One of the weirdest sea slugs known to science has been discovered at a staggering depth, swimming around right down in the Midnight Zone between 1,013 to 4,009 meters (3,323 to 13,153 feet). As well as becoming the first sea slug known to live in the deep-water column, it also uses its body like a big net, glows in the dark, and even jettisons little glowing decoy noodles to distract its predators. Bathydevius caudactylus really is a sea slug unlike all others.
There are a few rules to being an IFLScience writer, and one is that if someone sends you an email with the subject matter “mystery mollusk,” you don’t sit on it. So, when the Monterey Bay Aquarium Research Institute (MBARI) told us they’d finally unmasked the identity of a mystery mollusk that had been puzzling scientists since it was first spotted in the year 2000, we sat up and listened.
It took quite a while for us to figure out what kind of animal it is.
Bruce Robison
The team thought the mystery mollusk might be some kind of snail because its bizarre 5.6 to 14.5-centimeter (2.2 to 5.7-inch) body included what looked like a muscular foot. However, after carefully collecting a few animals and studying them in the lab, they made the shocking discovery that it was actually a sea slug.
“I’d say that I was puzzled, the first few times I saw Bathydevius, because it took quite a while for us to figure out what kind of animal it is,” said MBARI senior scientist Bruce Robison to IFLScience. “Relief is what I experienced once we were sure that we had a nudibranch, principally because it was so unexpected.”
Nudibranchs are among the most extra marine animals out there, with designs that come in just about every shape, color, and swimming style imaginable. So, to find a new species that MBARI’s scientists are describing as “unlike all other nudibranchs” is big news.
“Virtually everything about Bathydevius seems remarkable to me,” continued Robison. “The most striking initial impact comes from its unusual anatomy and configuration, which is very unlike all other nudibranchs. The most remarkable adaptation seems to me to be that it feeds on fast-moving shrimp with hair-trigger escape responses, when Bathydevius itself is a very slow-moving animal.”
[They] entrap their prey in what looks like a living grocery bag.
Steve Haddock
The sea slug captures its prey using a big gelatinous hood that is part of its body, a feeding strategy that’s also seen in the tidepool nudibranch Melibe. This got the team wondering if perhaps they were related, but even here Bathydevius turned up some surprises.
“The genetic story showed that they were not close relatives, indicating that we had yet another example of convergence,” MBARI Senior Scientist Steven Haddock told IFLScience. “Several unrelated animals, including the large jellyfish Deepstaria and fragile comb jellies, entrap their prey in what looks like a living grocery bag.”
“Instead of out-muscling their captives, they use passive resistance to absorb the energetic escape responses, and eventually subdue their prey. For me, seeing another independent origin of this strategy, highlights how adaptation to living in the vast water-column allows you to remain fragile and transparent, but still be a successful predator.”
Bathydevius caudactylus produces brilliant bioluminescence to deter predators. If threatened, a diffuse glow spreads over the oral hood and finger-like dactyls in the tail.
Image credit: © MBARI
Successful predators though they may be, these sea slugs still need to protect themselves from other hunters in the Midnight Zone, and they’ve evolved to have luminous granules in their tissues that create a “starry” appearance across its back. Even the finger-like dactyls on its tail glow, and – much like a lizard dropping its tail – it can jettison them to act as a shiny decoy when they’re under attack.
It was these dactyls that inspired the “caudactylus” part of its name, meanwhile “Bathydevius” is a hat-tip to how it deviates from the nudibranch norm in its choice of habitat, appearance, and penchant for glowing. This kind of bioluminescence is rare in nudibranchs, with Bathydevius marking the third time it has independently evolved in this group – and the seventh time among gastropods.
“Filming that display with our deep-sea low-light camera was a magical moment for everyone on the ship,” added Haddock.
And so the mystery of the mollusk is solved, but what deep-sea oddity will those scientists unmask next?
Source Link: “Mystery Mollusk” Unmasked As First-Known Nudibranch To Live In The Midnight Zone
