In late December, researchers were descending a remotely operated vehicle in the Atlantic Ocean off Argentina when, at a depth of roughly 800 feet, the control room fell silent. On the screens, a jellyfish appeared. But it was no ordinary sighting: It was a giant phantom jelly, a deep-sea species rarely seen in the world’s seas.
“There was a mixture of excitement and disbelief,” said María Emilia Bravo, a marine biologist at the University of Buenos Aires, who led the dive as part of an expedition aboard R/V Falkor (too), a research ship operated by the Schmidt Ocean Institute. The institute released its findings on Tuesday.
“Its ethereal and delicate presence in such an extreme environment was deeply surprising,” she said. The creature’s long arms made it difficult for the underwater vehicle’s operators on the ship to maneuver safely. “We were curious about getting to know it better and documenting it well.”
First collected in 1899, giant phantom jellies were not recognized as a species until 60 years later. Even then, specimens were typically found dead in trawling nets and rarely documented alive. Remote-operated vehicles made sightings of these creatures possible.
“Up until recently, nobody was able to actually see it in its natural habitat and in its full glory,” said Steve Haddock, a marine biologist at the Monterey Bay Aquarium Research Institute who was not involved with the dive.
During the last century, phantom jellies have been documented about 100 times.
These elusive creatures are among the largest jellyfish: Their bells can grow over three feet in diameter, and their arms can reach up to 30 feet in length. People often assume that undescribed deep-sea species are obscure, hidden animals, Dr. Haddock said. “But no,” he said, “it’s these giant things, the size of a giant squid practically, that have gone largely unnoticed.”
Phantom jellies hunt differently than other stinging jellyfish. They rely instead on their long, curtain-like arms to catch plankton and small fish which they reel into their mouths. But they also maintain a symbiotic relationship with some fish species, which are often seen swimming or hovering around a jelly’s bell or arms.
The midnight zone, the dark layer of the ocean that extends down to about 13,000 feet, is food-scarce and offers little refuge, so many smaller denizens find shelter among larger animals. Phantom jellies provide protection from predators, while the fish feed on the jelly’s parasites.
Despite the vastness of the ocean, these two species find each other and remain together for most of their lives, Dr. Haddock said: “It’s almost an ‘Odd Couple’ type story.”
In the Northern Hemisphere, the fish often seen with phantom jellies are pelagic brotula. The one observed by the team in Argentina, however, belonged to a different group, a genus of medusafish called Centrolophus.
This finding is particularly important for the Argentine deep sea, Dr. Bravo said. Although there are historical records of associations between jellyfish and fish, this interaction had never before been documented in the deep waters around Patagonia.
The Argentine-led expedition also documented dozens of new species, including corals and sea urchins, along the country’s continental shelf. Scientists traveled from Buenos Aires in the north to waters off Tierra del Fuego in the south. Their main goal was to find cold seeps: seafloor areas where methane and other chemicals serve as energy for microbes, which in turn sustain animals such as clams and mussels. They found one active seep measuring about half a square mile.
Alexa Robles-Gil is a science reporter and a member of the 2025-26 Times Fellowship class, a program for journalists early in their careers.
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