Their location off Panama represents species’ southernmost range of distribution
Off the Pacific coast of Panama, researchers studying biodiversity at the Hannibal Bank Seamount got more than they bargained for. From within the Deep Rover 2 submersible, moving through the oxygen-poor waters, they came across the astounding spectacle of thousands of deep-sea red crabs swarming on the seafloor.
Jesús Pineda, a biologist at Woods Hole Oceanographic Institution and chief scientist of the expedition, described the experience as unexpected and mesmerising. Travelling on board the M/V Alucia, he and his fellow researchers had been on an expedition to study the biological and physical processes of the seamount in April 2015 when they stumbled upon the swarming red crabs.
“When we dove down in the submarine, we noticed the water became murkier as we got closer to the bottom,” said Pineda.
Describing it as a turbid layer, he said that one could not see anything beyond it, and they did not know what was causing it. He continued: “As we slowly moved down to the bottom of the seafloor, all of the sudden we saw these things. At first we thought they were biogenic rocks or structures. Once we saw them moving – swarming like insects – we couldn't believe it.”
The researchers recently published their findings in the journal peerj. Through DNA sequencing, the red crabs were identified as Pleuroncodes planipes. They are usually found off the western coast of Baja California, Gulf of California, and the California Current. The location where they had been sighted was a new southernmost range of distribution for this species.
“To find a species at the extreme of their range and to be so abundant is very unusual,” said Pineda, lead author of the paper.
This aggregation of red crabs had taken place in an oxygen-deficient region along the northwestern flank of the Hannibal Bank Seamount at depths of 355 to 385 metres. “These crabs have been detected before in similar low oxygen conditions," said Pineda, suggesting that such low-oxygen waters might serve as a refuge for them from predators.
Link to study: peerj.com/articles/1770