Video: Huge Underwater Swarm of Crabs Stuns Scientists

A swarm of crabs has been unexpectedly spotted above a seamount off Panama. WHOI

Scientists have discovered a massive swarm of crabs above an underwater mountain off the coast of Panama, the like of which has never been seen before.

These types of crabs haven't been observed swarming like this previously, says Jesús Pineda, a biologist at Woods Hole Oceanographic Institution. The crabs were discovered in April 2015 by scientists, including Pineda, aboard a research vessel called the M/V Alucia.

The scientists had taken a manned submersible to the seafloor when they saw an odd, massive cloud of sediment, called a "turbidity layer." This can often be caused by natural phenomena such as ocean currents, but the researchers soon realized it was caused by something else—something alive, Pineda says. A sampling of the animals revealed that they were red crabs (Pleuroncodes planipes), according to a study describing the find, published April 12 in the journal PeerJ.

This came as quite a surprise, since these animals weren't thought to exist this far south, he says. And not only exist, but thrive in massive concentrations.

The aggregation formed by the red crab mimics those formed by swarming insects, and hasn't been seen before in this species or in many other types of crabs. Pineda says that such swarming could have various advantages, such as protecting the animals from predation (by decreasing the chance that any one individual could get picked off by a hungry yellowfin tuna, for example), or facilitating reproduction.

Dave Eggelston, a marine ecologist at North Carolina State University who wasn't involved in the paper, says that the animals were probably swarming in order to breed. "We have seen similar types of mating pods for snow and king crabs in the Gulf of Alaska," he says. He also notes that though the species are called red crabs, they are actually a type of squat lobster. (Squat lobsters look like lobsters, but are actually more closely related to hermit crabs," according to the Monterey Bay Aquarium.)

Pineda says, however, that some of the explanations for why crabs swarm are controversial, and ultimately, the exact reason for the swarm remains mysterious.