We use cookies to give you the best online experience. We use them to improve our website and content, and to tailor our digital advertising on third-party platforms. You can change your preferences at any time. 

During Beta testing articles may only be saved for seven days.

Create a list of articles to read later. You will be able to access your list from any article in Discover.

You don't have any saved articles.

Squiggly little mounds of sand are a common sight when you walk along a beach at low tide. These are the signs of a hidden but industrious worm that has an important place in the beach ecosystem. 

What's more, a quick look at those sand piles will help you decode a secret that they kept from scientists for rather a long time.

The sandy mounds are worm casts, effectively the poo, of a marine worm called a lugworm. Common on sandy and muddy-sandy shores around the coast of Britain, they live in burrows under the wet sand.

A lugworm spends most of its life in the safety of its burrow, which is U- or J-shaped. It lies with its head at the bottom of the tunnel and its tail pointing up towards the surface.

A lugworm in action: eating sediment from the front of its burrow and expelling a cast from its tail

For much of the day their burrows are under the sea, but at low tide they are exposed.

The worm feeds on micro-organisms and organic matter in the water and sandy sediment collected in the front end of its burrow. Indigestible material passes through the worm and is ejected from the back of the burrow via its tail. This worm poo forms the distinctive casts on the surface.

At a casual glance, one worm cast looks much like another. But if you take a closer look, on some beaches you might notice two distinct types. Some casts are messy, like squiggly spaghetti, while others form a neater spiral.

The two types of cast are an indication that there are two common lugworms living on Britain's shores.

The messier cast of the blow lugworm (Arenicola marina) on the left. The neater coils of the black lugworm (Arenicola defodiens) on the right. © National Museum Wales

The blow lugworm (Arenicola marina) produces the messier cast. They grow up to about 20cm in length and vary in colour, from pink or greenish to dark brown or black. Their burrows are between 20-40cm below the surface.

As well as the tell-tale cast, you can also usually spot a slight depression close by in the sand which marks the front entrance to its burrow. 

Blow lugworm casts often have an accompanying depression which marks the front entrance of the burrow. This is rarely seen for black lugworms. Image Gene Selkov (CC BY 2.0), via Flickr.

The black lugworm (Arenicola defodiens) produces the neater spiral cast. It is usually longer - up to 40cm or more - and darker in colour than the blow lugworm. Its burrow is deeper, between 40-70cm below the surface, and normally lacks the depression in the sand marking a front entrance.

Blow lugworm can be found higher up the beach and are easier to find, whereas black lugworm live further down the beach. You might only spot their neat casts when the tide goes out a long way.

Fisherman, who dig up lugworms for fishing bait, have long known of the differences between the two. They particularly prize the black lugworm, though it is much harder to extract from its deep burrow.

However, the black lugworm wasn't officially identified as a separate species from the blow lugworm until 1993, when researchers at Swansea University detailed their genetic differences.

The blow lugworm (left) and black lugworm (right) preserved in alcohol

Like their relatives, the earthworms, lugworms play an important role in their local ecosystem.

The lugworms' burrowing and feeding helps to aerate the sand and release nutrients back into the ground, helping other organisms to thrive.

Lugworms also have an important place in the foodweb. As well as digesting small organisms and helping to break down decaying matter, they are eaten by other animals.

Although lugworms, deep in their burrows, are fairly well protected, they are vulnerable when they expel waste from their bodies. To do so, they must bring their tail near to the surface to produce the sand cast. This is the point at which predators can attack.

If the burrow is underwater at the time, then fish and crabs will take a bite at the tail. When the burrow is exposed at low tide, birds such as curlews and godwits will swoop on them.

Fortunately for the lugworm, it has a fragile tail that can break off from the rest of it body if attacked. So often the predator ends up with a tasty morsel, but the lugworm survives.

Even so, quite a few lugworms do end up completely eaten and they are estimated to be an important food resource for many birds.

A curlew's long bill is ideal for extracting lugworms. Image by Ian Kirk from Broadstone, Dorset, UK, Curlew with lugworm, CC BY 2.0.

The lifecycle of these animals has been well studied and they can live for around five to six years.

When it comes to lugworm love, reproduction is done at a distance. Male and female lugworms live in different burrows and don't come out to mate. So, once a year in the autumn, when the tide is out, males release sperm on to the sand's surface. When the tide comes back in, the sea water washes the sperm into the burrows of the females and their waiting eggs.

The juvenile lugworms spend the winter in quiet channels, then in the spring they build burrows close together high up on the beach, known as nursery beds. Towards the end of that year, they float down the beach on the tide and make new burrows. By the second year they will be fully grown adults and ready to begin the cycle again.

So the next time you squish a cast under your toes at the beach, think of the secret world underneath your feet and the busy life of a lugworm.

Find out about the plants and animals that make the UK home.

If you see a snake blink at you, it's not a snake.

Take a look through the eyes of a dog, a spider and three other creatures.

Get to know the world's largest land-dwelling crustacean.

Meet the beetles that fire a boiling chemical brew at predators that get too close for comfort.

Get email updates about our news, science, exhibitions, events, products, services and fundraising activities. You must be over the age of 13. Privacy notice.

Follow us on social media