Thousands of 10-inch “penis fish” have mysteriously washed up on a beach in California following winter storms.

The wriggling animals are a species of marine spoon worm commonly known as the “fat innkeeper worm” or sometimes the “penis fish”.

The critters were spotted flopped across the sands of Drakes Beach in California, about 50km (31m) north from San Francisco.

The nightmarish scene was first reported by Bay Nature, which wrote that the creatures are quite common along the west coast in North America.

However because the animals normally live in U-shaped burrows under the sand, people visiting the beach are rarely aware of their existence.

The so-called 'penis fish'. Pic: Kate Montana
Image: The so-called ‘penis fish’. Pic: Kate Montana

Winter storms which battered northern California recently exposed a number of these worms after the powerful waves washed away several feet of sand.

Officially known as Urechis caupo, the animals are “perfectly shaped” for their lives underground according to biologist Ivan Parr, even if they appear hideous to us.

They can live for up to 25 years and are even eaten as food in countries such as Korea and China.