Researchers have filmed an unidentified animal floating through 9,100m-deep waters off the coast of Japan.
The strange-shaped whiteish creature is currently known as Animalia incerta sedis because scientists haven’t been able to classify it.
“Initially, the authors speculated that this organism might be a nudibranch,” says the study, published in Biodiversity Data Journal. Like sea slugs, its body could be divided into two symmetrical halves and had antennae-like projections that looked like a nudibranch’s rhinophores. Its ghostly appearance also reminded the team of the alabaster nudibranch (Dirona albolineata).
But other experts they spoke to weren’t so sure. “Some noted that the appendages appeared too rigid to belong to a nudibranch,” write the study authors, “while others speculated that they appeared to be of 'molluscan morphology', but could not speculate beyond that.”
At 9,131m, it was also twice as deep as the deepest known nudibranch, which was recorded at around 4,000m. At the time, that strange organism bewildered researchers too. Because they couldn’t identify it, they dubbed it the ‘mystery mollusc’.
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The study also documents the marine diversity found in the Japan, Ryukyu and Izu-Ogasawara trenches during a two-month expedition in 2022.
They recorded many different species, including carnivorous sponges, a 'supergiant' scavenging amphipod (Alicella gigantea), a snailfish feeding at a record 8,336m and meadows of more than 1,500 crinoids attached to the rocks.
Deep-sea scientists often trawl for samples, which can harm the animals. On this mission, the team tried to minimise any damage by taking photos and videos from submersibles and baited cameras.
“This combination enabled us to build the most comprehensive visual baseline yet for abyssal and hadal megafauna in the Northwest Pacific to date,” says a statement from the team, which is made up of researchers from the Minderoo-UWA Deep-Sea Research Centre and the Tokyo University of Marine Science and Technology.
Studying the habitat through image-based surveys was an important part of this mission. “While [trawls and physical samples] provide essential information, they can damage fragile organisms and rarely capture behaviour or ecological context,” they add.
“This study was not simply about observing deep-sea organisms, but also aimed to establish a foundation for future research at these depths. More than anything, the hadal zone remains one of Earth’s least explored and most intriguing frontiers.”
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Life in Japan's deep-sea trenches
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