Thousands of crabs are on the move in the Pacific and scientists don't know where they're headed or why

Crab swarmYoutube/ New Scientist ScreenshotA swarm of tuna crabs off the coast of Panama.

A newly released video from a small underwater vehicle found something quite surprising: A swarm of thousands of crabs all headed in the same direction, 1,263 feet deep in the Pacific Ocean, according to Motherboard

And it looks like something out of your worst nightmare.

Or, if you’re a scientist who studies this type of thing, it’s a dream come true. 

The scientists who captured the video have no idea what the crabs are doing, where they’re headed, or why they have all decided to move at the same time.

“It was unusual. In the past, we’ve noticed aggregations of breeding crabs hanging around the ocean floor, or migrating onto land if they’re terrestrial,” Jesús Pineda, a researcher at the Woods Hole Oceanographic Institute (WHOI) told Motherboard. “But these particular crabs weren’t responding to food, or migrating, or reproducing. This was something different.”

The video was filmed in April last year but only just released, as researchers were investigating the complex deep water ecosystems off the Hannibal Bank seamount off the coast of Panama.

DNA testing revealed the crabs to be members of the Pleuroncodes planipes species, which is sometimes known as the “tuna crab.” They’re a favourite snack of tuna and other large aquatic species like billfish and yellowtail.

The crabs are usually found in deep water off the Southern California coast, but they have never been spotted as far south as Panama, according to New Scientist

Tuna crabs’ range can extend northward (as far as Monterey, California) in El Niño years, when strong currents push larvae that direction.

But Pineda doesn’t think El Nino explains why a swarm this large would be found off the coast of Panama.

“I don’t see a mechanism for El Niño explaining what we observed,” he told Motherboard. “At the time of the cruise one year ago, a full El Niño had not been declared.”

And while scientists know that these crabs move up and down the water column to feed, usually retreating during the day to deep, low-oxygen waters to avoid predators, they say they have never seen anything like this swarm.

The pattern of the crab swarm, while common for terrestrial insects, is uncommon among bottom-dwelling organisms, according to Pineda. 

“Insect swarms tend to head in the same direction, but we don’t know where the red crabs were heading or why they were moving,” Pineda told New Scientist.

In a paper published for PeerJ, Pineda and his colleagues theorised that the crabs they spotted represent the southern end of the tuna crab species more commonly found in Southern California. 

The researchers plan to return to Panama to find out what exactly the swarm of crabs was doing and what role, if any, their movement might play in the complex seamount ecosystem.

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