"Mysterious mollusk"

Swimming nudibranch discovered in the deep sea

(Bild: MBARI)

Researchers have now identified a "mysterious mollusk" that lives in the deep sea as a new species of nudibranch - the very first to live at such depths. The animal is almost invisible, but sometimes shines brightly for a short time.

Because the mollusk lives at depths between 1000 and 4000 meters, in a zone known as the bathypelagic (Greek bathýs = deep), it has been named Bathydevius caudactylus. A striking feature of the nudibranch, which grows up to 15 centimetres in size, is the large gelatinous cap and the flat, paddle-like tail, which has nine to 16 finger-like appendages.

First spotted during a dive in 2000
Scientists from the Monterey Bay Aquarium Research Institute (MBARI) first spotted the strange animal in February 2000 during a deep-sea diving expedition with the remote-controlled underwater vehicle "Tiburon" off the coast of Monterey Bay, a bay on the Pacific coast in Florida. Since then, they have made more than 150 sightings of the "mysterious mollusc".

Anatomy and genetics showed that the animal was a nudibranch, albeit a highly unusual species.(Bild: MBARI)

Anatomy and genetics examined in the laboratory
The MBARI researchers were also able to capture a specimen and examine it more closely in the laboratory. Anatomy and genetics showed that the animal was a nudibranch, but a highly unusual species that has adapted to catch prey and thrives in the dark zone where there is no light.

At first glance, Bathydevius caudactylus looks more "like a megaphone with a feathered tail" than a sea slug, according to the scientists. And the mollusk has a foot like a snail, which initially led the researchers to wonder exactly what kind of mollusk it is. On its back, the mollusk has gills and two olfactory organs typical of snails, known as rhinophores.

"We have invested more than 20 years in understanding this fascinating nudibranch species," said MBARI researcher Bruce Robison. "The discovery is a new piece of the puzzle that can help us better understand the largest habitat on Earth, the deep sea."