"When we dove down in the submarine, we noticed the water became murkier as we got closer to the bottom," said the expedition's chief scientist, Jesus Pineda, a biologist at the Woods Hole Oceanographic Institution, in a press release. "There was this turbid layer, and you couldn't see a thing beyond it. We just saw this cloud but had no idea what was causing it."
"As we slowly moved down to the bottom of the seafloor, all of the sudden we saw these things," said Pineda, who was the study's co-author. "At first, we thought they were biogenic rocks or structures. Once we saw them moving — swarming like insects — we couldn't believe it."
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.
The team caught the video footage of the crabs while in a submarine, called the Deep Rover 2, last April. The submarine was part of a research cruise on the research ship M/V Alucia. On the cruise were scientists from Woods Hole Oceanographic Institution, Point Loma Nazarene University, San Francisco Estuary Institute and Smithsonian Tropical Research Institute.
The researchers were studying an area known as the Hannibal Bank Seamount, which they call an "ecological hot spot" for its rich biodiversity.
The team published its findings Tuesday in the journal PeerJ.