New Species of Bioluminescent Sea Slug Discovered

Marine biologists at Monterey Bay Aquarium Research Institute (MBARI) have described an exceptional sea slug (nudibranch) species, new to science, from depths of the eastern North Pacific Ocean. Named Bathydevius caudactylus, the species is bioluminescent and represents the third independent origin of bioluminescence among sea slugs.

Bathydevius caudactylus is the first sea slug species known to live in the deep water column.

The new species is currently known to live in the waters offshore of the Pacific coast of North America.

It can be found in the ocean’s midnight zone, an expansive environment of open water 1,000 to 4,000 m (3,300-13,100 feet) below the surface, also known as the bathypelagic zone.

The researchers first observed Bathydevius caudactylus in February 2000 during a dive with the institute’s remotely operated vehicle (ROV) Tiburon offshore of Monterey Bay at 2,614 m (8,576 feet) deep.

They leveraged MBARI’s advanced and innovative underwater technology to gather extensive natural history information about the mystery mollusk.

“Thanks to MBARI’s advanced underwater technology, we were able to prepare the most comprehensive description of a deep-sea animal ever made,” said MBARI senior scientist Bruce Robison.

“We’ve invested more than 20 years in understanding the natural history of this fascinating species of nudibranch.”

“Our discovery is a new piece of the puzzle that can help better understand the largest habitat on Earth.”

Bathydevius caudactylus has evolved unique solutions to find food, safety, and companions to survive in the midnight zone.

While most sea slugs use a raspy tongue to feed on prey attached to the seafloor, the species uses a cavernous hood to trap crustaceans like a Venus fly trap plant.

A number of other unrelated deep-sea species use this feeding strategy, including some jellies, anemones, and tunicates.

If threatened, Bathydevius caudactylus can light up with bioluminescence to deter and distract hungry predators.

On one occasion, the scientists observed the animal illuminate and then detach a steadily glowing finger-like projection from the tail, likely serving as a decoy to distract a potential predator.

“When we first filmed it glowing with the ROV, everyone in the control room let out a loud ‘Oooooh!’ at the same time,” said MBARI senior scientist Steven Haddock.

“We were all enchanted by the sight.”

“Only recently have cameras become capable of filming bioluminescence in high-resolution and in full color.”

“MBARI is one of the only places in the world where we have taken this new technology into the deep ocean, allowing us to study the luminous behavior of deep-sea animals in their natural habitat.”

Bathydevius caudactylus is described in a paper published in the journal Deep-Sea Research Part I: Oceanographic Research Papers.

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Bruce H. Robison & Steven H.D. Haddock. 2024. Discovery and description of a remarkable bathypelagic nudibranch, Bathydevius caudactylus, gen. et. sp. nov. Deep Sea Research Part I: Oceanographic Research Papers 214: 104414; doi: 10.1016/j.dsr.2024.104414