As they dove some 1,200 feet deep, the submarine illuminated the culprit: a swarm of red crabs scuttling across the seafloor.

“I was mesmerized,” said Dr. Pineda. “I just couldn’t believe it. It was very unexpected.”

This was the first time researchers recorded footage of the inch-long crabs carpeting Panama’s ocean floor, according to Dr. Pineda. He said it was unusual to watch the swarm crawl in the same direction, a behavior normally seen in insects.

Using DNA analysis they identified the crabs as Pleuroncodes planipes, or red crabs, which are usually found off the coast of Baja California. The team published their findings this week in the journal PeerJ.

But there is still a mystery: Dr. Pineda and his team said they aren’t sure why the crustacean congregation formed, and why it appeared so far south from where the crabs usually lurk.