Newly discovered sea slug is first known to swim in the ocean's Midnight Zone

Researchers at the Monterey Bay Aquarium Research Institute (MBARI) have unveiled a new species of sea slug, Bathydevius caudactylus, that is the first known nudibranch to inhabit the open water column of the deep sea. After reviewing more than 150 sightings from MBARI's remotely operated vehicles (ROVs) over the past 20 years, the team published a detailed description of this mysterious mollusk in the journal Deep-Sea Research.

First observed in February 2000 during a dive with an ROV offshore of Monterey Bay at a depth of 2,614 meters (8,576 feet), Bathydevius caudactylus puzzled scientists with its unique features and behaviors. Unlike typical nudibranchs, which usually crawl along the seafloor in coastal environments such as coral reefs, kelp forests, and tide pools, this sea slug swims freely through the ocean's midnight zone—a wide area of open water about 1,000 to 4,000 meters (3,300 to 13,100 feet) below the surface.

"Thanks to MBARI's advanced underwater technology, we were able to prepare the most complete description of a deep-sea animal ever made. We have 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 may help better understand the Earth's largest habitat," said Bruce Robison, MBARI's lead scientist, according to RPP Noticias.

Bathydevius caudactylus stands out with a large, gelatinous hood that it uses to capture prey, fingerlike projections at one end, and colorful internal organs in between. The sea slug exhibits bright bioluminescence, glowing a bioluminescent blue when threatened to distract hungry predators. On one occasion, researchers observed it illuminating and then shedding a finger-like projection that glowed continuously 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. We were all enchanted by the sight," said Steven Haddock, a senior scientist at MBARI, as reported by Popular Science. Haddock added, "Only recently have cameras become capable of filming bioluminescence in high-resolution and in full color."

The discovery of Bathydevius caudactylus is significant not only because it is the first known sea slug to inhabit the deep-water column but also because its unique characteristics warrant the creation of a new family, Bathydeviidae. Genetic analysis revealed that it is so distinct from other known nudibranchs that it does not fit within existing classifications. "The genetic story showed that they were not close relatives, indicating that we had yet another example of convergence," Haddock stated, according to IFLScience.

The sea slug's ability to swim freely in the midwaters is a departure from the norm for nudibranchs. Bathydevius caudactylus flexes its body up and down to swim or drifts motionless with the currents. Its transparent body helps it avoid detection by predators, allowing it to hide in plain sight. "Virtually everything about Bathydevius seems remarkable to me," Robison said, as quoted by IFLScience. "The most striking initial impact comes from its unusual anatomy and configuration, which is very unlike all other nudibranchs."

Researchers also observed that Bathydevius caudactylus uses a cavernous hood to trap crustaceans, working somewhat like a Venus flytrap. It can expel water rapidly from its hood, producing jet-like bursts that send it moving quickly backward to escape predators. Additionally, the nudibranch is hermaphroditic, possessing both male and female reproductive organs. The scientists report that it appears to descend to the seafloor to spawn, occasionally finding spawning individuals in proximity to one another on the seabed.

"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," Haddock noted, as per Popular Science. He added, "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 ocean's vast midwaters are the largest and least explored part of the planet's biosphere, presenting long-standing challenges for exploration due to their depth. Bathydevius caudactylus has evolved unique adaptations to survive in the harsh conditions of the deep sea, including its elastic hood and paddle-shaped tail. Its bioluminescence adds to its extraordinary adaptations to life in the deep sea, where it resides in dark waters.

"Our discovery is a new piece of the puzzle that can help better understand the largest habitat on Earth," Robison said, according to The New York Times. By exploring and documenting creatures like Bathydevius caudactylus, scientists aim to gain a better understanding of the adaptations and behaviors that enable life in the extreme conditions of the deep sea.

Currently, Bathydevius caudactylus is known to live in the coastal waters of the Pacific Ocean off North America. MBARI scientists have spotted it as far north as Oregon and as far south as Southern California. An observation of a similar-looking animal by NOAA researchers in the Mariana Trench in the western Pacific suggests that it may have a wider distribution.

The research team hopes that studying this species will shed light on the mysteries of the midnight zone. "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," Haddock said, according to Popular Science.

This discovery emphasizes the importance of continuing to explore the ocean's depths to uncover the secrets of Earth's largest and least understood habitat. As Robison noted, Bathydevius caudactylus adds valuable insights to our understanding of the planet's largest biosphere, helping science understand not only unfamiliar aspects of this vast habitat but also the natural history of some of its fascinating residents.

Sources: IFLScience, The Sun, The Scottish Sun, RPP Noticias, Interesting Engineering, Popular Science, The Daily Star Lebanon, The New York Times

This article was written in collaboration with generative AI company Alchemiq