Throngs of red crabs have been spotted blanketing the ocean floor off San Diego after ocean currents pushed them unusually close to shore.
When Anna Sagatov, an underwater filmmaker, was diving at night off La Jolla Shores Beach in San Diego in late April, she spotted an unexpected sight: the ocean floor was turning red with a horde of crabs. The swarm of crabs stretched as far as her dive light could reach, swirling and shifting with the currents, Sagatov said. The crimson horde spotted by Sagatov and others off San Diego is the pelagic red crab, according to the In-Fisherman website.
The shallow waters around Southern California aren't typically home to the pelagic red crab, which is more commonly found around Baja California, Mexico. But this marks the second time in six years that they've turned up in the region. Some experts speculate that the crabs may have been transported to the submarine canyon off San Diego by a nutrient-rich current spawned by El Niño, a climate pattern in which the ocean releases more heat into the atmosphere, causing dramatic shifts in ocean currents and air pressure in the equatorial Pacific Ocean. Such an event could signal changing climate patterns in the region. In the meantime, the crab congregation offers scientists and divers like Sagatov a rare glimpse of a marine creature that typically ends up in the bellies of tuna.
Sagatov witnessed the crabs engaging in cannibalistic behavior during her dives. Although pelagic red crabs are primarily filter feeders, they're also opportunistic predators late in their life cycle, and will readily eat their own kind.
Pelagic red crabs are more closely related to hermit crabs than true crabs, though they've evolved similar characteristics. They get their name from the fact that they're a favorite food of larger fish, including tuna. Late in their life cycle, pelagic red crabs live just above the continental shelf, becoming bottom feeders. They often drift through the water column in search of plankton during this stage, making them susceptible to being transported northward by winds, tides and currents. At the bottom of the Scripps Submarine Canyon, the crabs form massive piles, sometimes thousands deep. For local predators, it's an all-you-can-eat buffet.
The presence of the crabs this year and in 2018 is a bit of a scientific mystery, said Megan Cimino, an assistant research scientist at the University of California, Santa Cruz's Institute of Marine Sciences. When the crabs showed up the last time, her team found that their movement into California waters correlated with an unusually strong ocean current originating from Baja, but not necessarily tied to El Niño. This new event, she said, suggests "there's something different going on in the ocean." The exact relationship between the crab swarms and El Niño is unclear, but climate change could be leading to more variable ocean conditions.
Given the cold water temperatures in the Scripps canyon, the crabs aren't likely to stick around San Diego for long. Their mass die-off will either lead to a "red tide" event, in which their carcasses wash ashore and turn the beach and surrounding ocean water a reddish hue. Or the same currents that brought them in could carry the crabs back out to sea.
Summary
Swarms of pelagic red crabs have been spotted off the coast of San Diego, California, twice in the past six years. The crabs are typically found in warmer waters around Baja California, Mexico, and their presence in Southern California may be a sign of changing climate patterns. The crabs, which feed on plankton and other small organisms, are also opportunistic predators and will cannibalize their own kind. While their presence is a mystery to scientists, the crabs' unusual behavior offers researchers a rare glimpse into the life of a creature that typically ends up as food for larger fish.