oceanic. 
Snailfish and Eurythenes sp., feeding off the baited lander at 7,500 m depth. Credit to Jamieson et al., 2026, Minderoo-UWA Deep-Sea Research Centre, Inkfish and Caladan Oceanic.
Among the most striking encounters were dense aggregations of stalked crinoids at 9,137 m (forming a “crinoid meadow”), with hundreds of individuals anchored to rocks and terraces. We also observed carnivorous sponges at nearly 10 km depth, and filter-feeding isopods perched along rock edges.
Crinoid meadow at a 9,300 m depth. Credit to Jamieson et al., 2026, Minderoo-UWA Deep-Sea Research Centre, Inkfish and Caladan Oceanic. 
Crinoid on a rock at a 9,100 m depth. Credit to Jamieson et al., 2026, Minderoo-UWA Deep-Sea Research Centre, Inkfish and Caladan Oceanic. 
Isopods at a 7,500 m depth. Credit to Credit to Jamieson et al., 2026, Minderoo-UWA Deep-Sea Research Centre, Inkfish and Caladan Oceanic. 
Supergiant amphipod and Pseudoliparis fish at 7,000 m depth. Credit to Jamieson et al., 2026, Minderoo-UWA Deep-Sea Research Centre, Inkfish and Caladan Oceanic.
Meanwhile, baited landers revealed a different side of the ecosystem: the mobile scavenger and predator communities. They are deployed from the surface and free-fall to the seabed.
Once settled, they record everything that approaches the bait – mostly crustaceans and fish. We caught footage of supergiant amphipods (Alicella gigantea), predatory amphipods hunting smaller prey, and perhaps most excitingly, a snailfish feeding at depths down to 8,336 m – the deepest in-situ observation of a fish to date.
Together, these two approaches revealed complementary parts of the same system and show that the hadal environment supports a remarkable diversity of life, with organisms highly adapted to conditions considered extreme by human standards.
Rarities and mysteries
Even with high-definition footage and international taxonomic expertise, some animals remain enigmatic. Twice, we filmed a slow-gliding animal that we have not been able to confidently assign to any phylum. Some thought they looked like nudibranchs, others like sea cucumbers, but nobody could agree.
Then there were xenophyophores: giant, single-celled organisms that build elaborate homes out of sediment. While a few morphotypes could be identified, the overall diversity and number of species remain uncertain.
Are the studied trenches the same?
One of the most exciting aspects of this study was the ability to compare three separate subduction trenches. Broadly, many morphotaxa were shared across all three but local patterns differed. The Japan Trench hosted the highest number of observed morphotaxa, likely influenced by both sampling effort and environmental heterogeneity.
The Izu-Ogasawara Trench, the deepest surveyed, showed dramatic biological aggregations in its deepest parts. The Ryukyu Trench, despite being about as deep as the Japan Trench, lacked several taxa present in the other trenches. The differences in community composition and diversity likely reflect variations in depth, tectonic setting and nutrient input from surface waters.
Final thoughts
By integrating footage from submersible transects and baited lander deployments, we gained an unprecedented picture of the habitats and biodiversity of the Japanese subduction trenches.
Our observations show that these systems are shaped by a complex interplay of geological processes, productivity, disturbance and depth. 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.
More than anything, the hadal zone remains one of Earth’s least-explored and most intriguing frontiers. As technological capabilities advance, continued exploration will be essential for uncovering the mechanisms that sustain life at these depths.
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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