Beachgoers in Northern California have been stunned by the sight of what looks like thousands of disembodied penises washing up on the shoreline.
A recent storm in Drakes Beach, north of San Francisco, uncovered a mass of worms from their burrows deep under the sand.
The creatures, which are known as fat inkeeper worms or Urechis caupo, were soon dubbed "penis fish" due to their phallic characteristics and distinctive colouring.
Biologist Ivan Parr wrote in the journal Bay Nature that the same phenomenon had been reported in other areas on the California coast.
"Yes, the physical design of the fat innkeeper worm has some explaining to do," he wrote.
"[But] whether or not you feel privileged by its presence, Urechis caupo is an almost uniquely California experience, perhaps having the best claim for State Worm."
Long-term storm impact on species unknown
Mr Parr, who measured the worms as "10-inch" (or 25cm) and described them as "pulsing", said they were perfectly-shaped to burrow underground.
They feed by sucking plankton and bacteria into their burrow as sustenance, providing a food source for surrounding clams, shrimp, and crabs.
Seagulls, otters, sharks, and other sea creatures prey on the worm, which has a lifespan of up to 25 years.
Fossil evidence of the creatures dates back 300 million years.
This particular species is found only in Northern California, although a closely-related marine worm is considered a delicacy in Korea and China, and is used elsewhere as fishing bait.
Most of the creatures in this beaching event were dead, prompting Mr Parr to wonder whether further dislocations would have a long-term impact on the species.
"So far as I know, there are no programs explicitly taking stock of the worms," he said.
