A year ago, researchers in two small submarines were exploring a seamount — an underwater, flat-topped mountain — off the Pacific coast of Panama when they noticed a dense cloud of sediment extending 4 to 10 meters above the seafloor. One of the submarines approached closer, and the scientists could soon see what was kicking up the cloud: thousands of small, red crabs that were swarming together like insects.

“The encounter was unexpected and mesmerizing,” Jesús Pineda of the Woods Hole Oceanographic Institution in Massachusetts and colleagues write in a paper published April 12 in Peer J.

The team decided to investigate further. They sent an autonomous underwater vehicle to pass over the swarm several times, capturing images and video of the crabs. At the densest points in the swarm, there were more than 70 crabs in a square meter of ocean bottom, and this occurred consistently in a water depth of 350 to 390 meters. The crabs, all 2.3 centimeters in carapace length and larger, were moving together in the same general direction. Some would jump and swim for about 10 centimeters or so before landing back in the pack.
Using one of the submarines, the researchers collected some crabs from the swarm. Back in the lab in Woods Hole, they used DNA barcoding to identify the species: Pleuroncodes planipes. This is the same species of crab that has sometimes washed up in mass stranding events on California beaches, which the team confirmed by comparing the DNA barcodes to those of crabs from a stranding event in La Jolla, Calif., in June 2015.

For reasons that scientists still don’t fully understand, seamounts are ecological hot spots where plankton get trapped and feed a wide array of fish and marine mammals higher up in the food web. Fishermen have figured out that they can take advantage of this, but scientists are just now getting into the game and exploring these sites. Because of this, less than one percent of the world’s seamounts have been checked out by researchers. That probably explains why no one had seen a crab swarm like this before on a seamount.
But this is not the first time crabs have been seen swarming. Scientists have previously documented large aggregations of king crabs, spider crabs, tanner crabs and lyre crabs on the seafloor. Such behavior may be linked to reproduction.

And then there are the red crabs of Christmas Island in the Indian Ocean, which swarm in the millions during the wet season, coming out of the forests and making a long trek to the beach for a massive mating party.