Scientists discovered the strangest sea slug in the ocean

• In 2000, scientists spotted a bioluminescent mollusk in the deep sea.
• The animal was so unique that they had no idea what it was until they did genetic testing.

It's been over 20 years since researchers were first dazzled by a strange, glowing creature with vibrant organs and a fringed tail floating in the waters near Monterey Bay, California. They had no idea what it was.

"It looks like it's something made up of a bunch of spare parts left over from other animals," Monterey Bay Aquarium Research Institute senior scientist Bruce Robison, who led the research, told Business Insider.

It has a hood like a jellyfish and a "foot" like a snail, and it hunts prey like some anemones. But after genetic testing, scientists discovered that it wasn't any of these animals.

It belongs to a new family of sea slugs. Researchers from the MBARI dubbed it Bathydevius caudactylus and recently published their findings in the peer-reviewed journal Deep Sea Research Part I.

Bathydevius is unlike any sea slug the scientists had ever seen.

A unique new species of sea slug

Bathydevius typically lives thousands of feet below the surface, far from the coastal tide pools or kelp forests that many sea slugs prefer.

It also captures its prey in an entirely different way. Usually, sea slugs use their rough tongues to scrape food off the seafloor. But this mysterious, bioluminescent mollusk uses its hood to snatch its food mid-swim. Researchers still aren't sure how these slow-moving sea slugs catch the speedy crustaceans they devour.

These unusual features and behaviors are likely the result of its unique environment, deep below the surface in what's called the ocean's "midnight zone."

"Almost every aspect of Bathydevius reflects an adaptation to this habitat: anatomy, physiology, reproduction, feeding, behavior; it's unique," Robison told CNN.

Since first spotting it in February 2000, scientists exploring the oceans with MBARI's remotely operated vehicles have come across it over 150 times. It just kept popping up from Oregon to Southern California.

At first, no one knew how to classify the strange sea slug. Recent improvements in underwater cameras allowed scientists to film its vibrant bioluminescent colors, and genetic analysis showed how distant it is from other sea slugs on the family tree.

"It is radically different" from its relatives, Robison said, both in how it looks and its genetic makeup.

A sea slug in the midnight zone

In addition to trapping prey, the slug's hood can snap shut and propel the animal away from danger. Predators might overlook the sea slug's transparent body or be startled by its bioluminescence.

"It kind of acts as a burglar alarm," Robison said.

MBARI has been using ROVs since the late 1980s. It now uses cameras that can film bioluminescence in high resolution, allowing researchers to see how these and other luminescent creatures glow in the deep ocean.

Every year, researchers uncover thousands of new marine species, and ROVs have often given them a front-row seat to these unknown creatures.

Scientists are only beginning to understand how marine life in the deep ocean affects everything from salmon to swordfish to seals, all the fish and mammals we consume or care about, Robison said.

"If we're going to live in harmony with the rest of this planet, we at least need to know fundamentally how it's put together and how it works," he said.