MONTEREY, Calif.—More than two decades after spotting a mysterious, gelatinous, bioluminescent creature swimming in the deep sea, California researchers this week announced that it is a new species of sea slug.

The Monterey Bay Aquarium Research Institute posted video online of the new sea slug floating gently in the depths.

Using a remote vehicle, scientists with the institute first noticed what they called a “mystery mollusc” in February 2000 at a depth of 8,576 feet in the Pacific.

“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, the team initially struggled to place this animal in a group,” the institute said in a statement Tuesday.

After reviewing more than 150 sightings of the creature and studying it in a lab, researchers determined it was a new type of nudibranch, or sea slug. It lives in the so-called midnight zone, an area of deep ocean known for “frigid temperatures, inky darkness, and crushing pressure,” the statement said.

The findings were published in the journal Deep-Sea Research Part I.