The crabs are likely showing up further and further north because the water is unusually warm.
"They have this ability to transition from the seafloor through the water column," Sala said. "They're subject to current and internal waves and tides, so they can be pushed along with different water masses. Typically, when we do see larger numbers of tuna crab, it's during warm water intrusions."
Fisherman first spotted the little red crabs in Southern Californian waters last year, Sala said, and reports came in earlier this year of sporadic strandings on Catalina Island and elsewhere. Starting in mid-May, thousands washed up on San Diego beaches.
Donna Kalez, general manager of Dana Wharf Sportfishing, told the Orange County Register that boat captains spotted the creatures for weeks in the water. Now that they've washed ashore, “everyone is taking selfies with the red crab,” Kalez told the paper.
“They are all still alive. They are in the surfline and swimming up,” Kalez told the Register. “Once they get this close to shore, they can’t go anywhere, so they just wash in. They aren’t strong enough to swim out.”
The warmer water could be due to a combination of factors, Sala said, including an El Nino, a large-scale oceanic climate phenomena. A giant patch of warm water, nicknamed "The Blob" by researchers, has formed in the Gulf of Alaska and off California's coast as weather patterns have failed to suck the usual amount of heat from the ocean.
The red tuna crabs aren't the only odd creatures to wash up by the thousands in California because of warmer waters. "By-the-wind sailors," small bright blue jellyfish, covered California beaches in the spring, as warmer water brought them down from the north. Such jellyfish strandings happen every three to six years, Monterey Peninsula College marine biologist Kevin Raskoff told National Geographic.
Unlike those blue jellyfish, the red tuna crabs are getting pushed up from the south, and there are reports of them making it as far north as Monterey Bay back in the 1800s, Sala said. Other large stranding events took place as recently as 2002 and the late 1990s.
As for whether the tuna crabs will cover beaches even further up the coast this year, Sala said it's difficult to predict. "The ocean is mysterious," she said.