SAN DIEGO — When Anna Sagatov, an underwater cinematographer, goes on her regular night dives off San Diego’s La Jolla Shores, she’s used to spotting “occasional octopuses, nudibranchs and horn sharks.” But what she witnessed on a dive in late April was shocking: the sea floor turned red with what she described as an “overlapping carpet of crabs.” Spinning and shifting in the current, the creatures stretched “as far as my dive lights could illuminate,” she said.
The swarming red crustaceans she and other observers have spotted off the coast of San Diego are called tuna crabs, but they’re actually lobsters. And the shallows around Southern California are not their usual home.
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The animals usually live in the open sea, around Baja California in Mexico. But this is their second appearance in six years in the area. Some experts say they may have been pushed toward San Diego’s coastal canyons by nutrient-rich currents triggered by El Niño, when warmer oceans release extra heat into the atmosphere, creating fluctuating currents and fluctuations in atmospheric pressure over the equator Pacific.
The event may signal changes in the region’s climate. At the same time, the clustering of tuna crabs offers scientists and divers like Sagatov a close-up view of a sea creature that usually appears in the stomach of a tuna fish.
Some of the observations took twisted turns, such as when she began to notice what she called “mass cannibalism” among the red creepers. Although tuna are equipped to eat plankton, they are also opportunistic predators in the benthic stage of their life cycle, which can lead them to feed on their own species.
Tuna crabs are also known as red crabs, lobster krill and langoustine. They are more closely related to hermit crabs than to “true” crabs, although they have evolved similar characteristics. Their common name derives from their role as a preferred food source for large species such as tuna during their life cycle when they live in the open ocean.
In the final phase 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 undertake vertical journeys through the water column in search of plankton, making them susceptible to winds, tides and currents that 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 demersal tuna are consumed, hundreds of thousands of individuals go uneaten as the novelty of this new food source wears off.
That aggregation and what preceded it in 2018 are mysteries to science, said Megan Cimino, a research assistant at the Institute of Marine Sciences at the University of California, Santa Cruz. When the tuna last appeared, her team found that their movement in California was “associated with 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.”
Although the link between tuna schools and El Niño is not entirely clear, “when we think of climate change, the first thing that comes to mind may be warming temperatures, but climate change can also lead to more variable ocean conditions,” Cimino said. She called the tuna an “indicator species” capable of offering evidence of large-scale changes in ocean currents and composition that can have positive and negative effects on animals in the area’s waters.
Because of the cold water in Scripps Canyon, these crabs will not last long once they are established in San Diego. This mass die-off creates stranding events where tuna wash up on beaches in droves, turning the sand and surrounding waters red. Alternatively, the same currents that brought the swarm to San Diego could have kicked them back out to sea.
The end of this invasion could help scientists one day create a system for predicting future crayfish aggregations. There’s no telling yet exactly how long the tuna will stay or when they’ll return to California shores. But in a warming ocean, that could happen sooner than anyone expects.
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