A team of marine scientists has captured a rare video of thousands of pelagic red crabs (Pleuroncodes planipes) swarming in hypoxic (low-oxygen) waters at Hannibal Bank, a seamount and an ecological hotspot in the coastal eastern tropical Pacific Ocean off Panama.

The large aggregation of swarming red crabs was observed along the Northwest flank of the Hannibal Bank Seamount at depths of 1,164 to 1,263 feet (355 to 385 m) in an area of hypoxic water.

“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,” explained team leader Dr. Jesús Pineda of Woods Hole Oceanographic Institution (WHOI).

“The encounter was unexpected and mesmerizing,” he said.

Dr. Pineda and his colleagues were working aboard the M/V Alucia. They captured the video of the crab swarm during a dive in the manned submersible Deep Rover 2 in 2015.

“When we dove down in the submarine, we noticed the water became murkier as we got closer to the bottom,” Dr. Pineda said.

“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.

“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,” he said.

“Once we saw them moving — swarming like insects — we couldn’t believe it.”

According to the team, this documentation represents a new southernmost range of distribution for the pelagic red crab.

“No one had ever found this species that far south,” Dr. Pineda said.

“To find a species at the extreme of their range and to be so abundant is very unusual.”

The researchers describe their findings in a paper published online today in the journal PeerJ.

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Pineda J. et al. 2016. A crab swarm at an ecological hotspot: patchiness and population density from AUV observations at a coastal, tropical seamount. PeerJ 4: e1770; doi: 10.7717/peerj.1770