New species of sea slug discovered in the Monterey Bay region, nicknamed the “mystery mollusk”

MONTEREY BAY, Calif. (KION-TV) -- Monterey Bay Aquarium Research Institute (MBARI) announced Tuesday that they've discovered a new species of sea slug that lives in the deep sea known as Bathydevius caudactylus (or the "mystery mollusk").

"It has a big oral hood at one end that leads into the mouth and sort of a flat tail at the other end with some finger-like projections sticking out in the middle," said Bruce Robison, a senior scientist at the Monterey Bay Aquarium Research Institute. "It has a body that surrounds a bunch of brightly colored internal organs and a cylindrical foot that projects down below."

According to MBARI, the deep sea creature swims through the ocean's midnight zone and lights up with bright bioluminescence. "The mystery mollusk uses a cavernous hood to trap crustaceans like a Venus fly trap plant," MBARI wrote in its initial findings, whereas most other sea slugs use a "raspy tongue" to feed on seafloor prey.

Robison and his team of scientists actually first discovered the creature in February 2000 during a deep dive using the institute's remote operated vehicle (ROV) offshore Monterey Bay at about 8,576 feet below the surface.

MBARI says that after studying the creature in over 150 sightings over the past 20 years, they were finally able to confirm that it is, in fact, a sea slug.

“Thanks to MBARI’s advanced underwater technology, we were able to prepare the most comprehensive description of a deep-sea animal ever made," said Robison. "We’ve invested more than 20 years in understanding the natural history of this fascinating species of nudibranch. Our discovery is a new piece of the puzzle that can help better understand the largest habitat on Earth."

MBARI scientists discovered that the mystery mollusk is the first nudibranch (or sea slug) to live in a deep water column. They say that most live on the seafloor in coastal environments like tide pools, kelp forests and coral reefs. But, the mystery mollusk lives in open water 3,300 to 13,100 feet below the surface.

"Mystery mollusks are typically seen in open water far below the surface and far above the seafloor," MBARI scientists observed.

Other characteristics include moving through ocean waters by flexing their body up and down to swim or drift motionless with the currents. "To avoid being eaten, the mystery mollusk hides in plain sight with a transparent body," according to MBARI scientists. "Rapidly closing the oral hood facilitates a quick escape, similar to the pulse of a jelly’s bell."

When threatened, MBARI scientists say that the mystery mollusk uses bioluminescence to light up and deter predators. They say that in one occasion, scientists observed the sea slug illuminate and then detach a "steadily glowing finger-like projection" from its tail.

“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. “Only recently have cameras become capable of filming bioluminescence in high-resolution and in full color."

According to MBARI, the mystery mollusk, like other nudibranchs, is a hermaphrodite with both female and male sex organs. Scientists say that the animal uses its muscular foot to attach to the seafloor and release its eggs.

"It sort of looks like a bunch of spare parts that were thrown together from other animals to make up this one, which again, makes it very different from what we've ever seen before," said Robison. "Figuring out what it was was a real challenge because it didn't look like anything else. We kind of figured that it had to be a mollusk, largely because of the big foot that sticks down below the main part of the body."

Robison says that in his over 30 years of studying the mysteries of Monterey Bay, he hasn't seen anything else like the mystery mollusk. "This discovery is an indication of the kinds of discoveries that await us as we continue to explore the ocean, the deep ocean, even further," said Robison.