Thousands upon thousands of tiny sea creatures have been washing up and covering the beaches of southern California over the past two weeks. The creatures are tuna crabs, Pleuroncodes planipes, a species that grows to only between 1 to 3 inches long. Normally they live off of the Baja peninsula of Mexico, but warmer waters have driven them farther north.

"Typically such strandings of these species in large numbers are due to warm water intrusions," said Linsey Sala, collection manager for the Pelagic Invertebrates Collection at Scripps Institution of Oceanography, UC San Diego, in a news release. According to the San Diego Tribune, "Scientists are investigating the nature and cause of the massive pool of warm water that developed last year in the Pacific, from Mexico to Canada. The pool helped push sea surface temperatures in San Diego to unusually high levels for part of the winter and spring."

The density of the stranded crabs in some areas is apparent in the image below:

tuna crabs

Photo: Video screengrab, ABC 10 News

It's not unusual for these crabs to wash up during periods of warmer weather; in fact, they are an indicator species of warmer waters. Similar strandings occurred in 2002, an El Niño year, and also in 1997. However, exactly what caused this year's patch of warm water hugging the West Coast isn't known, and researchers are looking into the conditions that have caused it.

Meanwhile, beachgoers are being asked to avoid eating the crabs and leave the feast to the gulls. The crabs feed on phytoplankton that may contain toxins, so eating the crabs may cause illness.

tuna crab

Photo: Scripps Institution of Oceanography, UC San Diego

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Jaymi Heimbuch ( @jaymiheimbuch ) focuses on wildlife conservation and animal news from her home base in San Francisco.