MARIN COUNTY, CA — Thousands of pulsing pink worms were recently spotted on Drakes Beach in Marin County. The mass stranding was reported on Dec. 6, after a storm in the Bay Area.

The fat innkeeper worm, also known as "penis fish," is a type of spoonworm that is found from southern Oregon to Baja California, with most sightings reported between Bodega Bay and Monterey, according to biologist Ivan Parr, who wrote about the phenomenon in Bay Nature magazine.

The worms build U-shaped burrows in the muddy sand of low-zone mudflats, according to the Monterey Bay Aquarium. Food, shelter and water pumped by the worms attract other creatures such as the arrow goby, pea crabs and scale worms.