Fun facts about the 'penis fish'

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Kate Montana


It is called a fat innkeeper worm, although in social networks it is known as the 'penis fish', due to its peculiar elongated shape and bulging head.

This type of worm is arousing great interest, especially after last December 6, where thousands of specimens of this 'penis fish' were stranded on a beach in Costa Rica: an incident that has been unpleasant for many.
What do we know about this marine worm? Where does it live and how does it behave?

Image: holding a specimen of 'penis fish' in Bodega Bay in June 2019./Kate Montana


The fat innkeeper worm otherwise known as Urechis caupo lives in large U-shaped burrows in sand, mud and other soft sediments. Their underground homes date back 300 million years. In addition to being old, they are quite long-lived, as they can live up to 25 years.This particular type of innkeeper worm is perfectly designed to slide under wetlands. 


The innkeeper worm has an interesting way of feeding itself: it produces a mucous net that captures small pieces of food that float in the water. After a while, it reabsorbs this net again, also swallowing the food that has been trapped in it.


Who feeds on this repulsive sea worm? It's a delight for the bat ray, leopard sharks, and even for some marine mammals such as otters, which absorb them and take them out of their burrows. Like a snack!

One of the problems with the innkeeper worm is that the food, shelter and running water pumped by it attract guests to its burrow, such as the goby fish (Pholidichthys leucotaenia), the scaly worm (Polynoidae), or some types of crab, with which it is forced to share space.


Urechis caupo produces offsprings through sexual reproduction: males and females respectively deposit sperm and egg in the water, where fertilization takes place. Once the egg is fertilized, it grows into a larva that swims freely until it begins to build its own U-shaped burrow.

The 'penis fish' invasion

As we explained at the beginning of the article, thousands of 'penis fish' were found stranded on a Costa Rican beach on December 6; but that was not the only case. Scenes similar to the image above have been seen in other areas, such as Pajaro Dunes, Moss Landing, Bodega Bay and Princeton Harbour, in recent years.

What is causing this flood of worms stranded on the beaches? As photographer and biologist Ivan Parr reports on, it's very possible that these are the consequences of strong storms, such as those that occurred during the El Niño years: "These storms are perfectly capable of besieging the intertidal zone, breaking sediments and leaving their contents stranded on the coast," he explains.

Where can you find the 'penis fish'?

The U. caupo is endemic to North America. It usually populates the area from southern Oregon to Baja California, and most sightings have occurred between Bodega Bay in California and Monterey in Mexico.

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