There's a massive 'crab swarm' off the coast of Panama

A team of scientists conducting research on the seafloor off the coast of Panama made a surprising discovery last April when they came across something in low-oxygen waters.

"When we dove down in the submarine, we noticed the water became murkier as we got closer to the bottom," said Jesús Pineda, a biologist at Woods Hole Oceanographic Institution (WHOI) and lead author of a study published in the biomedical science journal PeerJ. 

"There was this turbid layer," he added, in remarks provided by WHOI, "and you couldn't see a thing beyond it. We just saw this cloud but had no idea what was causing it."

Turns out, as the scientists soon realized, it was a massive swarm of crabs.

"As we slowly moved down to the bottom of the seafloor, all of the sudden we saw these things," Pineda said. "At first, we thought they were biogenic rocks or structures. Once we saw them moving—swarming like insects—we couldn't believe it."

The crabs are Pleuroncodes planipes, or tuna crabs, and are usually found around Baja California. The Panamanian coast is a long way from home.

"No one had ever found this species that far south," Pineda said. "To find a species at the extreme of their range and to be so abundant is very unusual."

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