MBARI researchers discover remarkable new species of sea slug living in deep sea

Researchers at the Monterey Bay Aquarium Research Institute (MBARI) have identified a new species of deep-sea nudibranch, Bathydevius caudactylus, also known as the “mystery mollusk.”

This remarkable sea slug was first observed over two decades ago at a depth of 2,614 meters (8,576 feet) off Monterey Bay, California, and is the first known nudibranch to live exclusively in the ocean’s bathypelagic, or “midnight,” zone.

With a translucent, gelatinous hood and paddle-like tail, the mystery mollusk uses bioluminescence to deter predators, emitting a bright glow and sometimes shedding a luminescent tail fragment as a decoy.

It captures prey, primarily crustaceans, by using its hood like a trap—a feeding method similar to that of some deep-sea jellies and anemones. The discovery of Bathydevius caudactylus, published in Deep-Sea Research Part I, adds a new family, Bathydeviidae, to the nudibranch lineage.

MBARI’s advanced underwater technology enabled the detailed documentation of this unique species. The study offers valuable insights into life in the deep-sea environment and highlights the diversity of largely unexplored ocean habitats.