Scientists Have No Explanation for This Mysterious Swarm of Thousands of Crabs

A "crusade of crabs" isn't technically the correct collective noun for the ocean-dwelling crustaceans (for the record, it's "cast"), but I propose that it should be. At least with special regard to this mesmerizing footage of thousands of determined pelagic red crabs (Pleuroncodes planipes) swarming in unison across the seafloor.

The miraculous feat of nature was captured on film by a team of biologists off Panama's Pacific coast at the Hannibal Bank seamount. Jesús Pineda, a senior scientist at the Woods Hole Oceanographic Institution, was aboard the the manned submersible Deep Rover 2 at a depth of approximately 1,263 feet when he and his crew spotted the swarm.

As the newly released video reveals, thousands of crabs were filmed moving in sync with one another, seemingly away from and toward nothing.

"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. But these particular crabs weren't responding to food, or migrating, or reproducing. This was something different," Pineda told me about the event.

The team of biologists, which included staff from Point Loma Nazarene University, San Francisco Estuary Institute, and the Smithsonian Tropical Research Institute, were researching the hotspots of diversity found on Panama's nutrient-rich underwater seamounts. Less than one percent of these ecosystems have been studied, but they're of special interest to biologists because their high levels of biomass present exciting opportunities for new discoveries and insights into how deep-water organisms thrive.

Pineda removed a few specimens from the swarm, and using DNA sequencing he was able to determine the species as the red crab. Named for their striking color, these crustaceans are also sometimes called "tuna crabs" due to their tendency to feed on yellowfin tuna.

As to why this underwater murmuration might've occurred, Pineda and his colleagues are still looking for a definitive answer.

One potential theory has to do with the feeding behavior of this species of crab. Red crabs have been observed moving up and down in the "water column"—a term used to describe the various ecological features of different ocean strata—to find and eat prey.

"People have described schools of crabs six miles long in the water column. Some of these crabs have been reported to migrate vertically, which means that during the day, they can be found on the bottom of the ocean in the sediment. But at night, they ascend in the water column to feed on copepods and plankton near the surface. One possibility about this swarm is that it's a school of crabs that's just sitting on the bottom during the day," Pineda told me.

This particular area off the Panama coast features especially "hypoxic" (low oxygen) water levels, according to a statement. Red crabs have been detected in hypoxic areas before, and it's also possible the swarm was seeking refuge from predators in a habitat where few predatory species are able to survive.

Scientists not affiliated with the expedition have theorized that El Niño conditions may have contributed to the phenomenon. Thousands of red crabs have washed up on beaches in San Diego in the past, and have been positively linked to increased ocean temperatures. But according to Pineda, this explanation doesn't describe what he saw at the Hannibal Bank seamount.

"Red crabs are very abundant in southern Baja California. People have noticed during El Niño years that ocean currents change and cause the mass transport of water [in a northerly direction]," he explained to me. "With that, larvae is also transferred, resulting in huge schools of crabs that suddenly show up in southern California. But I don't see a mechanism for El Niño explaining what we observed. At the time of the cruise one year ago, a full El Niño had not been declared."

Pineda said he and his colleagues hope to eventually discover what was driving the exodus of thousands of red crabs that day. The team has just published their findings from last year's expedition in the journal PeerJ.

Their plan is to return to the Hannibal Bank seamount and continue their research on the ecosystem's astounding levels of biodiversity. And who knows—maybe they'll get lucky enough to come across another crusade of crabs. Or maybe we'll just all have to chalk this up as being another one of the ocean's many great mysteries.