When Anna Sagatov, an underwater cinematographer, goes on her usual evening dive at La Jolla Shores in San Diego, she’s used to seeing “the occasional octopus, nudibranch and horn shark.” But what he witnessed in late April was startling: a seafloor turned red by what he described as a “layered carpet of crabs.” Twisting and turning in the current, the creatures stretched “as far as my dive lights could shine,” he said.
The swarming red crustaceans he and other observers have seen off the coast of San Diego are called tuna crabs, but they are actually squat lobsters. And the shallows around Southern California are not their usual home.
The animals usually live in the high seas, around Baja California in Mexico. But this is their second appearance in six years at the venue. Some experts say they may have been pushed near San Diego’s coast by nutrient-dense currents caused by El Niño, when warmer oceans release additional heat into the atmosphere, creating choppy waves. and air pressure fluctuations over the equatorial Pacific.
The event may indicate changes in the region’s climate. At the same time, the aggregation of tuna crabs offers scientists and divers like Ms. Sagatov of a close-up of a sea creature that usually appears inside a tuna’s stomach.
Some of the observations were meandering, like when he started noticing what he called “mass cannibalism” among red crawlers. While tuna crabs are equipped to eat plankton, they are also opportunistic predators the benthic stage of their life cycle, which may cause them to eat their own species.
Tuna crab is also known as red crab, lobster krill and langosilla. They are more closely related to hermit crabs than to “true” crabs, even if they exist develop similar features. Their common name comes from their role as a favored food source for large species such as tuna during 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 settle 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 wind, tides and currents, which may have pushed many of the animals north.
On the floor of Scripps Canyon, these crabs form piles, thousands of individuals thick. For local predators, this is a welcome bounty. Although many bottom-dwelling tuna crabs are depleted, hundreds of thousands of individuals remain inedible when the novelty of a new food source wears off.
This merger and the one before it in 2018 are mysteries to science, said Megan Cimino, an assistant researcher at the Institute of Marine Sciences at the University of California, Santa Cruz. When the tuna crabs last appeared, his team found that their movement in California was “correlated with unusually strong ocean currents coming off Baja,” sometimes but not always coinciding with El Niño.
He said the new event “signals something strange is happening in the ocean.”
Although the link between tuna crab aggregations and El Niño is not exactly clear, “when we think about climate change, the first thing that comes to mind may be warming temperatures, but the climate change may result in more changes in ocean conditions” as well, said Dr. Cimino. He called the tuna crab an “indicator species” that could suggest evidence of large-scale changes in ocean currents and composition that could have both positive and negative effects on local aquatic life.
Because of the cold water in Scripps Canyon, these crabs do not last long after settling in San Diego. This mass dying creates stranding events where tuna crabs wash up on the beaches in droves, turning the sand and the surrounding water red. Alternatively, the same currents that brought the swarm to San Diego could have kicked them back out to sea.
Ending this invasion could help scientists one day create a forecasting system for future aggregation of tuna crab. There’s no telling how long the tuna crabs will last, or when they’ll return to the California coast. But with a warming ocean, it may be sooner than anyone expects.
