Thousands of 'penis fish' washed up on a Point Reyes beach. Yes, really.

Southern (California) Sea Otter eating Fat Innkeeper Worm close-up in Elkhorn Slough, California. Southern (California) Sea Otter eating Fat Innkeeper Worm close-up in Elkhorn Slough, California. Photo: Hal Beral/Getty Images Photo: Hal Beral/Getty Images Image 1 of / 8 Caption Close Thousands of 'penis fish' washed up on a Point Reyes beach. Yes, really. 1 / 8 Back to Gallery

You may have never heard of a fat innkeeper worm, also commonly known as a penis fish, but from this moment on, you'll never be able to unsee them.

I had also never heard of such creatures, until Bay Nature reported on a mass stranding of the 10-inch worms on Drakes Beach in Point Reyes Friday. Naturalist Ivan Parr attributes the phenomenon to the aftermath of a strong storm.

If you want to get scientific about it, these thick, pink worms are called Urechis caupo. They live in shallow water along the West Coast, between Oregon and Baja California. They're most commonly seen here in Northern California, Parr writes.

This may just be the weirdest thing you've seen today!



Thousands of these marine worms, called fat innkeeper worms—or "penis fish"—washed up on Drake's Beach after a recent storm. But why? https://t.co/MwY6xkN3kb pic.twitter.com/vGMpSvGoAT — Bay Nature magazine (@BayNature) December 11, 2019

The worms aren't shaped so — err, distinctly? — for just any reason. It's for survival. They dig U-shaped burrows on beaches and mudflats and use the tunnels to help catch food. Parr explains:

When the tide is in, the worm slides up to the chimney of its burrow and exudes a sticky mucous net from a ring of glands. ... The worm continues to secrete as it slips lower into the burrow, generating a slime-net that stretches from the chimney to the worm’s mouth. Using contractions (peristalsis) to pump water through its burrow, the worm sucks plankton, bacteria, and other bits into this net. When, like any vacuum, the net gets clogged, the worm slurps it all back into its mouth, taking in the particles it wants to eat and discarding the rest into the tunnel.

Their predators include otters (as seen in the gallery above), as well sharks, rays and gulls. They are also eaten raw or stir-fried in Korea and China.

Read more about the lives of penis fish in the wild by reading Bay Nature's full story.

Alix Martichoux is an SFGATE digital editor. Read her latest stories and send her news tips at alix.martichoux@sfgate.com.