Tuna crabs, neither tuna nor crabs, are swarming near San Diego

SAN DIEGO — When Anna Sagatov, an underwater cinematographer, goes on her usual night dives off the coast of La Jolla in San Diego, she is accustomed to spotting “occasionally octopuses, nudibranchs and horn sharks.” But what she witnessed on a dive in late April was shocking: the sea floor turned red from what she described as an “overlapping carpet of crabs.” Swirling and shifting in the current, the creatures stretched “as far as my dive lights could shine,” she said.

The swarms of red crustaceans she and other observers have seen off the coast of San Diego are called tuna crabs, but they are actually stocky lobsters. And the shallow waters around Southern California are not its usual home.

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The animals normally live in the high seas, near Baja California, Mexico. But this is their second appearance in six years in the area. Some experts say they may have been pushed into the canyons off the coast of San Diego by nutrient-dense currents triggered by El Niño, when warmer oceans release additional heat into the atmosphere, creating variable currents and atmospheric pressure fluctuations above. the equatorial Pacific.

The event may signal changes in the region’s climate. At the same time, the aggregation of tuna crabs gives scientists and divers like Sagatov a close-up of a sea creature that usually appears inside a tuna’s stomach.

Some of the observations took a twisted turn, like when she began to notice what she called “mass cannibalism” among the red creepers. Although tuna crabs are equipped to eat plankton, they are also opportunistic predators in the benthic phase of their life cycle, which can cause them to feed on their own species.

Tuna crabs are also known as red crabs, lobster krill, and langostilla. They are more closely related to hermit crabs than to “true” crabs, although they have developed similar characteristics. Its common name derives from its role as a preferred food source for large species such as tuna during the period of their life cycle when they live in the open ocean.

In the final stage of their life cycle, crabs descend from the open ocean and live just above the continental crust as bottom dwellers. At this stage, they will make vertical journeys through the water column in search of plankton, making them susceptible to winds, tides and currents, which may have pushed many of the animals north.

At the bottom of Scripps Canyon, these crabs form writhing piles thousands of individuals thick. For local predators, this is a welcome reward. Although many bottom-dwelling tuna crabs are consumed, hundreds of thousands of individuals remain uneaten as the novelty of this new food source wears off.

This aggregation and the one that preceded it in 2018 are mysteries to science, said Megan Cimino, an assistant researcher at the Marine Science Institute at the University of California, Santa Cruz. When tuna crabs last appeared, her team discovered that their movement in California was “related to unusually strong ocean currents originating in Baja,” sometimes, but not always, coinciding with El Niño.

She said the new event “signals that something different is happening in the ocean.”

While the link between tuna crab aggregations and El Nino isn’t exactly clear, “when we think about climate change, the first thing that comes to mind might be rising temperatures, but climate change can also result in more variable ocean conditions.” , Cimino said. She called tuna crabs an “indicator species” capable of suggesting evidence of large-scale changes in ocean currents and composition that could have positive and negative effects on animals in the region’s waters.

Because of the cold water in Scripps Canyon, these crabs won’t last long after establishing themselves in San Diego. This mass die-off creates stranding events in which tuna crabs wash up on beaches en masse, turning the sand and surrounding waters red. Alternatively, the same currents that brought the swarm to San Diego could expel them back out to sea.

Ending this invasion could help scientists one day create a prediction system for future tuna crab aggregations. It is not yet possible to say exactly how long the tuna crabs will remain or when they will return to California’s shores. But in a warming ocean, this could happen sooner than expected.

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