As a group of scientists descended in a submersible off the coast of Panama, they noticed a strange cloud hovering just above the ocean floor. When they took a closer look, what they saw was astonishing: a massive swarm of thousands of red crabs marching through the sand and silt at the bottom of the ocean.
“We just saw this cloud but had no idea what was causing it,” Jesús Pineda, a biologist at Woods Hole Oceanographic Institution said in a statement. “At first, we thought they were biogenic rocks or structures. Once we saw them moving, swarming like insects, we couldn’t believe it.”
Pineda and his colleagues spotted the teeming swarm of crabs at the base of a site called the Hannibal Bank seamount, an underwater mountain about 1,200 feet below the ocean’s surface. While Hannibal Bank is a well-known hotspot for a diverse array of underwater creatures, marine biologists had never before seen so many red crabs in one place and as far south, Alan Yuhas reports for The Guardian.
Typically, red crabs are found off of the coast of Baja California in Mexico, though they have been known to range further north to central California during El Niño years when the waters get warmer. While the crabs have been seen gathering in large herds before, this was the largest, densest group ever reported, with Pineda and his colleagues counting about seven crabs per square foot, Mindy Weisberger reports for LiveScience.
“Nothing like this has ever been seen, where we have this very dense swarm at the bottom,” Pineda said in a statement. “We have no idea why they might be doing this.”
Intriguingly, the swarm was also spotted in a region with very low levels of oxygen in the water where it is hard for sea creatures to survive. Red crabs have been spotted in similar “hypoxic regions” before, though, and Pineda guesses that the crabs may have huddled in this region to hide from predators.
In a study published this week in the journal PeerJ, Pineda and his colleagues noted that studying oxygen-poor environments like this could be helpful in learning about how climate change is affecting the oceans, as these regions are becoming more common as the waters get warmer, Yuhas reports.
While this sighting was a surprise to the scientists, it just goes to show how much more there is to learn about life beneath the waves. Underwater mountains like the Hannibal Bank seamount are plentiful, but fewer than one percent of them have been studied closely, Yuhas writes. During the same dive, Pineda and the research team also spotted a diverse array of sea creatures like rays, octopuses, soft corals and sea urchins, and plan on returning to the site to see what more they can learn from the teeming ecosystem.