Tuna crabs swarm near San Diego, neither tuna nor crabs

When Anna Sagatov, an underwater cameraman, goes on her usual night dives off La Jolla Shores in San Diego, she’s used to spotting “the occasional octopus, nudibranch and horned shark.” But what she witnessed in late April was shocking: the sea floor turned red with what she described as an “overlapping carpet of crabs.” The creatures circled and shifted in the current, stretching “as far as my diving lights could illuminate,” she said.

The swarming red crustaceans she and other observers spotted off the coast of San Diego are called tuna crabs, but they’re actually stubby 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 this area. Some experts say they may have been pushed into the canyons near San Diego’s coast by nutrient currents triggered by El Niño, when warmer oceans release additional heat into the atmosphere, creating variable currents and air pressure fluctuations over the equatorial Pacific.

This event could signal climate change in the region. At the same time, the aggregation of tuna crabs offers scientists and divers like Ms. Sagatov a close-up view of a sea creature that usually appears in a tuna’s stomach.

Some observations changed, like when she began noticing what she called “mass cannibalism” among red creepers. While 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 prey 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 evolved similar traits. Their common name comes from their role as a favorite 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 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 vulnerable to winds, tides and currents that may have pushed many animals north.

At the bottom of Scripps Canyon, these crabs form squirming 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, an assistant research scientist at the Institute of Marine Sciences at the University of California, Santa Cruz. The last time the tuna crabs showed up, her team found that their movement in California “was associated with unusually strong ocean currents coming from Baja,” sometimes but not always coinciding with El Niño.

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

Although the link between tuna crab aggregations and El Niño is not completely clear, “when we think about climate change, the first thing that comes to mind might be warming, but climate change can lead to more variable ocean conditions,” said Dr. Cimino. She called tuna crabs an “indicator species” capable of suggesting evidence of large-scale changes in ocean currents and composition that can have both positive and negative effects on animals in the region’s waters.

Because of the cold water in Scripps Canyon, these crabs do not last long after settling in San Diego. This mass die-off creates stranding events where tuna crabs wash up on beaches in droves, turning the sand and surrounding waters red. The same currents that brought the swarm to San Diego could blow them back out to sea.

The end of 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 shores. But in a warming ocean, that could be sooner than anyone expects.