Scientists studying biodiversity off the coast of Panama were treated to a red crab migration that looks more like a swarm of insects than anything you would expect to see in the ocean.
Insect swarm turns out to be crabs
During their final dive last summer in the manned submersible Deep Rover 2, scientists studying at the Hannibal Bank Seamount off the coast of Panama, were treated to a red crab swarm in low-oxygen waters above the sea floor.
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Jesus Pineda, a biologist at Woods Hole Oceanographic Institution (WHOI), along with his colleagues from Point Loma Nazarene University, San Francisco Estuary Institute, and Smithsonian Tropical Research Institute recounted their story in a paper published April 12, 2016, in the journal PeerJ. The swarm was discovered on the last dive of a month-long study of the Seamoun last April.
“When we dove down in the submarine, we noticed the water became murkier as we got closer to the bottom,” said Pineda, 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.”
“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.”
These crabs really had no business being there and the find came as a proper shock to those present on the dive. DNA testing and sequencing determined that the crabs were Pleuroncodes planipes, which if the team were in a submersible off the coast of Baja would not have caused the surprise the researchers experienced. In stumbling across the swarm the researchers essentially added to the southern range of this species of decapod crustaceans.
Red crab migration – What are you doing this far south and this deep?
“No one had ever found this species that far south,” Pineda said. “To find a species at the extreme of their range and to be so abundant is very unusual.”
It’s likely that the low-oxygen waters (hypoxic) where the swarm was found (about 1,200 feet deep) provide the crabs with protection from predators. The crabs they found are also known as tuna crabs, not because they love to eat tuna, but rather because yellowfin tuna seem to find this specific crab a bit of a delicacy.
“These crabs have been detected before in similar low oxygen conditions,” Pineda said. “It could be that these low oxygen waters provide a refuge for this species from predators.”
Last summer, owing to El Niño conditions, thousands of the same species of crab from the Hannibal expedition found themselves stranded on a beach in San Diego. Well within the the habitat range of the crab, but a long ways from the Seamount off the coast of Panama.
Seamounts are essentially underwater mountains that are known for their biodiversity and that’s what the team was doing there. Somewhat surprisingly, scientists aren’t quite certain why seamounts are more often than not, ecological “hotspots” that host a magnificent diversity of marine life.
Onboard the submersible during the swarm encounter were researchers from all walks of life including: physical oceanographers, biologists, software scientists, and robotics engineers. In addition to the manned submersible, the team had two unmanned submersibles at their disposal to collect samples from the seamount.
“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 coauthor of the paper. “The fact that we could combine the use of submersibles to explore, observe and sample, and then use an AUV to follow-up those observations of the crab aggregation and get quantitative data is really powerful.”