The first nudibranch known to dwell in the ocean’s bathypelagic or midnight zone, far beyond the reach of sunlight, has been described by researchers from Monterey Bay Aquarium Research Institute (MBARI) in California.
Bathydevius caudactylus – or “Deep Deceiver” – is bioluminescent and has a large hooded structure that can shoot jets of water at one end, a flat tail fringed with finger-like projections or dactyls at the other, and brightly coloured internal organs in between.
Based on its snail-like foot, the creature was initially referred to by the marine biologists as “the mystery mollusc”.
It was first observed almost a quarter-century ago, in February 2000, on a dive using the MBARI ROV Tiburon outside Monterey Bay at a depth of 2,614m. However, it is only now, after more than 150 ROV sightings have been reviewed, that a detailed description has been published.
“Thanks to MBARI’s advanced underwater technology, we were able to prepare the most comprehensive description of a deep-sea animal ever made,” commented the institute’s senior scientist and team-leader Bruce Robison. “We’ve invested more than 20 years in understanding the natural history of this fascinating species of nudibranch.”
Gelatinous hood
While most nudibranchs move around on solid surfaces using their muscular foot, this sea slug uses its gelatinous hood to propel itself through the water, swimming with up-and-down undulations of the entire body, from hood to tail.
If threatened, the tips of its forked tail glow from bioluminescence and can also detach – possibly a tactic to distract predators, the scientists speculate.
“When we first filmed it glowing with the ROV, everyone in the control room let out a loud ‘Oooooh!’ at the same time,” said MBARI senior scientist Steven Haddock. “We were all enchanted by the sight.
“Only recently have cameras become capable of filming bioluminescence in high resolution and in full colour. MBARI is one of the only places in the world where we have taken this new technology into the deep ocean.”
Genetic analysis
Eighteen individuals were also captured for genetic analysis. This revealed the creature to be a nudibranch, but one so distantly related to known species that it was deemed to belong in a family of its own.
It was given a scientific name based on Deep Deceiver because of its obscure identity and the finger-like tail-tips. Typical length is 14.5cm, while the elastic hood can measure up to 9cm across.
The red organ that can be seen in images is the stomach, the orange organ the digestive gland and the small white patch the brain. Analysis of each nudibranch’s stomach contents revealed the remains of mysid shrimp, which appeared to have been captured through use of the hood, though this process has yet to be observed.
Like other nudibranchs, the Deep Deceiver is hermaphrodite and appears to descend to the seabed to spawn, staying attached by its foot while releasing its eggs. The full report has just been published in the journal Deep-Sea Research Part I.
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