Researchers have made an exciting discovery of a new sea slug species found in the deep ocean. This creature, known as the ‘mystery mollusc,’ formally named Bathydevius caudactylus, glides through the ocean’s midnight zone with a sizable gelatinous hood and a paddle-like tail, radiating dazzling bioluminescent light.
Researchers have made an exciting discovery of a new sea slug species found in the deep ocean. This creature, known as the “mystery mollusc,” formally named Bathydevius caudactylus, glides through the ocean’s midnight zone with a sizable gelatinous hood and a paddle-like tail, radiating dazzling bioluminescent light. The study detailing this animal has been published in the journal Deep-Sea Research Part I.
“Thanks to MBARI’s cutting-edge underwater technology, we have crafted the most thorough description of a deep-sea creature ever compiled. Over the past 20 years, we’ve dedicated our efforts to understanding the natural history of this captivating nudibranch. Our finding provides valuable insight into one of Earth’s largest habitats,” stated Bruce Robison, Senior Scientist at MBARI, who led the investigation of the mystery mollusc.
The initial sighting of the mystery mollusc by MBARI occurred in February 2000 during a dive with the institute’s remotely operated vehicle (ROV) Tiburon, at a depth of 2,614 meters (8,576 feet) offshore from Monterey Bay. The team utilized advanced MBARI technology to collect extensive information on the natural history of this unique mollusc, eventually publishing a comprehensive description after analyzing over 150 observations from MBARI’s ROVs over two decades.
The creature features a large, hooded body at one end and a flat tail covered with finger-like extensions at the other, adorned with vibrant internal organs in between. The researchers initially faced difficulties categorizing it due to its unique features, which led them to refer to it as the “mystery mollusc.”
After carefully collecting a specimen, MBARI scientists examined the animal in a laboratory setting. Through in-depth anatomical and genetic studies, they unraveled the mystery and confirmed that this remarkable creature is a nudibranch.
Most nudibranchs, or sea slugs, typically reside on the ocean floor. While they are commonly found in coastal areas like tide pools, kelp forests, and coral reefs, a few species inhabit the deep abyssal seafloor, and some are pelagic, living in the water near the surface.
The mystery mollusc is the first nudibranch identified to inhabit the deep water column. It resides in the midnight zone of the ocean, a vast area of open water located between 1,000 and 4,000 meters (3,300 to 13,100 feet) beneath the surface, often referred to as the bathypelagic zone.
Currently, the mystery mollusc has been observed in the Pacific waters off the coast of North America, with sightings reported from as far north as Oregon and as far south as Southern California. Observations of a similarly appearing organism by NOAA researchers in the Mariana Trench in the Western Pacific imply that the mystery mollusc may have a broader distribution.
This unique mollusc has developed special adaptations to seek out food, evade dangers, and find companionship in the dark midnight zone.
Unlike most sea slugs that use a rough tongue to feed on organisms attached to the sea floor, the mystery mollusc employs a spacious hood to trap crustaceans, much like a Venus flytrap. Several other unrelated deep-sea species also utilize this feeding technique, including various jellies, anemones, and tunicates.
The mystery mollusc is typically encountered in open waters, situated far beneath the surface and well above the seabed. It propels itself by flexing its body up and down or may drift passively with ocean currents. To evade predators, it can blend into its surroundings with a transparent body. The rapid closing of its oral hood allows for a swift getaway, reminiscent of a jellyfish’s pulse.
If threatened, the mystery mollusc can emit bioluminescent light to scare off and confuse predators. One remarkable observation involved the creature illuminating itself before shedding a glowing finger-like projection from its tail, likely acting as a decoy to divert a predator’s attention. “When we first captured it glowing on the ROV, there was a collective ‘Oooooh!’ from everyone in the control room. We were all captivated by the sight,” remarked MBARI Senior Scientist Steven Haddock. “Only recently have cameras developed the capability to capture high-resolution, full-color footage of bioluminescence. MBARI is among the few locations globally that have taken this new technology deep into the ocean, enabling us to study the luminous behaviors of deep-sea creatures in their natural settings.”
Similar to other nudibranchs, the mystery mollusc is hermaphroditic, containing both male and female reproductive systems. Observations suggest that it descends to the ocean floor for reproduction; MBARI researchers noted some specimens using their muscular foot to secure themselves to the muddy seabed while releasing eggs.
A thorough examination of specific gene sequences confirmed that the mystery mollusc is distinct enough from other known nudibranchs to warrant the formation of a new family, Bathydeviidae. While two shallow-water nudibranchs—the lion’s mane nudibranch (Melibe leonina) and the veiled nudibranch (Tethys fimbria)—also capture their meals with a hood, this phenomenon appears to be convergent evolution, as the mystery mollusc is only distantly related to these species. Genetics indicate that this mollusc may have branched off on its own from the rest of the nudibranch family tree much earlier.
“The mystery mollusc excites me as it highlights how much we continue to learn as we explore deeper into the ocean, especially below 2,000 meters. The existence of such a relatively large, unique, and glowing animal that belongs to an unrecognized family emphasizes the significance of utilizing advanced technology to catalog this expansive environment. The more knowledge we gain about deep-sea ecosystems, the better we can make informed decisions regarding ocean conservation and management,” expressed Haddock.
The mystery mollusc is just one of many intriguing discoveries that MBARI has unearthed in the midnight zone. To date, MBARI’s technology has documented over 250 previously unrecorded deep-sea species.
“Deep-sea creatures truly captivate people’s imaginations. These are our neighbors sharing this blue planet with us. Each new finding is a chance to raise awareness about the deep sea and inspire the public to protect the remarkable animals and ecosystems that lie deep beneath the surface,” remarked Robison.
This research received funding from the David and Lucile Packard Foundation, which has continuously supported MBARI’s efforts to enhance marine science and technology in understanding our changing oceans.
Mystery mollusc (Bathydevius caudactylus) fact sheet
Common name: Mystery mollusc
Scientific name: Bathydevius caudactylus Pronunciation: bath-ee-dee-vee-us caw-dack-till-us
Habitat: midwater, in the bathypelagic zone
Depth range: 1,013 to 4,009 meters (3,323 to 13,153 feet)
Geographic range: currently identified from the Northeastern Pacific Ocean, spanning Oregon to Southern California, likely more widespread
Size: 14.5 centimeters (5.6 inches) in total length
Diet: crustaceans, including mysid shrimp
Swimming: Bathydevius caudactylus swims.
The Bathydevius caudactylus moves its entire body up and down, from its hood to its tail. By quickly closing its hood, the creature can swiftly propel itself backward. Most sightings of this species occur in the water column at depths ranging from 1,013 to 3,272 meters (3,323 to 10,735 feet), where they are seen either swimming slowly or floating passively. Bathydevius caudactylus is neutrally buoyant, allowing it to maintain its position in the water without sinking or floating when at rest.
Feeding: Bathydevius caudactylus employs a gelatinous hood to capture crustaceans. This flexible, bowl-shaped hood can expand up to 9 centimeters (3.5 inches) in diameter. The animal consumes its meals through a funnel-shaped mouth located at the back of the hood. Unlike many bottom-dwelling nudibranchs and snails, Bathydevius caudactylus does not possess a radula, which is a tongue-like structure for feeding. It prefers nutrient-rich prey and has adapted to slowly metabolize food, especially in environments where meals are rare.
Physiology: Scientists studied the oxygen consumption of Bathydevius caudactylus using the Midwater Respirometer System created by MBARI researchers. The findings reveal that its metabolism is considerably lower than that of other nudibranchs, resembling instead the respiration rates observed in deep-sea jellyfish. This reduced metabolic rate is indicative of a slower lifestyle in the deep ocean.
Bioluminescence: Studies conducted both in the ocean and in the lab have documented the bioluminescence of Bathydevius caudactylus. The animal’s tissues contain luminous granules that create a star-like effect across its back, with a soft glow present in the oral hood and along the tips of its finger-like dactyls on the tail. To evade predators, this species can shed some of its luminescent dactyls as a diversion, similar to how lizards lose their tails. These dactyls can regenerate, and some individuals have been observed with dactyls of varying lengths. Bioluminescence is rare among nudibranchs and snails, making Bathydevius caudactylus an example of a unique evolutionary occurrence—it’s one of the few known cases of bioluminescence in these animals.
Reproduction: As a hermaphrodite, Bathydevius caudactylus has both male and female reproductive systems. Spawning has been documented at seafloor depths between 2,269 and 4,009 meters (7,444 to 13,153 feet). Although this species is generally solitary, some spawning individuals have been spotted close to one another on the ocean floor. One specimen observed by MBARI researchers released a string of eggs in a laboratory setting. These eggs hatched after three days, producing trochophore larvae that feature round bodies and long, hair-like cilia.
Etymology: The genus name Bathydevius highlights the “devious” nature of this deep-sea species, which misled researchers with its distinct characteristics that set it apart from other known nudibranchs. The species name caudactylus refers to the unique finger-like projections, known as dactyls, found on its tail.
