Over 30,000 animals captured on video in a deep trench never seen before

The survey filmed 29,556 animals in Japan’s deepest ocean trenches, revealing how life copes with darkness and intense pressure below the surface.

An international team used a full ocean depth submersible to visit three trenches off Japan and film the seafloor in rare detail.

Deep and hidden world

The work was led by Dr. Denise Swanborn, a deep sea ecologist at The University of Western Australia’s Minderoo Deep Sea Research Center (MDSR).

Her research focuses on how the deepest ocean communities respond to shifting food supplies and physical disturbance. Below regular deep sea levels lies the hadal zone, the part of the ocean deeper than about 20,000 feet.

These narrow trenches cover only a tiny share of seafloor area yet hold some of the most unusual communities on Earth.

At those depths, pressures climb to well over a thousand times what we feel at sea level and temperatures hover just above freezing. Animals here tend to grow slowly, live long lives, and depend on scarce food drifting down from far above.

Filming life in the hadal zone

To reach those depths, the team used a crewed submersible, a small research vehicle designed to carry people safely underwater for long periods.

Each dive followed the seafloor between about 23,000 and 32,000 feet down while cameras recorded everything moving across the sediment and rocks.

The dives targeted three subduction trenches, long arcs where one tectonic plate sinks beneath another, known as the Japan, Ryukyu, and Izu Ogasawara trenches. Together they form a chain of deep gullies running along Japan’s eastern and southern margins.

Back on shore, the scientists paused the footage again and again to log every visible animal. They sorted those animals into 70 broad morphotypes across 11 major animal groups.

They also defined eight distinct habitat types that describe seafloor texture and slope. These categories captured everything from flat muddy plains to steep rocky walls and piles of loose debris.

Food supply reshapes trench animals

Even at these depths, food starts at the surface as particulate organic carbon, tiny bits of dead plankton and waste. Those particles fall through the water column and eventually settle onto the seafloor as a slow but steady food source.

Earlier studies show that trenches sitting under productive surface waters tend to receive stronger pulses of this sinking food.

In the new work, seafloor areas that received more of these particles supported denser communities and more deposit feeders than leaner sites.

These contrasts emerged even at similar depths because food supply changed from trench to trench. Depth alone could not explain which species were present or how many individuals the seafloor could support.

Along some slopes, filter feeding animals stretched out arms or tentacles to capture particles directly from the water. Elsewhere, burrowing worms and sea cucumbers churned through the sediment, processing the thin organic coating that had settled on the mud.

Earthquakes carve and reset habitats

The Japanese trenches sit along a zone where tectonic plates collide, so underwater earthquakes and landslides are common. 

After the 2011 Tohoku Oki earthquake, an earlier investigation showed muddy flows burying animals and spreading sediment across parts of the Japan Trench.

Such pulses of moving sediment can strip away existing communities, bury others, and send carbon rich material into trench axes where deposit feeders thrive. Over time, this reshuffling creates a patchwork of disturbed and stable areas along each trench.

She explained in an article that seismically active trench sections held dense, low diversity groups, while more stable slopes supported richer communities.

“Within trenches, at the same depth band, differences in historical seismic disturbance and seafloor stability created different communities,” said Dr. Swanborn.

Different trenches, different animals

In the axis and forearc of the Japan Trench, the videos showed swarms of mysid shrimps and broad patches of sea cucumbers on muddy areas. At similar depths in the Ryukyu Trench, brittle stars dominated the seafloor and sea cucumbers were scarce.

In the Izu Ogasawara Trench, the team found meadows of stalked crinoids, animals that anchor to hard rock and filter particles from water. 

Farther south and deeper, the trench floor supported thickets of carnivorous cladorhizid sponges and roaming sea cucumbers instead of fish.

Records of such animals below about 29,500 feet are rare, and earlier work described only scattered samples from a few trenches.

Finding fields of stalked animals and sponges at these depths suggests the right mix of hard surfaces and flowing water can create deep sea hotspots.

Why deep trenches matter

Hadal trenches act as storage sites for sinking organic matter, helping lock away carbon that might otherwise cycle higher in the water column.

Knowing which habitats host the most diverse and abundant communities helps researchers estimate how much life is supported by this hidden carbon sink.

The new results also show that biodiversity in the deepest ocean depends on both slow background conditions and sudden shocks.

Regional food supply sets the stage, while earthquakes and landslides rewrite the seafloor in ways that favor either hardy opportunists or more varied communities.

Because full depth video surveys are still rare, each new expedition adds important pieces to the picture of how life functions under extreme pressure.

These discoveries in Japan’s trenches provide a guide for exploring other deep ocean regions where food supply and disturbance may combine in different ways.

The study is published in the Journal of Biogeography.

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