Off the coast of Monterey Bay, California, researchers have made a groundbreaking discovery deep within the ocean’s depths—an entirely new species of nudibranch named Bathydevius caudactylus, colloquially referred to as the mystery mollusc. This remarkable sea slug captures attention not only for its bioluminescent glow and unique gelatinous features but also for its elusive lifestyle, thriving at depths ranging from 1,000 to 4,000 meters (3,300 to 13,100 feet) beneath the surface, known as the midnight zone.
The Monterey Bay Aquarium Research Institute (MBARI) led the effort, with senior scientist Bruce Robison at the forefront of the research. "Thanks to MBARI’s advanced underwater technology, we were able to prepare the most comprehensive description of a deep-sea animal ever made," Robison remarked. The creature was first spotted back in February 2000 during an expedition using MBARI's remotely operated vehicle (ROV), Tiburon, which descended to depths previously unexplored.
Upon encountering the mystery mollusc, researchers were struck by its distinctive appearance—a voluminous hood at one end and a paddle-like tail adorned with finger-like projections. Mesmerizingly colored internal organs interrupted the silhouette, leaving scientists puzzled over its classification. Initially nicknamed the mystery mollusc due to its ambiguous lineage, the team later collected specimens for closer examination. Detailed examinations of its anatomy and genetic material eventually confirmed its identity as part of the nudibranch family.
Traditionally, nudibranchs are found on the ocean floor or near coastal regions; they are familiar to those who explore tide pools, kelp forests, and coral reefs. The mystery mollusc, distinctively, is the first known nudibranch residing primarily within the open water column of the deep sea. This new classification reveals the depth and diversity of the nudibranch family and highlights the adaptability of marine life.
The newly discovered species appears to be relatively widespread, with known habitats stretching along the Pacific coast from Oregon to Southern California and hints of its presence around the Mariana Trench via observations made by NOAA researchers.
Bathydevius caudactylus has evolved unique adaptations to thrive under challenging conditions. Unlike many nudibranchs, which forage along the seafloor, this mollusc employs its expansive gelatinous hood as a trap to ensnare crustaceans, reminiscent of the way carnivorous plants like the Venus flytrap capture prey. This adaptation is invaluable for survival, particularly considering the sporadic availability of food at such extreme depths.
Movement through the water is facilitated by the mystery mollusc’s flexible body, allowing it to swim by undulating from its hood to its tail or by drifting with the currents. To evade predators, the mollusc relies on its transparent body for camouflage, which effectively hides it against the backdrop of the deep blue sea. Interestingly, the creature can escape threats by rapidly contracting its hood—a tactic reminiscent of how jellyfish pulse their bells to flee.
One of the standout features of Bathydevius caudactylus is its bioluminescence; when threatened, it can emit brilliant light to deter predators. One fascinating account from the research team describes observing the slug illuminate and sever one of the glowing projections on its tail, possibly using it as a decoy to distract attackers. Senior scientist Steven Haddock expressed the wonder felt by the research team upon witnessing this dazzling behavior: "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."
Like its relatives, the mystery mollusc is hermaphroditic, possessing both male and female reproductive organs. During spawning, these slugs descend to the ocean floor, where they use their muscular foot to attach themselves briefly before releasing eggs. Scientists witnessed this behavior and collected samples for study, noting the spawned eggs developed rapidly, transitioning to larvae with distinct morphological features.
Genetic analysis revealed Bathydevius caudactylus is distinct enough to warrant classification under a new family, Bathydeviidae, highlighting its evolutionary significance. Although other nudibranchs, like the lion’s mane nudibranch and veiled nudibranch, share some feeding methods with Bathydevius, they are not closely related; this shared trait appears to be the result of convergent evolution.
The discovery of this deep-sea slug emphasizes the wealth of knowledge still to be uncovered about marine ecosystems, particularly those below 2,000 meters, areas previously inaccessible or difficult to study. The knowledge gained about Bathydevius caudactylus adds to the growing catalog of deep-sea species—over 250 documented by MBARI to date—and underlines the significance of technological advancement in marine research.
"Deep-sea animals capture the imagination. These are our neighbors who 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," Robison said, highlighting the urgent need for ocean conservation. The research was supported by the David and Lucile Packard Foundation, which has long championed MBARI's dedication to advancing marine science.
For marine biologists and conservationists, the findings associated with Bathydevius caudactylus represent not only scientific milestones but also hopes for the future of ocean stewardship. Studying such unique creatures sheds light on the ecological roles they play and underlines the importance of preserving deep-sea environments.
This incredible nudibranch reminds us of the mysterious and often misunderstood world lurking beneath the ocean's surface. With each expedition, researchers drill down on the secrets of the deep—revealing the beauty and complexity of life thriving far from sunlight and familiar terrestrial ecosystems.
