Absolute darkness. Crushing pressure. Icy cold. The Pacific Ocean’s “midnight zone”—between 3,300 and 13,100 feet deep—is not a welcoming place. But that hasn’t deterred one delicate, baffling “mystery mollusk” species from setting up shop in this inhospitable water column.
For more than 20 years scientists at California’s Monterey Bay Aquarium Research Institute (MBARI) have occasionally encountered this five-inch translucent creature with a bizarre medley of traits. Its face is surrounded by an oversized hood that it uses to enfold prey and jet-propel itself like a jellyfish. Its tail is fringed with tentacles, and if provoked, it can detach one. When touched, its hood and tail glow with a constellation of blue-green dots like an underwater planetarium.
Now scientists have determined that this deep-sea enigma is a nudibranch, or sea slug—but one so odd that it merits the creation of an entirely new nudibranch family, the researchers report in Deep Sea Research, Part I: Oceanographic Research Papers. Dubbed Bathydevius caudactylus, it’s the first nudibranch known to live in the deep-sea water column rather than, for example, lurking on the seafloor or floating near the surface.
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A mystery mollusk observed by MBARI’s remotely operated vehicle Tiburon in the outer Monterey Canyon at a depth of approximately 1,900 meters.
© 2021 MBARI
The animal features a unique grab bag of traits of other nudibranchs, says study co-author and MBARI marine biologist Steven Haddock. Haddock was present when scientists first spotted the mollusk, during exploration using a remotely operated vehicle in 2000. “We were all spitballing what we thought it was,” he recalls.
In the two decades since then, the researchers have observed more than 100 B. caudactylus and studied some in their laboratories. Genetic analysis revealed the creature probably belongs to a family that split from the other nudibranchs long ago—so even though it shares some features with other species, it evolved its eclectic range of traits independently. “Similar features can evolve multiple times, but to see it happen in such a unique kind of organism under such different circumstances than what we see in other nudibranchs is pretty cool,” says Jessica Goodheart, a mollusk researcher at the American Museum of Natural History in New York City. “Maybe [such features] can evolve much more easily than we anticipated.”