A research team studying biodiversity at the Hannibal Bank Seamount off the coast of Panama captured the unique video.
The video shows the swarm of five-inch-long crabs, which resemble small lobsters, amid clouds of sediment at a depth of 355 to 385 meters.
“This phenomenon was just so striking and so shocking,” said Jesus Pineda, the expedition’s chief scientist, who spotted the swarm from a submersible. “I’d never seen anything like that in my life. I was mesmerized.”
The researchers are still not sure why they gathered in this area in such large numbers, but Pineda said they may be amassing in the deep, low-oxygen waters to avoid predators, such as yellowfin tuna.