The video isn’t as dramatic as Hollywood would like it to be. No glowing leviathan, no lunging predator. Nine kilometers below the surface, somewhere off the coast of Japan, a tiny, pale, faintly glowing shape drifts through black water. Even so, it’s difficult to watch the video without getting a little startled by the realization that the people who took it, who have dedicated their professional lives to cataloging what lives down there, still have no idea what they’re looking at.
Animalia incerta sedis is the temporary name they have given it. The taxonomic equivalent of a shrug, it is a Latin acronym for an animal of uncertain placement. Working out of the Tokyo University of Marine Science and Technology and the Minderoo-UWA Deep-Sea Research Center, the team recorded the creature at 9,131 meters in the Izu-Ogasawara Trench during a 2022 expedition that was just published in the Biodiversity Data Journal.
| Detail | Information |
|---|---|
| Mystery organism | Animalia incerta sedis (provisional designation) |
| Depth of sighting | 9,131 metres |
| Location | Izu-Ogasawara Trench, off Japan |
| Lead institutions | Minderoo-UWA Deep-Sea Research Centre; Tokyo University of Marine Science and Technology |
| Expedition partners | Inkfish and Caladan Oceanic |
| Expedition duration | Two months in 2022 |
| Trenches surveyed | Japan, Ryukyu, Izu-Ogasawara |
| Method used | Baited cameras and submersibles (no trawling) |
| Previous deepest nudibranch | Around 4,000 metres |
| Published in | Biodiversity Data Journal, 2026 |
The researchers initially believed they were examining a nudibranch, a type of sea slug. The head had two thin projections that resembled rhinophores, the tiny antennae that nudibranchs use to taste water, and the body was symmetrical on both sides. Some of them were reminded of the alabaster nudibranch found off the Pacific Northwest by the whitish, nearly translucent skin. Then the doubts began. The appendages appeared too stiff and rigid to belong to a soft-bodied slug, according to some of the experts they consulted. Some speculated that the animal appeared to have what they loosely referred to as “molluscan morphology,” but they were unable to elaborate. The issue of depth is another.
About 4,000 meters was the depth of the deepest nudibranch ever found. This object was more than twice as deep in the water. It’s either something completely different or a nudibranch that has subtly evolved to withstand crushing pressure no one anticipated it could withstand. Maybe something fresh.
It’s worthwhile to consider how the video was actually recorded. For many years, deep-sea biologists have relied on trawl nets to retrieve specimens; while this method is effective, it often tangles anything delicate by the time it reaches the deck. In contrast, the Japan expedition used submersibles and baited cameras to observe animals while they were stationary. The distinction is evident.
Carnivorous sponges, a “supergiant” amphipod named Alicella gigantea, a snailfish feeding at a record 8,336 meters, and dense meadows of crinoids—more than 1,500 of them—anchored to the rocks like underwater wildflowers were all recorded by the team over the course of two months. Photographed at 6,300 meters, the six-legged shrimp-like creature Cerataspis monstrosus appears to be examining the camera, much like an inquisitive cat might.
The hadal zone, or anything below 6,000 meters, continues to be one of the regions of the planet that we know about in theory but not in practice. There are maps. There are charts. However, the ocean begins to behave more like a place we’ve only glimpsed and less like a place we’ve mapped the deeper you go. Observing the results of this trip gives us the impression that our knowledge of what exists down there is still in its infancy.
The team has framed the work with caution. As of yet, they are not claiming a new species. Before anyone attempts to make management or conservation decisions regarding these trenches, the paper aims to establish a visual baseline, or a record of what is truly present in these trenches. That is the appropriate response. It’s probably easier to be honest. They sought the deep, and the deep provided them with something they were unable to identify.
