Unexplained swarming of thousands of red tuna crabs off Panama, well south of their normal range

Researchers spotted thousands of tuna crabs swarming off the coast of Panama, well south of their normal range, while surveying the ocean floor in a manned submersible.

The scientists were able to capture the phenomenon on video but are unable to explain it.

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The scientists explain:

When we dove down in the submarine, we noticed the water became murkier as we got closer to the bottom. 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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Vimeo Video

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.

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

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The spotted crabs are also known as pelagic red crabs or tuna crabs, and are common along the coast of Baja California. The species has never been observed so far south.

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The encounter occurred in April of 2015. Just a couple months later, thousands of tuna crabs washed ashore in Orange County, California as well as along baja California.

This was most probably related to the hot waters El Nino that was found responsible for other strange animal appearances along the California coast over the last 18 months.

A crab swarm at an ecological hotspot: patchiness and population density from AUV observations at a coastal, tropical seamount

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