It’s a deep-sea crab invasion. A unique video has captured a surprise swarm of red crabs on the sea floor at the Hannibal Bank seamount off the Pacific coast of Panama.

Jesús Pineda from the Woods Hole Oceanographic Institution and colleagues were exploring the diverse ecosystem at the underwater mountains when a submersible spotted thousands of red crabs in cloudy waters at a depth of 355 metres. The video, taken last April but only just released, shows the crabs all shuffling in the same direction.

“It was quite unexpected,” says Pineda, “I didn’t know that these swarms existed.”

The crabs were later identified as Pleuroncodes planipes through DNA tests of a few specimens collected from the swarm. The species is abundant off the coast of Baja California and had washed up by the thousands on beaches further north in San Diego around the same time, due to waters warmed by El Niño.


“We have known that populations of these crabs can explode following periods of warmer temperatures,” says Nathaniel Evans of the Florida Museum of Natural History in Gainesville. “But we hadn’t observed what is going on in deeper habitats.”

The crabs are known to retreat to deep water low in oxygen, similar to the conditions at Hannibal, possibly to avoid predators. But they have never been documented this far south.

Going mountaineering – under water Jesús Pineda, Yogesh Girdhar, Woods Hole Oceanographic Institution

One theory is that so far we’ve simply not noticed them. A similar species, Pleuroncodes monodon, lives near the Pacific coast of Central and South America and is hard to distinguish visually, so Pleuroncodes planipes could have been misidentified.

The reason for the swarming is still a mystery. The animals are known to move up and down a water column, often settling at the bottom during the day, but they haven’t been observed in such a large group.

Other crustaceans, such as king crabs, tanner crabs and spiny lobsters, aggregate when feeding or during mating season or migration.

Pineda doesn’t think any of these reasons explain the crab swarm, which was tightly packed with all individuals moving in the same direction.

Where are they going? Jesús Pineda, Yogesh Girdhar, Woods Hole Oceanographic Institution

“Insect swarms tend to head in the same direction but we don’t know where the red crabs were heading or why they were moving,” says Pineda.

The landscape where they were found is featureless, with uniform environmental factors like temperature and oxygen that might otherwise drive the animals to relocate.

The swarm, is denser in the middle with fewer individual towards the edges. “This type of pattern is very rare for bottom organisms,” Says Pineda.

The team plans to return to Hannibal seamount to investigate further. They also plan to look at how it might fit in with the rest of the ecosystem.

“We hope to test hypotheses about the role these crabs play in the productivity of the seamount,” says Pineda. “Large fish are known to aggregate at Hannibal and they may be feeding on the crabs.”

Journal reference: PeerJ, DOI: 10.7717/peerj.1770

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