Monterey Bay Aquarium Research Institute discovers a new deep-sea creature.

The deep-sea is the largest habitat on Earth and remains a dark, mysterious place.

Aptly named the “mystery mollusc,” this translucent, gelatinous creature with a paddle-like tail and visible internal organs is the latest deep-sea discovery from the Monterey Bay Aquarium Research Institute. It is the first species of its kind found to inhabit the waters of the deep.

Researchers were initially stunned by its diet, behavior, bioluminescence and morphology, which showed no close relation to any known families.

Its scientific name, Bathydevius caudactylus, reflects the incredibly unique morphology, paving the way for a new family, class and species. 

“Our discovery is a new piece of the puzzle that can help better understand the largest habitat on Earth,” said MBARI Senior Scientist Bruce Robison in a press release, who led efforts to describe the mystery mollusc.

(You can see a video MBARI captured of the mystery mollusc below.)

MBARI first discovered the creature in early 2000 using their advanced underwater remote operated vehicle (ROV) Tiburon, at a depth of 8,576 feet in Monterey Bay. Over the last two decades, researchers have cataloged 150 sightings of these creatures underwater, capturing footage with ROVs. After successfully collecting a specimen to examine in the lab, they were finally able to confirm that the animal is a nudibranch.

Nudibranchs, also known as “naked snails,” are a type of mollusc with soft bodies which shed their shells early in life. They are a type of sea slug, often noted for their vibrant colors and fingerlike projections that protrude from their backs. 

The mystery mollusc too shares these fingerlike projections emerging from its paddle-like tail, although unlike many other nudibranchs, which commonly live in coastal waters like tide pools and coral reefs, the mystery mollusc is found in what is known as the bathypelagic zone, or the ocean’s “midnight zone” found 3,300 to 13,100 feet below the surface.

The mystery mollusc is the first nudibranch known to live in this deep, expansive environment of the open ocean, descending to the sea floor to spawn and feeding off crustaceans using its gelatinous hood. 

“When we first filmed it glowing with the ROV, everyone in the control room let out a loud ‘Oooooh!’ at the same time. We were all enchanted by the sight,” said MBARI Senior Scientist Steven Haddock in the press release. 

“Only recently have cameras become capable of filming bioluminescence in high-resolution and in full color. MBARI is one of the only places in the world where we have taken this new technology into the deep ocean, allowing us to study the luminous behavior of deep-sea animals in their natural habitat.”