Deep sea mystery mollusc discovered by robot in “midnight zone” was difficult to define, till now

High tech remotely operated equipment has revealed new creatures in the deep ocean. One particular "mystery mollusc" appeared particularly tricky to define, in more ways than one. But scientists have cracked its secret.

Mystery mollusc of the deep sea.
MBARI researchers have described a remarkable new species of nudibranch from the depths of the midnight zone. Nicknamed the “mystery mollusc,” Bathydevius caudactylus swims with a fingered tail, uses a cavernous hood to capture food, and glows with brilliant bioluminescence. Image: © 2014 MBARI.

Researchers from Monterey Bay Aquarium Research Institute discovered the deep sea slug through the use of their advanced underwater technology. Senior

Scientist at MBARI Bruce Robison led the mission to describe the mysterious creature, explained that as a result, “we were able to prepare the most comprehensive description of a deep-sea animal ever made. 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.”

It was originally observed by a remotely operative vehicle (ROV) called Tiburon. This underwater robot dived deep offshore of Monterey Bay at a depth of around 2,614 metres in 2000. An ROV like this differs from autonomous underwater vehicles (AUVs) being actively piloted in real-time by an operator, as opposed to AUVs which are usually preprogrammed for missions.

After a robot observed the creature in 2000, it was revisited 150 times to better understand it. Only recently have scientists been able to define it - as similar to a type of sea slug, but a unique one.

The technology provided them information about the mollusc’s mysteries but would require more sightings. Over time, they gained as many as 150 sightings from the ROV. After two decades of these investigations in the deep ocean, they have finally published a description of the animal in detail.

Remotely operated vehicles (ROV) can access parts of the deep sea humans safely may not be able to.
Remotely operated vehicles (ROV) can access parts of the deep sea humans safely may not be able to.

It turned out to be tricky to define because of how unusual it kept appearing to be. The creature had a fringed tail, colourful inside organs, and substantial hooded structure at one end. Therefore, it was tricky to put into any animal group. Based on its snail-like foot, the closest animal it could be related to is a snail, so they nick named it a “mystery mollusc.”

How the type of animal came to light

The scientists had to gently collect a specimen and analyse it in the lab to dive deeper scientifically into its definition. They found that incredibly, it is a nudibranch – a group which include sea slugs.

Nudibranchs are commonly in coastal settings like tide pools, coral reefs and kelp forests though a few live deep in zones of the sea. This particular “mystery mollusc” is the first that is known to live in the deep water column of the ocean - also known as the ocean’s midnight zone. This is an expansive environment of open water from around 1,000 to 4,000 metres below the surface.

It is even more peculiar that compared to other sea slugs that use a tongue to feed on prey, this creature uses its hood to trap prey – a bit like what we see on the land surface in carnivorous plants that traps flies. Even more strangely, it can light up with bioluminescence to deter predators when threatened. The sight of this happening stunned the research crew around the ROV control room.

Senior Scientist Steven Haddock reflected that everyone was “enchanted by the sight”. He explained that it has only been “recently have cameras become capable of filming bioluminescence in high-resolution and in full colour. 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 behaviour of deep-sea animals in their natural habitat.”

Also curiously, the mystery mollusc is technically both male and female and it descends to the bottom of the sea to spawn. This deep sea animal has not only captured the imagination of the scientific crew who observed it over the past two decades, but now the world. It shows how many more discoveries lay in wait in the deep ocean, and how much this is an important environment to protect.

Source of the news:

Discovery and description of a remarkable bathypelagic nudibranch, Bathydevius caudactylus, gen. et. sp. nov. Deep Sea Research. DOI: 10.1016/j.dsr.2024.104414