Marine researchers operating the underwater robot SuBastian off the coast of Argentina recorded a rare giant phantom jellyfish escorting 19 juvenile rudderfish at a depth of 250 meters. The deep-sea footage, captured during an expedition between December 2025 and January 2026, provides the first visual evidence of an interspecies partnership that scientists suspected for over 50 years.
Why were the fish traveling with the jelly?
The young rudderfish likely used the massive creature as a drifting shelter to avoid predators in the hazardous open ocean. Observers noted the juvenile fish swimming beneath the bell and even nibbling along the tentacles. Researchers believe the fish gain vital protection and potential food, though it remains unclear whether the jellyfish derives any direct mutual benefit from hosting them.
During the dive from the research vessel Falkor (too), the six-meter-long Stygiomedusa gigantea drifted peacefully through the twilight mesopelagic zone. While one fish was seen nibbling tissue or organisms on an oral arm, scientists led by María Emilia Bravo noted that the massive arms primarily provide a protective barrier for young fish navigating open water.
Confirming a half-century deep-sea mystery
Marine biologists first proposed this interaction in 1973 after discovering both species tangled together inside Atlantic fishing nets. Without live visual records, that relationship remained unverified for decades. This expedition finally confirmed the living partnership in Patagonia, proving that juvenile rudderfish actively accompany the elusive predator in deep waters rather than colliding by chance during net trawls.
The study, published in the journal Ecology, underscores how flexible ocean interactions can be across different depths and environments. With fewer than 120 sightings of the giant phantom jellyfish recorded globally since 1899, robotic cameras now offer unprecedented insight into hidden ecological relationships across fragile marine habitats.
