Imagine yourself slowly sinking beneath the waves. As you fall, the Sun shrinks, fades, fades away. Cold water puts pressure on you and the pressure increases. You are now in pitch darkness, so deep that sunlight cannot reach you. And suddenly, to your surprise, you see a flash of light from a creature no one has ever seen before.

Recently, researchers from the Monterey Bay Aquarium Research Institute (MBARI) in the United States discovered a naked mollusk floating freely in the water column, illuminated by a bioluminescent glow and adorned with a billowing cap. This is Bathydevius caudactylus, the first known teleost of its kind: It lives not in shallow water or on the seafloor, but in open water, in the bathypelagic zone, less than 2,200 meters (about 7,220 feet) below the surface of the Pacific Ocean.

“Most jellyfish live bottom-dwelling in shallow water, so it was very surprising to find a jellyfish this deep in the water column and so far from the bottom,” MBARI marine scientist Bruce Robison told ScienceAlert. “We knew of several species that lived on the deep seafloor, but none of them had been reported in deep, intermediate waters.”

Cephalopod sea snails are one of the most charismatic molluscs. They are often brightly colored and have elaborate appendages and patterns. They can often be found on reefs, where some species gnaw corals, while others feed on algae, sponges, anemones, jellyfish, and even other nudibranchs.

They are diverse and versatile, but this versatility was believed to be limited to a specific area. This is one of the reasons why MBARI researchers dubbed the animal the “mysterious mollusk” when they encountered it while using the Tiburon remote-controlled research vehicle at a depth of 2,614 metres.

“When we first filmed the flash with the UAV, everyone in the control room said, ‘Ooooo!'” Haddock said. he said. “We were all mesmerized by this spectacle.”

The creature they saw had a sea slug-like leg, a large, gaping head, and a fringed tail that resembled waving fingers. Her pink internal organs could be seen through the ethereal, translucent skin of her body. The device glowed with a bioluminescent glow as it approached and the creature felt threatened.

This behavior appears to be defensive: Researchers once saw one of the tendrils on the slug’s tail constantly glowing, spinning around, and eventually breaking off and floating freely; similar to how a lizard throws its tail back to distract a predator.

Researchers encountered a total of 157 individuals of this species during dives between 2000 and 2021; 32 of them were examined in detail, and 18 were collected for further studies in the laboratory. This research involved a genome analysis that allowed them to identify the strange creature as a tarsier; but this was so different from all other tarsidae that a new family, Bathydeviidae, had to be created.

“The most exciting aspect of this discovery is that we were able to provide the most complete initial description of a new deep-sea species (anatomy, respiration, bioluminescence, reproduction, nutrition, genetics, behavior) ever presented,” Robison told ScienceAlert. )”.

Researchers observing Batydevius in its natural habitat saw the animal use its large head to catch prey, indicating a diet rich in crustaceans. Batydevius, which does not have a hard, tongue-like appendage like most other nudibranchs, swallows its food with its mouth behind its hood.

Two other species of loons use similar headgear, but they are far removed from Bathydevius in the loon family; This suggests that this feature evolved independently in the newly discovered species.

Like other free-swimming species, nudibranchs are hermaphrodites. On one occasion, researchers were lucky enough to observe part of Bathydaeus’ reproductive strategy: At a depth of 2,755 meters, two individuals anchored to the seafloor to lay frilled egg strips. Other individuals have been observed similarly clinging to the seafloor at depths of up to 4,009 metres.

In another case, one of the captured individuals was ready to spawn; He did this in an aquarium by releasing a ribbon that drifted and then attached itself to the bottom of the aquarium. After three days, the mucosa of the band dissolved and the eggs turned into larvae. This is the most comprehensive study to date of a new deep-sea species, and it suggests we may be too conservative in our assumptions about what the natural world is capable of.

“Bathydevius is radically different from all other teleosts because it is well adapted to life in a completely different habitat; “Evolution overcame the challenges of surviving where it lived and adapted very well to thrive there,” Robison says.

“This shows us that tibia evolution is much more flexible than we previously thought.”

The study was published in the journal Deep-Sea Research Part I.