When Anna Sagatov, an underwater cinematographer, goes on her regular night dives off San Diego’s La Jolla Shores, she’s used to spotting “the occasional octopus, nudibranch and shark.” But what she witnessed on her dive in late April was shocking: the sea floor turned red with what she described as a “folding carpet of crustaceans.” Swirling and moving in the current, the creatures stretched “as far as my diving lights could illuminate,” she said.
The swarming red crabs she and other observers spotted off the San Diego coast are called tuna crabs, but they’re actually chunky lobsters. And the shoals around Southern California are not their usual home.
The animals usually live in the open sea, around Baja California, Mexico. But this is their second appearance in six years in that area. Some experts say they may have been pushed to San Diego’s coastal canyons by nutrient-rich currents driven by El Niño, when warmer oceans release extra heat into the atmosphere, creating changing currents and air pressure fluctuations over the equatorial Pacific.
The event could signal changes in the region’s climate. At the same time, the gathering of tuna crabs offers scientists and divers like Ms. Sagat a close-up view of the sea creature that usually appears in the stomach of tuna.
Some observations took turns, like when she began noticing what she called “mass cannibalism” among the red creepers. Although tuna crabs are equipped to feed on plankton, they are also opportunistic predators in the benthic phase of their life cycle, allowing them to feed on their own species.
Tuna crabs are also known as red crabs, lobster krill and langostilla. They are more similar to hermit crabs than “true” crabs, although they have evolved similar traits. Their common name derives from their 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, crustaceans descend from the open ocean and live just above the continental crust as bottom dwellers. At this stage, they will embark on vertical journeys through the water column in search of plankton, making them vulnerable to winds, tides and currents, which may have pushed many animals northward.
At the bottom of Scripps Canyon, these crabs form twisting piles, thousands of individuals thick. For local predators, this is a welcome reward. While many bottom-dwelling tuna crabs are consumed, hundreds of thousands remain uneaten when the novelty of this new food source wears off.
This aggregation and the one that preceded it in 2018 are a mystery to science, said Megan Cimino, a research assistant at the Institute of Marine Sciences at the University of California, Santa Cruz. When tuna crabs 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 crab aggregations and El Niño is not entirely clear, “when we think of climate change, the first thing that comes to mind might be warming temperatures, but climate change can also result in more variable ocean conditions,” said Dr. Cimino. She called tuna crabs an “indicator species” that can suggest evidence of major changes in ocean currents and composition that can have both positive and negative effects on animals in the area’s waters.
Because of the cold water in Scripps Canyon, these crabs won’t last long after settling in San Diego. This mass die-off creates stranding events where tuna crabs wash up on beaches in droves, staining the sand and surrounding waters red. On the other hand, the same currents that brought the swarm to San Diego could push them back out to sea.
The end of this invasion could help scientists one day create a prediction system for future gatherings of tuna crabs. 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, that could be sooner than anyone expects.