The Fat Innkeeper Worm

A landlord of the mud flats

This worm digs a curved tunnel in mud. The tunnel looks like a letter U. The worm never leaves its home.

It shares the tunnel with other animals. That is why people call it the innkeeper.

Family tree

Annelida, Urechisidae

The fat innkeeper worm is a sea worm. Its name is Urechis caupo. It has no segments like an earthworm. A tube dwelling neighbour is the Mediterranean Fanworm.

25 cm in a U shaped tunnel

How to tell one

The body is soft and sausage shaped. It grows to about 25 cm long. The skin is pink and slightly wet. A short spoon sits at the front end.

There are no legs and no eyes. Tiny hairs cover the whole body. Ring muscles let it move in waves.

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Pacific mud flats

Where it lives

It lives on the Pacific coast of North America. It digs in mud flats and river mouths. The tunnel may go 50 cm down.

It lines the walls with smooth mucus. The mud is often low in oxygen. It stays in one place all its life.

A net of mucus

What it eats

It does not hunt for its food. It spins a net of mucus inside the tunnel. Then it pumps sea water through the net.

Small plants and animals stick to it. The worm then swallows the net whole. It makes a fresh net many times a day.

Spawning once a year

The breeding cycle

Males and females release cells into the water. The eggs drift as tiny larvae. The larvae settle in the mud nearby.

A young worm digs its own tunnel. Adults spawn once a year in summer. Many young are eaten by fish.

Filled mud flats and bait digging

Conservation

This worm has no Red List entry. Mud flats are filled in for building. Digging for bait removes many worms.

Birds, rays and otters eat them. Clean estuaries keep their numbers up. Few people notice this hidden animal.

Quick hits

Five things to remember

• A goby fish lives inside its tunnel.
• A small crab also shares the burrow.
• A scale worm hides in the same home.
• In 2019 a storm washed thousands ashore.
• Fishers use it as bait for other fish.