When three scientists descended more than a thousand feet underwater in an exploration vessel off the coast of Panama, they expected to see many kinds of life. They'd chosen to explore the Hannibal Seamount, a flat-topped undersea mountain that's the ocean equivalent of a tropical jungle, rich with a diversity of animals and plants found nowhere else. Their submersible, the Deep Rover 2, has the look of a giant, transparent bubble; with the help of spotlights, they could peer out from every angle. But as they approached the northwest flank of the seamount, they saw something inexplicable on the ocean floor.

It looked like an underwater dust storm. As they got closer, the researchers realized it was an enormous swarm of crabs, kicking up sediment from the seafloor. They had never seen anything like it.

Biologist Jesús Pineda recalled:

When we dove down in the submarine, we noticed the water became murkier as we got closer to the bottom. 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. At first, we thought they were biogenic rocks or structures. Once we saw them moving—swarming like insects—we couldn't believe it.

When they came back up with a few captured crabs, the researchers sent an ROV back down to take more pictures. They weren't even sure what species they'd seen, but DNA analysis revealed the swarm was Pleuroncodes planipes, or red crabs. These creatures are a couple of inches long, and they love to float up and down in the water column to eat plankton, spending some of their time close to the surface and some close to the seafloor. This crab mob was fairly deep, in a very low-oxygen region of the water, which was unusual. Plus, they were far from their typical haunts off the coast of Mexico.

The researchers describe their experience in a paper published in PeerJ, noting that "dense patches" of red crabs have been observed before, ranging from 7 to 16 kilometers across. But this is the first time a patch has been caught on video and also the first time they've been spotted in waters so far south of their usual range.

Through careful analysis of the footage, the researchers determined that the patch they'd seen extended for tens of meters in every direction, and at its densest points there were about 77 crabs per square meter.

What caused the crabs to swarm in such an unusual location? There are a lot of possibilities.

The crabs interacted with each other a lot, suggesting that this swarm may have had social causes, especially because patches can be related to social activities like mating. Sometimes the crabs moved in the same direction, like an insect swarm; but at other times the crabs seemed to be crawling around in pretty much random directions. They only formed the dense patch in one specific area, which might indicate that they were feeding on plankton that were abundant there. But patches like this are also formed during migrations, too.

Roughly around the same time that the researchers spotted the crab swarm at Hannibal, a massive number of crabs washed ashore in Southern California, casualties of an El Niño weather event. It turned out they were the same species as the red crabs the researchers observed. This might mean that the red crabs form patches and swarms seasonally, and one unfortunate group got beached.

We'll need to do more research to know for sure why the red crabs were swarming when and where they did. But we can be certain that those patches they form are an essential part of the undersea ecosystem—and such a big crowd will provide some damn tasty meals for animals who fancy a crustacean snack.

PeerJ, 2016. DOI: 10.7717/peerj.1770