Invertebrates
Innkeeper Worm
Urechis caupo · also called fat innkeeper worm, penis fish
In 2019 a storm dug thousands of them out of a California beach, and the internet met the innkeeper worm: a fat, pink, sausage-shaped animal that filters its food from seawater with a disposable mucus net and shares its burrow with a small menagerie of lodgers.
Overview
Urechis caupo is a marine spoon worm, 10 to 20 cm long, that lives in a U-shaped burrow in the mudflats and sandflats of the North American Pacific coast. It is a filter feeder, straining bacteria and plankton from the water it pumps through its tunnel.
The mucus net
To feed, it spins a cone of fine mucus across the burrow, pumps water through it for up to an hour, then swallows the clogged net whole and spins a fresh one — one of the most efficient particle filters in nature.
The innkeeper
Its burrow shelters lodgers — a pea crab, a scale worm, a small clam, and the arrow goby — that live off its water current and food scraps and give it nothing back. That tolerated freeloading is where the name comes from.
Status
Not Evaluated and common along its stretch of coast. Harmless to handle; dug mainly for fishing bait. Its constant pumping helps aerate the mudflat for other burrowers.
Innkeeper Worm: common questions
Why is it called the penis fish?
It is a nickname based purely on the animal's shape, and it is not a fish at all. The innkeeper worm is a smooth, plump, pinkish, cylindrical animal usually 10 to 20 centimetres long, and out of its burrow, lying on the sand, it looks unmistakably phallic, which is why the informal name has stuck and why the 2019 mass stranding in California went viral. Biologically it is a marine worm, an echiuran or spoon worm, that spends its whole life in a burrow filtering food out of seawater. The proper common name, innkeeper worm, comes from something far more interesting.
What are the lodgers, and why does the worm keep them?
The innkeeper worm shares its burrow with several much smaller animals, most reliably a pea crab, a polychaete scale worm, a small clam or two, and the arrow goby, a slender fish that shelters in the tunnel and darts out to feed. These commensals live off the constant flow of oxygenated water the worm pumps and off the food particles that escape or are left over. As far as biologists can tell, the worm gets nothing in return; it simply tolerates the tenants, which is why it was named the innkeeper. It is a classic textbook example of commensalism, where one partner benefits and the other is unaffected.
How does the mucus-net feeding actually work?
The worm secretes mucus from a ring of glands near its front end and, moving backward, draws it out into a fine, funnel-shaped net that spans the burrow tunnel and is anchored to the wall. It then pumps seawater through this net using body-wall muscle contractions, and the net strains out bacteria, plankton, and organic particles, down to a very small size. When the net is loaded, after anywhere from a few minutes to an hour, the worm moves forward, rolls up the whole clogged net, swallows it, and spins a fresh one. It is one of the most efficient particle filters known in the animal kingdom.
Can you eat an innkeeper worm?
Related spoon worms are eaten in East Asia, where a species called gaebul is a Korean seafood delicacy served live and sliced, prized for a chewy texture and a briny sweetness, and similar worms are eaten in parts of China and Japan. Urechis caupo, the North American innkeeper worm, is not part of a food tradition there and is mainly dug as bait for surf fishing, being a tough, durable bait that fish find hard to resist. It is harmless to handle, has no bite or sting, and is not toxic.