"When we dove down in the submarine, we noticed the water became murkier as we got closer to the bottom," project leader Jesús Pineda, a biologist at Woods Hole Oceanographic Institution, said in a press release. "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."
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"As we slowly moved down to the bottom of the seafloor, all of the sudden we saw these things," he continued. "At first, we thought they were biogenic rocks or structures. Once we saw them moving - swarming like insects - we couldn't believe it."
Genetic testing identified the crab species, which is known to be abundant off the coast of Baja California. Apparently the crustaceans have a secret life further south too. Information about the discovery is published in the journal PeerJ.
Pineda and his team encountered the swarm while studying the Hannibal Bank Seamount off the Panamanian coast. Seamounts are underwater mountains that usually rise at least 3,280 feet above the ocean floor.
The large aggregation of red crabs was observed along the Northwest flank of the seamount in acidic water with very low levels of oxygen.
"These crabs have been detected before in similar low oxygen conditions," Pineda explained. "It could be that these low oxygen waters provide a refuge for this species from predators."
The frenzy to stay in this crowded spot makes sense when considering the nickname for these crustaceans: tuna crabs. Yellowfin tuna love to eat them, as do many birds, marine mammals and other fish. Humans can eat them too, but do so at risk. That is because the crab's food source consists of phytoplankton that can contain high levels of toxins.
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Despite their efforts to stay alive, the crabs might have perished not long after the video was recorded. Just a few months after the expedition, thousands of red crabs washed ashore onto Southern California beaches during a massive stranding associated with El Niño warming conditions.
DNA testing was employed once again. It revealed that the Panamanian and the Californian crabs are, in fact, the same species.
Pineda and his team plan to return to the Hannibal Seamount to study the region more. The researchers indicated that the deep water, low oxygen and acidic areas of the seamount could reveal how marine life communities might look in the future as the ocean responds to climate change.
Photo by Jesus Pineda, Yogesh Girdhar, Woods Hole Oceanographic Institution