Thousands of marine worms dubbed "penis fish" for their shape and color appeared this week on a California beach after a strong winter storm exposed them.

The creatures - more commonly known as fat innkeeper worms - covered Drakes Beach, 50 miles (80.5 kilometers) north of San Francisco. 

Despite their eye-catching appearance, the pulsating worms which reach around 10 inches are adept at digging U-shaped burrows on beaches and marshes, using the tunnels to catch food.

Found almost exclusively in California, they are themselves considered a culinary delicacy in South Korea, with reputed aphrodisiac effects.

"Yes, the physical design of the fat innkeeper worm has some explaining to do," wrote biologist Ivan Parr. "But the fat innkeeper is perfectly shaped for a life spent underground."

"They are preyed on by otters, flounders, sharks, rays, gulls, and humans," the latter of whom eat them as sashimi, fried or grilled.

AFP