Researchers first encountered the bioluminescent animal about 8,500 feet down during a remotely operated vehicle (ROV) dive in 2000. They were intrigued by it immediately but had no idea what it was, nicknaming it the "mystery mollusc." Over the next 20 years, researchers observed the blob-like mystery animal more than 150 times during ROV dives.
Eventually, researchers realized they'd discovered an "exceptional" sea slug: Bathydevius caudactylus. Generally, most sea slugs "live on the seafloor..." But not the new species.
Bathydevius caudactylus has a "mostly transparent, gelatinous" body, "voluminous, bell-shaped" head and flat "fringed" tail. It lives in the "cold, dark realm" of the open water column, the first nudibranch known to live in such a habitat.
Its internal organs are brightly colored and clearly visible. Photos show the sea slug, which looks more like a misshapen jellyfish than a slug.
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