Marine biologist Mark Benfield and his collegaues were using the undersea robots to search for signs of environmental damage in the Gulf of Mexico in 2011, when the mysterious deep-sea creature swam into view.
Dr Benfield released video of the ‘sea serpent’ and published a description of its behaviour in a scientific journal this week.
It’s only the second time that a living oarfish has been seen in at depth – most reports are based on the remains of dead or dying animals that float to the surface.
Dr Benfield estimated the specimen was up to seven metres long. The largest species, also known as the “king of herrings”, has been reported to grow up to 17 metres long.
They’ve also been reported to deliver an electric shock when touched, like an electric eel.
Oarfish usually swim vertically in the water, with their head facing the surface and their tail pointing towards the sea floor. They feed on krill and other small sea creatures.
The oarfish head is crowned by long red spines, and it swims by undulating the long dorsal fin along its back.
Dr Benfield’s project is a collaboration between several oil and gas companies in the Gulf of Mexico, who lend their underwater robots, or ROVs, for scientific exploration when they’re not needed for work on undersea rigs.