When Anna Sagatov, an underwater cinematographer, goes on her usual night dives off La Jolla Shores in San Diego, she’s used to spotting the “occasional octopus, nudibranch and horn shark.” But what she witnessed on a late April plunge was shocking: a seafloor turned red by what she described as an “overlapping carpet of crabs.” Swirling 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 been spotting on San Diego’s coast are called tuna crabs, but they are actually squat lobsters. And the shallows around Southern California are not their usual home.

The animals typically live on the high seas, 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 to San Diego’s near-shore canyons by nutrient-dense currents set off by El Niño, when warmer oceans release additional heat into the atmosphere, creating variable currents and air pressure fluctuations over the equatorial Pacific.

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

Some of the observations took twisted turns, like when she began to notice what she called “mass cannibalism” among the red crawlers. While tuna crabs are equipped to eat plankton, they are also opportunistic predators in the benthic stage of their life cycle, which can cause them to feed on their own species.

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