Some of the most bizarre life forms on Earth lurk in the deeper realms of the ocean. So little was known about either of these creatures that it took twenty years to find out exactly what it was. Things only got stranger from there.
The organism's distinctive, radiant presence was observed during multiple deep-sea missions between 2000 and 2021, but was simply referred to as a “mystery mollusk.” A team of researchers from the Monterey Bay Aquarium Research Institute (MBARI) has now reviewed extensive footage of previous mysterious mollusk sightings and used MBARI's remotely operated vehicles (ROVs) to observe it and collect samples. They named it and finally confirmed that it is a nudibranch: the first and only nudibranch known to live at such depths.
Bathydevius caudactylus, as this nudibranch is now called, it lives 1,000–4,000 meters (3,300–13,100 ft) deep in the bathypelagic or midnight zone of the ocean. It moves like a jellyfish, eats like a Venus flytrap, is bioluminescent and its genes are so different that it can be classified as the first member of a new phylogenetic family.
“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 said in a study recently published in Deep sea research.
Is that a…?
Slugs are gastropods, which literally translates to “stomach foot,” since the “foot” they crawl around on when they're not swimming is right under their guts. They are part of a larger group that includes land and water snails. B. caudactylusHowever, it seems to look more like a jellyfish than a sea slug. It usually swims using a face mask that opens and closes to propel itself backward through the water in a manner similar to many jellyfish.
The hood of B. caudactylus can also act something like a Venus flytrap. Although it is not a hinged structure like the plant's leaves, it is used to capture prey. Typically small crustaceans, the prey is then pushed towards the mouth at the back of the hood.
The slug also seems to have a unique way of avoiding becoming food itself. Projections at the end of its tail, known as dactyls, can detach when necessary, much like the tails of some lizard species. The MBARI team thinks these dactyls may be a lure intended to deceive predators while the nudibranch swims away. They regenerate later.
