The deep sea is the largest living space on Earth. Each dive into the ocean’s inky depths teaches us something new about this environment. MBARI’s advanced underwater technology is revealing the stunning community of life that thrives in the deep, including many species previously unknown to science. Over the past 37 years, MBARI researchers and our collaborators have discovered more than 250 new species—but a species described in 2024 may be our most amazing discovery yet.
Bathydevius caudactylus is a remarkable new species of nudibranch, or sea slug, that swims through the ocean’s midnight zone with a large gelatinous hood and paddle-like tail, and lights up with brilliant bioluminescence.
MBARI researchers first observed the mystery mollusc in February 2000 during a dive with the institute’s remotely operated vehicle (ROV) Tiburon offshore of Monterey Bay at 2,614 meters (8,576 feet) deep. The team leveraged MBARI’s advanced and innovative underwater technology to gather extensive natural history information about the mystery mollusc. After reviewing more than 150 sightings from MBARI’s ROVs over the past 20 years, they published a detailed description of this animal.
The mystery mollusc (Bathydevius caudactylus) lives in the midnight zone, with unique adaptations to survive in an environment of frigid temperatures, inky darkness, and crushing pressure. Image: © 2014 MBARI
With a voluminous hooded structure at one end, a flat tail fringed with numerous finger-like projections at the other, and colorful internal organs in between, the team initially struggled to place this animal in a group. Because the animal also had a foot like a snail, they nicknamed this the “mystery mollusc.”
After gently collecting a specimen, MBARI researchers were able to take a closer look at the animal in the lab. Through detailed investigations of anatomy and genetics, they began to solve the mystery, finally confirming that this incredible animal is a nudibranch.
Most nudibranchs live on the seafloor. Nudibranchs are common in coastal environments—including tide pools, kelp forests, and coral reefs—and a small number of species are known to live on the abyssal seafloor. A few are pelagic and live in open waters near the surface.
The mystery mollusc is the first nudibranch known to live in the deep water column. This species lives in the ocean’s midnight zone, an expansive environment of open water 1,000 to 4,000 meters (3,300 to 13,100 feet) below the surface, also known as the bathypelagic zone.
The mystery mollusc is currently known to live in the waters offshore of the Pacific coast of North America, with sightings on MBARI expeditions 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 the mystery mollusc may have a more widespread distribution.
The mystery mollusc 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 mystery mollusc 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.
Mystery molluscs are typically seen in open water far below the surface and far above the seafloor. They move through these waters by flexing their body up and down to swim or simply drifting motionless with the currents. To avoid being eaten, the mystery mollusc hides in plain sight with a transparent body. Rapidly closing the oral hood facilitates a quick escape, similar to the pulse of a jelly’s bell.
If threatened, 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. Only recently have cameras become capable of filming bioluminescence in high resolution and 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.
Like other nudibranchs, the mystery mollusc is a hermaphrodite, possessing both male and female sex organs. This species descends to the seafloor to spawn. MBARI researchers observed some animals using their muscular foot to attach to the muddy seafloor in order to release their eggs.
Detailed examination of specific gene sequences confirmed that the mystery mollusc is unique enough from other known nudibranchs to merit the creation of a new family, Bathydeviidae. Two shallow-water nudibranchs—the lion’s mane nudibranch (Melibe leonina) and the veiled nudibranch (Tethys fimbria)—use a hood to capture prey; however, this appears to be convergent evolution of a similar feeding method, as the mystery mollusc is only distantly related to these species. In fact, genetics suggests the mystery mollusc may have split off first on its own branch of the nudibranch family tree.
“Deep-sea animals capture the imagination. Each new discovery is an opportunity to raise awareness about the amazing ocean animals and environments at risk from human actions.”
—Senior Scientist Bruce Robison
The mystery mollusc is just one of many fascinating discoveries MBARI has made in the deep sea. Each reminds us how much we still have yet to learn about the ocean and its inhabitants.
The ocean is the heart of our planet and our climate system, and teems with life. Human actions threaten its future. The same threats that face coastal waters—overfishing, plastic pollution, climate change, and habitat destruction—extend to the depths below.
We urgently need to document the diversity of life deep beneath the surface before it becomes lost forever. MBARI science and technology are helping establish a baseline understanding of ocean health and deep-sea biodiversity so we can better assess the impacts of threats like climate change, pollution, and mining. Each new discovery is a new piece of the puzzle.
This work was funded as part of the David and Lucile Packard Foundation’s longtime support of MBARI’s work to advance marine science and technology to understand a changing ocean.
Research Publication:
Robison, B.H. and S.H.D. Haddock. 2024. Discovery and description of a remarkable bathypelagic nudibranch, Bathydevius caudactylus, gen. et. sp. nov. Deep-Sea Research Part I, 214: 104414. https://doi.org/10.1016/j.dsr.2024.104414