It swims through the layer of ocean known as the midnight zone with a large gelatinous hood and paddle-like tail while lighting up in ‘brilliant luminescence’. This is the remarkable new species of sea slug and subject of what scientists have heralded the “most comprehensive description of a deep-sea animal ever made.”

The discovery of this fascinating creature – named Bathydevius caudactylus but nicknamed the ‘mystery mollusc’ by authors of its new paper published this month in Deep Sea Research Part One – was made by researchers at the Monterey Bay Aquarium Research Institute (MBARI). 

And while this intriguing species of nudibranch may only measure no more than 15 centimetres in length, its discovery has been championed as a crucial new piece of the puzzle helping scientists better understand the deep-sea environment.

“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’s senior scientist, Bruce Robinson, who led the efforts to describe the mollusc. 

“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.”    

The ‘mystery mollusc’ was first sighted by MBARI researchers more than two decades ago, during a dive back in February 2000 with the Institute’s remotely operated vehicle (ROV), Tiburon, some 2.5km beneath the surface of the waters of Monterey Bay. In the 20 years since that first discovery, MBARI’s ROV sighted the creature 150 more times, enabling the team to eventually publish its detailed description of the animal.

With a voluminous hooded structure at one end, a flat tail fringed with numerous finger-like projections at the other, and colourful internal organs in between, the team initially struggled to place the animal in a group. It was the creature’s snail-like foot that earned it the nickname ‘mystery mollusc’.

Through a very gentle collection and study process – which included a detailed investigation of the anatomy and genetics of the creature – researchers finally unravelled the mystery, confirming the species as a nudibranch. 

Most nudibranchs, also known as sea slugs, live on the seafloor and are common in coastal environments such as tide pools, kelp forests, and coral reefs. A small number of the species are known to live on the abyssal seafloor while a few are pelagic and live in open waters near the surface.

This mystery mollusc, however, is the first nudibranch known to live in the deep water column, found in the layer known as the ocean’s midnight zone – an expansive environment of open water between 1,000 and 4,000 metres below the surface, otherwise known as the bathypelagic zone. Down here, the creature has had to evolve some unique solutions to finding food, safety, and companions to survive life.

While most sea slugs will use their raspy tongue to feed on prey attached to the seafloor, this mollusc uses its cavernous hood to trap crustaceans like a Venus fly trap plant. Meanwhile, their transparent bodies allow them to hide in plain sight and avoid becoming something’s dinner option. Should the threat occur, however, the mystery mollusc can light up with bioluminescence to deter and distract hungry predators. 

On one occasion, researchers 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 ‘ooooh’ at the same time. We were all enchanted by the sight,” said MBARI senior scientist, Steven Haddock. “Only 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.”

Like other nudibranchs, the mystery mollusc is a hermaphrodite, possessing both male and female sex organs. It would appear that the animals descend to the seafloor to spawn where researchers have observed some animals using their muscular feet to attach to the muddy seafloor in order to release eggs. 

“What’s exciting to me about the mystery mollusc is that it exemplifies how much we are learning as we spend more time in the deep sea, particularly below 2,000 metres,” said Haddock.

“For there to be a relatively large, unique, and glowing animal that is in a previously unknown family really underscores the importance of using new technology to catalogue this vast environment. The more we learn about deep sea communities, the better we will be at ocean decision making and stewardship.”

The new species is just one of more than 250 deep-sea species that MBARI technology has been able to document, all of which were previously unknown to science.

“Deep sea animals capture the imagination. These are our neighbours that share our blue planet. Each new discovery is an opportunity to raise awareness about the deep sea and inspire the public to protect the amazing animals and environments found deep beneath the surface,” said Robinson.

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