In a fascinating video, researchers have revealed a new and bizarre see-through species of sea slug floating around the midnight zone of the deep sea.
Nicknamed the "mystery mollusc" by the scientists, this 5-inch creature is more formally known as Bathydevius caudactylus. It has been officially described for the first time in a new paper in the journal Deep Sea Research Part I: Oceanographic Research Papers.
It was first discovered in California by Monterey Bay Aquarium Research Institute (MBARI) researchers in 2000. Over 150 individuals of this new species have been observed in the years since, with this video showing several of the times the rare mollusk has been captured on camera.
"Thanks to MBARI's advanced underwater technology, 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," said Bruce Robison, a senior scientist at MBARI, in a statement.
"Our discovery is a new piece of the puzzle that can help better understand the largest habitat on Earth," he added.
The first sighting of this sea slug -- also called nudibranchs -- was in 2000 at a depth of 8,576 feet in waters near California's Monterey Bay.
The researchers describe how a grand total of 157 of these mollusks have been spotted since, 137 of which were found floating in the water column between 3,323 and 10,735 feet. Another 20 of the sea slugs were spied on the seafloor between depths of 7,444 and 13,153 feet.
This places the species firmly in the midnight or bathypelagic zone, which is a pitch-black section of the deep ocean between 3,300 and 13,100 feet below the surface.
The strange creature is see-through, with a large, jellylike hood, fringed tail and a bioluminescence that makes it glow in the dark of the midnight zone. Initially, researchers struggled to identify which family of animals the mollusk belonged to, hence its charming "mystery mollusc" nickname. After catching one of the creatures and studying its body and genetic materials, they confirmed that they were nudibranchs.
"Bathydevius caudactylus is clearly adapted for pelagic life and is the first nudibranch to be reported from the vast bathypelagic habitat," the researchers wrote in the paper.
Nudibranchs are soft-bodied marine mollusks known for their striking colors and elaborate patterns. They range in size from a few millimeters to over a foot in length, and they display a vast array of shapes, from smooth and sluglike to those with ornate frills and spikes.
This new species has been spotted in deep waters along the West Coast, between Oregon and as far south as Southern California, but another may have been spotted in the Mariana Trench in the Western Pacific.
"Deep-sea animals capture the imagination," Robison said. "These are our neighbors 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."
Bathydevius caudactylus, like many other deep-sea creatures, is bioluminescent, which allows it to deter predators.
"When we first filmed it glowing with the [remotely operated vehicle], everyone in the control room let out a loud 'Oooooh!' at the same time. We were all enchanted by the sight," said Steven Haddock, also an MBARI senior scientist, in the statement.
"Only recently have cameras become capable of filming bioluminescence in high-resolution and in full color," he continued. "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."
Interestingly, Bathydevius caudactylus may have evolved bioluminescence independently from the rest of its nudibranch cousin species.
"In a molecular phylogeny, it resolves as a distinct sister to the rest of known nudibranchs," the researchers wrote, noting that the creature represents the third independent evolution of bioluminescence among nudibranch species.
Haddock said: "What is 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 meters. 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 catalog this vast environment."
"The more we learn about deep-sea communities, the better we will be at ocean decision-making and stewardship," he said.
Robison, B. H., & Haddock, S. H. (2024). Discovery and description of a remarkable bathypelagic nudibranch, Bathydevius caudactylus, gen. et. sp. nov. Deep Sea Research Part I Oceanographic Research Papers, 104414. https://doi.org/10.1016/j.dsr.2024.104414