A wild storm has caused creatures dubbed "penis-fish" to cover a beach in California.

"Fat innkeeper worm" or Urechis caupo appeared on Drakes Beach, California last week when extremely bad weather forced them out of their underwater burrows, according to a marine life expert.

Writing for Bay Nature, Ivan Parr said the strong storms may have forced them out, leaving them exposed to predators.

"The same phenomenon has been reported over the years at Pajaro Dunes, Moss Landing, Bodega Bay and Princeton Harbor," he said.

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"I've heard my share of imaginative theories from beachcombers, such as flotsam of a wrecked bratwurst freighter.

"In truth, these are living denizens of our beaches rudely, yet also mercifully, mostly called 'fat innkeeper worms'."

Parr acknowledged that phallic shape of the fish "has some explaining to do", but explained that they are perfectly shaped for a life spent underground.

"Within a beach or mudflat, it digs a U-shaped burrow extending a few feet in length but no wider than the worm itself."

Parr said this is why is known as an "innkeeper" – although those who have come across the creatures have dubbed them "penis-fish".

Other marine life benefits from the worm's burrowing skills including clams, other worms, crabs, shrimp and even a fish called the arrow goby.

Holding the Fat Innkeeper Worm (Urechis Caupo) can sometimes require two hands. Photo / Kate Montana/Creative Commons 4.0
Holding the Fat Innkeeper Worm (Urechis Caupo) can sometimes require two hands. Photo / Kate Montana/Creative Commons 4.0

The toothless worm uses its spatula-shaped limb for feeding and swimming in its seaside habitat, Parr explained.

Fossil evidence reveals that the creatures' U-shaped burrows have been around for more than 300 million years.

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Some can live for 25 years, but they have many threats including humans, otters, flounders, sharks, rays and seagulls.

These worms are a delicacy in East Asia, where people actually enjoy eating a penis-fish or two with a dash of salt.