Glowing and Swimming Sea Slugs Newly Discovered in the Deep Sea

Monterey Bay researchers have discovered a remarkable new species of sea slug that lives in the deep sea. The glowing nudibranch swims through the ocean’s midnight zone with a large gelatinous hood and paddle-like tail and lights up with brilliant bioluminescence.

This transparent sea slug has a bright red stomach that looks like a raspberry. It captures tiny, fast-swimming shrimp and other prey with a cavernous hood. When threatened, it can emit bioluminescence — a light produced by a chemical reaction that is uncommon in sea slugs — and shed a glowing “decoy” piece of its tail as a distraction.

What’s unique about this newly discovered family of sea slugs, also known as Bathydevius caudactylus, is that it’s one of the first known species living in the deepest parts of the ocean — what researchers call the midnight zone because of its perpetual darkness — an expansive environment of open water 3,300 to 13,100 feet below the surface.

Most sea slugs live on the sea floor and in shallow water, in tide pools, kelp forests and on coral reefs. “To find one out in the deep water column, living entirely away from the sea floor, was a big surprise,” said Bruce Robison, a senior scientist with the Monterey Bay Aquarium Research Institute who led efforts to describe the new nudibranch species.

Researchers first identified this fascinating animal in 2000 during a deep-sea dive in Monterey Bay with a remote-driven underwater vehicle at a depth of over 8,500 feet. After reviewing more than 150 sightings from MBARI’s vehicles over the past twenty years, researchers were finally able to publish a detailed description of this animal.

Their genetic makeup and anatomy make them different from other sea slugs we know, Robison said. With a gelatinous hood on one end and a flat paddle-like tail with “fingers” at the other and colorful internal organs in between, researchers initially struggled to categorize this animal in a group. “The way it looks is also quite different from all of the other known species. It’s a funny-looking critter,” he added.

Because the animal also has a bottom like a snail, they nicknamed this the “mystery mollusc.”

After gently collecting a specimen, MBARI researchers were able to take a closer look at this mystery sea slug in the lab. “For the most part, we did all of our investigations in the animal’s natural habitat without disturbing them, which is a more natural approach to studying deep-sea animals like this,” Robison explained.

The mollusc is currently known to live in the waters offshore of the Pacific coast of North America, from Oregon to Southern California. A similar sighting by NOAA researchers in the Mariana Trench, which is located in the western Pacific Ocean, suggests it may have a broader range.

Through detailed investigations of anatomy and genetics, they began to solve the mystery, finally confirming that this incredible animal was a nudibranch.

Unlike most other sea slugs with “teeth” on their tongues to help break down food, the Bathydevious lacks that raspy tongue, also known as a radula. “It appears that they just engulf their food whole. They get it inside that big hood and down the gullet and into the stomach directly without breaking it up,” Robison said, adding that this species relies on chemical means to digest the food rather than chewing or breaking it up, similar to how a Venus fly trap would capture food, a technique seen in some other deep-sea creatures like jellies, anemones and tunicates.

“And that brings up one of the most perplexing aspects of what we’ve learned and haven’t learned about these animals,” he added.

The mystery mollusc is just one of many fascinating discoveries MBARI has made in the midnight zone. The institute has documented over 250 new deep-sea species previously unknown to science.

“We’ve been working here in Monterey Bay for more than 30 years, and we’re still making significant discoveries. But think what else is out there in the rest of the world’s oceans. It’s staggering to imagine what’s out there yet waiting to be discovered,” Robison said.

“Deep-sea animals capture the imagination. These are our neighbors that share our blue planet. Each new discovery is an opportunity to raise awareness about the deep sea and inspire the public to protect the amazing animals and environments found deep beneath the surface.”