Our first sighting of this unique sea slug stumped our scientists.

In early 2000, we encountered a curious critter during a deep-sea dive offshore of Monterey Bay. With a voluminous hooded structure at one end, a flat tail fringed with numerous finger-like projections at the other, and colorful internal organs in between, we initially struggled to place this animal in a group. A muscular foot suggested this might be a swimming snail, so we nicknamed this the “mystery mollusc.”

By leveraging advanced underwater technology, we were able to gather extensive natural history information about this remarkable animal from more than 150 sightings in the wild and detailed investigations of anatomy and genetics in the lab.

Meet Bathydevius caudactylus, the first nudibranch, or sea slug, known to live in the deep water column.

Most nudibranchs live on the seafloor. They are common in coastal environments, including tide pools, kelp forests, and coral reefs. A small number of species are known to live on the abyssal seafloor. A few are pelagic and live in open waters near the surface. Bathydevius caudactylus swims slowly through the midnight zone, an expansive environment of open water 1,000 to 4,000 meters (3,300 to 13,100 feet) below the surface.

While most sea slugs use a raspy tongue to feed on prey attached to the seafloor, the mystery mollusc uses a cavernous hood to trap prey like a Venus fly trap plant. Crustaceans are on the menu for Bathydevius caudactylus, though we are still not quite sure how such a slow swimmer catches such speedy prey.

If threatened, Bathydevius caudactylus can light up with bioluminescence. On one occasion, we observed a mystery mollusc illuminate and then detach a glowing finger-like projection from the tail, much like a lizard dropping its tail as a decoy to distract predators.

Bathydevius caudactylus is typically spotted swimming or floating in the water column, but descends to the seafloor to reproduce. We have observed several spawning individuals attached to the muddy seafloor with their muscular foot.

The deep sea is the heart of our planet and our climate system. These inky depths teem with life. Human actions affect habitats and animals deep underwater—many that we have yet to explore and discover. MBARI science and technology are helping establish a baseline understanding of ocean health and deep-sea biodiversity so we can better understand the impacts of threats like climate change, pollution, and mining. Each new discovery is a new piece of the puzzle.