A creature filmed at 9,000 m in the Japan Trench — why can no one classify it?

Imagine this. Nine thousand metres below the surface, where no light reaches and pressure is 900 times that at sea level. There, cameras captured an animal that has stumped taxonomists worldwide.

In 2022, a joint team from the Minderoo-UWA Deep-Sea Research Centre and Tokyo University of Marine Science and Technology boarded the DSSV Pressure Drop. Their destination: the Japan, Ryukyu and Izu-Ogasawara trenches. Over two months, they catalogued 108 distinct organism groups — among them world records and an animal that science admits it cannot classify.

Deep-sea surveys without nets

Traditionally, deep-sea fauna have been studied by trawling the seabed. However, trawl nets damage fragile organisms and rarely capture behavioural or ecological information.

This team combined two approaches: crewed submersible transects for observing seafloor-associated animals, and free-fall baited landers to attract and film deep-sea scavengers.

This combination enabled us to build the most comprehensive visual baseline yet for abyssal and hadal megafauna in the Northwest Pacific to date

— Research team (Minderoo-UWA Deep-Sea Research Centre / Tokyo University of Marine Science and Technology)

World records broken one after another

Snailfish — feeding at 8,336 m

A baited lander captured a snailfish (Pseudoliparis sp.) feeding at 8,336 metres — the deepest in-situ fish observation ever recorded.

The landers also recorded the 'supergiant' scavenging amphipod Alicella gigantea across all three surveyed trenches.

Carnivorous sponges — predators at 9,744 m

In the Izu-Ogasawara Trench, crewed submersibles filmed carnivorous sponges (family Cladorhizidae) at 9,568–9,744 metres — the deepest in-situ carnivorous sponge observation on record.

A 'meadow' of 1,500 crinoids

At the Boso triple junction, 9,137 metres deep, submersibles discovered over 1,500 stalked crinoids anchored to rock terraces. The team calls this sight 'crinoid meadows.'

An animal that defies classification

The expedition's greatest mystery is an organism filmed twice at 9,137 metres, gliding slowly along the seabed.

Described in the paper as Animalia incerta sedis — an animal of uncertain taxonomic placement — it bears visual similarities to nudibranchs and sea cucumbers, yet global experts could not confidently assign it to any known phylum.

In other words, it is known to be an animal, but no one can say what kind.

Human traces reach even the deep sea

The deep trenches may seem beyond human reach. Yet the team reports finding evidence of human-derived debris during the survey.

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

— Research team

Video opens a new era for hadal research

Historically, our understanding of abyssal and hadal ecosystems 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

— Research team

This study was not simply about observing deep-sea organisms, but also aimed to establish a foundation for future research at these depths. The hadal zone remains one of Earth's least-explored and most intriguing frontiers

— Research team

The team plans to develop comprehensive illustrated guides to support future imagery-based biodiversity surveys.

For more deep-sea species discoveries, see 'More than 110 new species from the Coral Sea discovered.'

For more on in-situ deep-sea finds, see 'A time capsule at 3,000 m — first megalodon tooth found in situ.'

A note from the author: Personally, what struck me most wasn't the mystery creature but the crinoid meadows. Over 1,500 crinoids lined up on a rock ledge at 9,000 metres — just imagining it sends a shiver down my spine.

Also, DSSV Pressure Drop — isn't that an impossibly cool name for a research vessel?