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A mystery mollusk sea slug in the deep ocean


A video about the newly described sea slug Bathydevius caudactylus, from the Monterey Bay Aquarium Research Institute.

  • Scientists identified a new deep-sea sea slug species, Bathydevius caudactylus, using remotely operated vehicles (ROVs).
  • Bathydevius caudactylus, found in the dark, cold depths of the ocean, is the only known sea slug living in deep open waters.
  • This mostly gelatinous sea slug uses a unique head hood to catch crustaceans. It’s bioluminescent and can swim, or drift in the currents.

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A discovery 20 years in the making

In February 2000, scientists using cameras on a remotely operated vehicle (called ROV for short) spotted an unusual sea slug swimming in the deep sea. They nicknamed it the mystery mollusk. Since then, there have been over 150 ROV sightings of this animal. And scientists were able to collect several specimens for detailed study in laboratories. Now, more than 20 years since that first sighting, they have enough information to announce that this sea slug is in fact a new species.

Researchers at the Monterey Bay Aquarium Research Institute (MBARI) published their findings in the December 2024 issue of the peer-reviewed journal Deep-Sea Research Part I: Oceanographic Research Papers.

They named the sea slug species Bathydevius caudactylus. This animal lives in a deep ocean zone cast in perpetual darkness because sunlight never reaches such great depths. It has a head dominated by a large gelatinous hood, a trunk containing internal organs, and a tail that acts like a paddle. Bathydevius are small creatures, with captured specimens ranging in size from 2.2 to 5.7 inches (56 to 145 millimeters). Moreover, they are bioluminescent, meaning these organisms can light up in the dark due to light-producing chemicals in their bodies.

Paper co-author Steven Haddock remarked in a statement:

What is exciting to me about the mystery mollusk is that it exemplifies how much we are learning as we spend more time in the deep sea, particularly below 2,000 meters [6,500 feet]. For there to be a relatively large, unique, and glowing animal that is in a previously unknown family really underscores the importance of using new technology to catalog this vast environment. The more we learn about deep-sea communities, the better we will be at ocean decision-making and stewardship.

Bathydevius is a sea slug known as a nudibranch

When scientists first saw Bathydevius, they did not know what to make of it. That’s because they were puzzled by its appearance. For instance, a hood-like structure dominated the animal’s head. Its flat tail had several tentacle-like protrusions. In addition, within its almost transparent trunk were reddish internal organs.

Sea slug: Transparent undersea organism with a large clear hood, reddish internal organs, and short tentacles on the tail.
A view of the newly identified sea slug species Bathydevius caudactylus. Image via MBARI. Used with permission.

Using ROVs, scientists carefully captured several of these delicate mystery mollusks for detailed anatomical and genetic studies. This work allowed them to confirm that their mystery mollusk was a nudibranch. Also known as sea slugs, nudibranchs are often found in shallower water such as tide pools and coral reefs. Some live in open water, near the ocean surface. In addition, a few nudibranch species live on the ocean floor.

However, Bathydevius was different. It was first encountered in the deep open waters of Monterey Bay. Since then, it has been observed in deep waters off the North American Pacific coast, ranging offshore from Oregon to Southern California. A similar-looking sea slug was spotted in the Mariana Trench, the deepest ocean trench, in the Western Pacific. Could that also be Bathydevius? Scientists aren’t sure, but if it is, that would greatly expand its range.

Bathydevius is a unique sea slug

What makes Bathydevius unique? It is the only sea slug known to live in deep open water. This animal has been found in depths from 3,300 to 13,100 feet (1,000 to 4,000 meters) in ocean waters known as the bathypelagic zone. This section of the ocean is always dark because sunlight never reaches such great depths. It is also very cold, about 39° Fahrenheit (4° Celsius). Pressures ranges from 100 to 400 atmospheres, depending on depth.

Like other nudibranchs, Bathydevius is a hermaphrodite, which means that it has both male and female sex organs. Scientists have observed them spawning, by attaching themselves to the seafloor with their muscular foot and laying ribbons of eggs.

The scientists also found, based on genetic studies of specimens they caught, that this creature is unique enough to belong to its own taxonomic family. (Family is a higher-level classification rank for closely related species. In this case, Bathydevius caudactylus is the only species in the family Bathydeviidae.)

A sea slug adapted for deep ocean waters

Most sea slugs feed using a rough-surfaced tongue, called a radula, taking prey attached to objects such as rocks. Bathydevius, however, uses its hood as a trap to catch crustaceans like shrimp, not unlike a Venus flytrap.

Translucent creature with large, wide-open hood, small body with internal organs visible, and tentacled tail.
The sea slug species Bathydevius caudactylus with its hood extended open. Image via MBARI. Used with permission.

It swims slowly in the water by undulating its body. Sometimes, it simply drifts in the currents. Because Bathydevius has an almost transparent body, it is able to go largely undetected by predators. But if it needs to make an escape, it closes its hood rapidly, which propels its body backward.

Bathydevius is bioluminescent. It can create a light display, possibly to deter predators. The light comes from granules embedded in its body and appears like tiny blue stars illuminating the shape of the animal.

An animation of bioluminescence, appearing as tiny bright dots, in the sea slug that shows its shape as it moves in the water.
Bioluminescent spots in Bathydevius caudactylus, captured using a low-light camera system. Image via MBARI. Used with permission.

Haddock remarked in the statement:

Only recently have cameras become capable of filming bioluminescence in high-resolution and in full color. MBARI is one of the only places in the world where we have taken this new technology into the deep ocean, allowing us to study the luminous behavior of deep-sea animals in their natural habitat.

Bottom line: After 20 years of research, scientists have identified and named a new species of sea slug that lives in the deep ocean. They’ve called it Bathydevius caudactylus.

Source: Discovery and description of a remarkable bathypelagic nudibranch, Bathydevius caudactylus, gen. et. sp. nov.

Via Monterey Bay Aquarium Research Institute (MBARI)

Read more: The adorable leaf sheep sea slug: Lifeform of the week

Posted 
November 20, 2024
 in 
Earth

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