Researchers diving in waters off the coast of Panama captured unique video of a massive swarm of red crabs swimming near the seafloor.
Expecting to learn more about the biodiversity at the Hannibal Bank Seamount, the team was both surprised and mesmerized by the swarm, which represents a new southernmost range for the species, Pleuroncodes planipes.
"When we dove down in the submarine, we noticed the water became murkier as we got closer to the bottom," said Jesús Pineda, a biologist at Woods Hole Oceanographic Institution (WHOI) and lead author of the paper. "There was this turbid layer, and you couldn't see a thing beyond it. We just saw this cloud but had no idea what was causing it."
Researchers aboard the submersible Deep Rover 2 recorded the unexpected sight about 1,200 feet below the ocean's surface, in an area low in oxygen.
"As we slowly moved down to the bottom of the seafloor, all of the sudden we saw these things," Pineda added. "At first, we thought they were biogenic rocks or structures. Once we saw them moving - swarming like insects - we couldn't believe it."
DNA sequencing later confirmed the crabs were Pleuroncodes planipes, a species abundant in waters off the west coast of Baja California. The recent discovery, however, revealed the crabs swim further south, too.
"No one had ever found this species that far south. To find a species at the extreme of their range and to be so abundant is very unusual," Pineda said. "These crabs have been detected before in similar low oxygen conditions. It could be that these low oxygen waters provide a refuge for this species from predators."
The crabs are also vulnerable to changing ocean currents, as they migrate up and down in the water column. A few months after the April expedition, thousands of red crabs washed ashore in Southern California during a massive stranding associated with El Niño warming conditions. Genetic testing revealed the Panamanian and the Californian crabs are in fact the same species.
Seamounts are underwater mountains, home to thriving communities of unique species. However, less than 1 percent of these unique habitats have been studied.
"This study is an example of how we can effectively use the multiple tools now available to study the deep-sea habitat," said Walter Cho, a biologist at Point Loma Nazarene University and co-author of the paper. "The fact that we could combine the use of submersibles to explore, observe and sample, and then use an AUV (autonomous underwater vehicle) to follow-up those observations of the crab aggregation and get quantitative data is really powerful."
Pineda and his team plan to return to the Hannibal Seamount to study the area more, as the deep water, low oxygen and acidic areas of the seamount may shed light on how marine life communities will respond to climate change.
Their findings were recently published in the journal PeerJ.
