OAcross the ocean depths lies a diaphanous creature that looks like a jellyfish, but is actually something else entirely. Pelagothuria natatrixwhich means swimming sea cucumber, belongs to a group of animals best known for crawling along the seabed like giant rubbery worms.
This sea cucumber was first named at the end of the 19th century, but for a long time it was known only from a few damaged specimens hauled up in scientific trawls. “They’re extremely fragile, almost to the point of being somehow intangible,” says Chris Mah, a biologist at the Smithsonian Institution in Washington DC. “The fact that they are gelatinous makes them extremely difficult to study.”
In 2014, Mah sparked what he describes as a rediscovery of the species when he was browsing a database of deep-sea images and spotted an umbrella-shaped umbrella. Pelagothuria which was mislabeled as a jellyfish. Until then, he says, only a handful of scientists knew about the species. Mah’s sighting encouraged others to seek them out when surveying the high seas.
Three years later, a team of scientists working in the Pacific Ocean got a spectacular view of these gassy creatures in their natural environment. Working on the Okeanos Explorer research vessel, the team watched video footage of Pelagothuria teleported in real time from a deep diving robot.
During nine dives, between American Samoa and Hawaii, they spotted nearly 100 of these swimming sea cucumbers, at depths ranging from 196 to 4,440 meters and often in very low oxygen areas in the ocean. seawater. Mah suggests it could be Pelagothuriato avoid predators that are more oxygen-hungry and could easily suffocate.
How Pelagothuria survives in these harsh conditions is still a mystery, but it likely has something to do with his jelly body. Many animals living in the deep sea have bodies made up of mostly water with a small amount of collagen mixed in. This gelatinous paste requires little energy to manufacture and maintain, and is therefore ideal for animals living at depths where food is often scarce. Jelly animals are also inherently buoyant, so they don’t need to waste precious energy and oxygen swimming vigorously to stay afloat; they can just drift.
Of approximately 1,200 species of sea cucumbers, Pelagothuria is the only one known to spend most of his time swimming. It uses the web around its mouth to propel itself through the water column.
Several other sea cucumber species are occasional swimmers. “They live on the bottom, but they can swim whenever they want,” says Mah. The approach of a predatory starfish can set a sedentary sea cucumber into action. Even a few brief seconds of clumsy swimming can be enough to escape.
It may be so Pelagothuria The ancestors started out, then evolved into better swimmers until they adopted a full-time jellyfish lifestyle. It’s a case of convergent evolution in which distant organisms — in this case, sea cucumbers and jellyfish — have solved challenges with a similar outcome.
“The gelatinous lifestyle is definitely something you see a lot in underwater animals,” Mah says. “It’s such a common adaptation that each organism will have its own story as to how it got there.”