New glowing species of sea slug that inhabits ocean's deep sea 'midnight zone' discovered

Researchers have discovered a new species of glowing sea slug deep in the ocean’s midnight zone.

Monterey Bay Aquarium Research Institute (MBARI) scientists said in a press release on Tuesday that while Bathydevius caudactylus is classified as a sea slug, it was nicknamed the "mystery mollusk" because the creature was unlike any other that’s been encountered before.  

The mystery mollusk’s genus name, Bathydevius, is a play on the deep-sea animal’s "devious" nature that fooled researchers, the researchers said.

Bathydevius is the first nudibranch, or sea slug, known to live in the deep sea. The sea slug’s body is made up of a large gelatinous hood and paddle-like tail. It can glow with bioluminescence. 

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Bathydevius caudactylus is classified as a sea slug. (MBARI)

It lives at an extreme depth of 1,000 to 4,000 meters, or 3,300 to 13,100 feet, below the surface in the ocean’s midnight zone, creating a unique challenge for scientists who’ve worked for decades to catalog the mysterious animal.

"We’ve invested more than 20 years in understanding the natural history of this fascinating species of nudibranch," said MBARI Senior Scientist Bruce Robison. "Our discovery is a new piece of the puzzle that can help better understand the largest habitat on Earth."

Bathydevius caudactylus can glow with bioluminescence, an ability that researchers say is to deter predators in the deep sea. (MBARI)

Researchers found that Bathydevius evolved to find unique ways to survive in its environment.

It uses a cavernous hood to trap crustaceans "like a Venus fly trap plant," like some jellies, anemones and tunicates. The mysterious creature also hides from predators in plain sight, taking advantage of its transparent body. 

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If it feels threatened, however, the sea slug will light up with bioluminescence to deter and distract hungry predators. Researchers once observed the creature illuminate and detach a "a steadily glowing finger-like projection from the tail, likely serving as a decoy to distract a potential predator."

Bathydevius caudactylus is believed to release its eggs on the seafloor. (MBARI)

Bathydevius, like other nudibranchs, is a hermaphrodite, having both male and female sex organs. Researchers said that the animal will descend to the seafloor to spawn, using their muscular foot to anchor to the muddy seafloor before releasing their eggs.

MBARI scientists said that it was only thanks to the facility’s advanced underwater technology that they were able to compile the most comprehensive description of any deep-sea creature to date.

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"What is exciting to me about the mystery mollusk is that it exemplifies how much we are learning as we spend more time in the deep sea, particularly below 2,000 meters," said MBARI Senior Scientist Steven Haddock. "For there to be a relatively large, unique, and glowing animal that is in a previously unknown family really underscores the importance of using new technology to catalog this vast environment."