Descending in a submersible in waters off Panama, scientists noticed something strange happening near the seafloor. It was a drifting fog of sediment, disturbed by something below. Diving deeper, the scientists found the cause: crabs, thousands of them, swarming in a way never before recorded.
“We just saw this cloud but had no idea what was causing it,” said Jesús Pineda, a biologist at Woods Hole Oceanographic Institution in Massachusetts and the lead author of a paper on the crabs published on Tuesday.
“At first, we thought they were biogenic rocks or structures. Once we saw them moving, swarming like insects, we couldn’t believe it.
“Nothing like this has ever been seen, where we have this very dense swarm at the bottom,” Pineda said. “We have no idea why they might be doing this.”
Pineda said he pored through scientific literature, looking for similar behavior among the many kinds of crabs, lobsters, shrimp and marine creatures that dwell near the bottom of the ocean. He found nothing.
His team of researchers were studying life at the Hannibal Bank Seamount, off the Pacific coast of Panama. They published their findings in the journal PeerJ.
A DNA test identified the crabs as Pleuroncodes planipes, better known as the red crab, for its color, or the tuna crab, for the fish that devours it in bulk. Each crab is only one to five inches long, and the species’ range was thought to be restricted to the waters off the southern coast of California and Mexico.
The discovery of a swarm of the crabs so far south and in such huge numbers, Pineda said, “is very unusual”.
The swarm was found about 1,200ft below the surface in an area of hypoxic water, which has extremely low oxygen and where most marine species struggle to survive. The crabs have been found in other hypoxic regions, and the researchers hypothesized that their ability to survive in such extreme conditions allowed them to take refuge from tuna and other predators.
The scientists noted that studying this region could be helpful for climate change research, as warming oceans make low-oxygen environments more common and kill off coral and rich ecosystems in more shallow waters.
Red crabs scavenge detritus and bodies but also eat plankton and, like sardines and anchovies elsewhere, are food for all manner of fish, birds, mammals and men. People in parts of Latin America eat them as crunchy snacks, Pineda said.
As larvae and juveniles, the crabs live in gigantic schools up and down the water column. Pineda hypothesized that as adults they fed in higher water during the night and swam together into the depths during the day.
“These crabs have a very interesting lifestyle,” he said.
Last June, thousands of the same species washed up on beaches in San Diego, in a massive stranding associated with the warm Pacific waters of an El Niño.
The Hannibal Bank Seamount is an underwater mountain – less than 1% of such mountains, dotted around the Pacific with volcanoes, sea vents and bizarre varieties of life, have been studied.
Pineda’s team also found octopuses, rays, scallops, soft corals and colorful sea urchins, and they plan to return for more study. They are also using underwater drones to map the mountain and the water currents around it.
Read article at The Guardian