Tuna crabs, neither tuna nor crabs, swarm near San Diego

When Anna Sagatoff, an underwater cinematographer, goes on her regular night dive off San Diego's La Jolla Shores, she's accustomed to spotting “octopuses, nudibranchs and horn sharks.” But what she saw as it sank in late April was shocking: the seafloor turned red by what she described as “an interwoven carpet of crabs.” The creatures swirl and change in the current, extending “as far as dive lights can illuminate,” she said.

The swarming red crustaceans she and other observers spotted on the San Diego coast are called crabs, but they are actually squat lobsters. The shallow waters surrounding Southern California are not their usual habitat.

These animals usually live in the high seas, around the state of Baja California in Mexico. But this is their second appearance in the region in six years. Some experts say they may have been pushed into San Diego's nearshore canyons by nutrient-dense currents created by El Niño, when warmer oceans release extra heat into the atmosphere, creating shifting currents and fluctuations in air pressure over the tropical Pacific Ocean.

This event may indicate changes in the region's climate. Meanwhile, the cluster of tuna crabs offers scientists and divers like Ms. Sagatoff a close-up of a sea creature that normally appears inside a tuna's stomach.

Some observations took circuitous turns, such as when she began noticing what she called “mass cannibalism” among the red reptiles. While tuna crabs are equipped to eat plankton, they are also opportunistic predators Benthic stage of their life cycle, which can cause them to feed on their own species.

Tuna lobster is also known as red crab, krill lobster and langostella. They are more closely related to hermit crabs than to “true” crabs, although they do exist Evolved Similar features. Its common name derives from its role as a preferred food source for larger 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 point, they would make 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.

On the floor of Scripps Valley, these crabs form writhing mounds thousands of individuals thick. For local predators, this is a welcome bonus. While many bottom-dwelling tuna crabs are consumed, hundreds of thousands of individuals remain uneaten as the novelty of this new food source wears off.

Megan Cimino, an assistant researcher at the Marine Sciences Institute at the University of California, Santa Cruz, said that this gathering and the one that preceded it in 2018 constitute a mystery to science. When the tuna crabs last appeared, her team found that their movement in California was “associated with unusually strong ocean currents coming from Baja,” which sometimes but not always coincide with El Niño.

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

While the relationship between crab populations and El Niño is not entirely clear, “when we think about climate change, the first thing that comes to mind may be rising temperatures, but climate change could lead to more variable ocean conditions,” Dr. Cimino said. . She described tuna crabs as “indicator species” capable of suggesting evidence of large-scale changes in ocean currents and composition, which may have both positive and negative impacts on animals in the region's waters.

Because of the cold water in the Scripps Valley, these crabs will not persist long after they settle in San Diego. This mass death creates stranding events Tuna crabs wash up on beaches in large numbers, turning the sand and surrounding water red. Alternately, the same currents that brought the swarm to San Diego could push them out to sea.

The end of this invasion could help scientists create a world one day Forecasting system For future tuna crab populations. It is not yet possible to determine how long the tuna crabs will remain, or when they will return to California beaches. But with the ocean warming, that may be sooner than anyone expects.