Filmed Twice, Identified Never: The Deep-Sea Animal That Fits No Known Branch of Life
Marine biologists are openly disagreeing about whether the creature is a sea slug, a sea cucumber, or a category of animal that humans haven't discovered yet.
By Ethan Denma
Thursday, May 14, 2026
Credit to Jamieson et al., 2026, Minderoo-UWA Deep-Sea Research Centre, Inkfish and Caladan Oceanic.
EARTH, Laniakea Supercluster—Something with no name and no known relatives just drifted past a camera nine kilometers under the Pacific. Twice.
Scientists have looked at the footage. Scientists have argued about the footage. Scientists have (for now) seemingly given up and filed it under Animalia incertae sedis—Latin for "of uncertain placement,” perhaps best colloquially translated as we'll get back to you.
The creature is a true deep-sea mystery. In a study published in the Biodiversity Data Journal on March 3, 2026, researchers from the Minderoo-UWA Deep-Sea Research Centre at the University of Western Australia and the Tokyo University of Marine Science and Technology concluded, simply, that the animal cannot be confidently assigned to any known phylum.
Phyla are the broadest divisions of animal life. Sponges are a phylum (the sea-floor blobs with no organs and no nervous system) and arthropods are a phylum (insects, spiders, lobsters, crabs, etc). So are chordates, the group that contains every fish, bird, lizard, and human (me, you, your cat). There are roughly 35 of them, depending on who's counting. To find an animal that fits none of them is, in taxonomic terms, definitely odd.
The pale, soft-bodied, and vaguely flat animal was spotted in the northwestern Pacific Ocean, somewhere in the pitch-black water column above the Izu-Ogasawara Trench, moving with the unhurried movements of a thing that has nowhere in particular to be.
From the footage, it appears almost ghostly—whitish, faintly flattened, drifting rather than swimming, with no obvious head, fins, or limbs to anchor it to any familiar group. It is also more than twice as deep as the deepest nudibranch ever recorded, which would seem to rule out one of the leading guesses.
"Twice, we filmed a slow-gliding animal that we have not been able to confidently assign to any phylum,"study lead author Denise Swanborn wrote in a guest post for the journal's publisher Pensoft in April. "Some thought they looked like nudibranchs, others like sea cucumbers, but nobody could agree."
The 2022 "Ring of Fire Expedition," aboard the deep-submergence vessel Pressure Drop and funded by Caladan Oceanic and Inkfish, was, by any measure, a haul. The team logged about 460 hours of seabed footage at depths between 4,534 and 9,775 meters, cataloguing 108 distinct organism groups across the Japan, Ryukyu, and Izu-Ogasawara trenches. The footage also produced the deepest in-situ observation of a fish ever recorded (a snailfish in the genus Pseudoliparis, feeding at 8,336 meters), along with hadal meadows of stalked crinoids and carnivorous sponges thriving below 9,500 meters.
But it's the unassigned glider, captured at 9,137 meters near the Boso triple junction where three tectonic plates collide, that has taken over the discourse. The paper itself notes that while the organism shares some visual traits with nudibranchs or sea cucumbers, "its identification remains a mystery."
Without a physical specimen for DNA sequencing, the team can't yet say whether it represents a deeply weird member of an existing phylum, a known group caught in an unfamiliar pose, or something genuinely new to science. The next expedition will, presumably, try to catch one.
Until then, the hadal zone keeps its secret.
"More than anything," Swanborn wrote, the deep ocean "remains one of Earth's least-explored and most intriguing frontiers."
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