MONTEREY, Calif. — In a remarkable development after over 20 years of observation, researchers from California have identified a mysterious bioluminescent creature previously observed in the deep sea as a new species of sea slug.
The Monterey Bay Aquarium Research Institute shared footage of this newly classified sea slug gracefully moving through the ocean’s depths.

The intriguing organism was first documented by scientists using a remote-operated vehicle back in February 2000 at a staggering depth of 8,576 feet (2,614 meters) in the Pacific Ocean.
Described as a “mystery mollusc,” this creature was recognized for its unique features: a large, hood-like structure at one end, a flat tail adorned with multiple finger-like projections at the opposite end, and vivid internal organs noticeable along its body. Initially, the research team faced challenges in categorizing this unusual marine life, as noted in a statement released on Tuesday.

Through extensive analysis that included more than 150 sightings of the creature and dedicated lab studies, the researchers ultimately classified it as a new type of nudibranch—commonly known as a sea slug. This fascinating species inhabits the “midnight zone” of the ocean, an environment characterized by chilling temperatures, pitch-black darkness, and immense pressure.

The significant findings were later detailed in the journal Deep-Sea Research Part I, marking an important milestone in our understanding of deep-sea biodiversity.