A fascinating new creature has emerged from the depths of the ocean. Researchers at the Monterey Bay Aquarium Research Institute (MBARI) have introduced the world to Bathydevius caudactylus, popularly called the "mystery mollusc." This newly discovered sea slug is about the size of an apple and lives in extreme depths of the ocean between 1,000 and 4,000 meters. The culmination of over two decades of research, the discovery was published in the journal Deep-Sea Research.

The mystery mollusc, officially classified as a nudibranch, differs from any of its shallow-water relatives. It has a translucent body and carries a gelatinous hood. The mollusc has an enormous paddle-like tail and bioluminescent internal organs that emit light. This luminescence not only serves as a protection system but also helps with the feeding mechanism, thus it stands out among deep-sea creatures.

Rather than feeding on the ocean floor like other nudibranchs, Bathydevius caudactylus uses its hood as a trap to capture small crustaceans, mimicking the Venus flytrap’s mechanism.  Its finger-like projections on the tail and distinct body shape initially puzzled researchers trying to categorize it within the marine ecosystem.

The journey to officially identify this species began in February 2000, during a research mission using MBARI’s remotely operated vehicle (ROV) named Tiburon. Spearheaded by researchers Bruce Robison and Steven Haddock, the mission of Monterey Bay in central California recorded the first sighting of the species. Over the years, more than 150 observations were made, gradually piecing together its natural history.

Genetic studies revealed enough differences from other nudibranchs to warrant the creation of an entirely new family, Bathydeviidae. "Anatomy, diet, behavior, bioluminescence, and habitat distinguish this surprising nudibranch from all previously described species, and genetic evidence supports its placement in a new family," the MBARI research team announced.

Bioluminescence is the hallmark of this mollusc, and it is one of the most important survival mechanisms for this creature. It not only deters predators but also helps it in its unique hunting method by producing light in the darkness of the deep ocean.

"This discovery adds a vital piece to the puzzle of understanding the largest habitat on Earth," Robinson noted. "We’ve invested more than 20 years in understanding the natural history of this fascinating species of nudibranch."

The study on Bathydevius caudactylus is actually a testament to the wonders left unexplored in the planet's oceans, offering peeks into the mysteries behind deep-sea life.