Natural England - Intertidal biogenic reefs

Intertidal biogenic reefs

Reefs on the seashore, made from the hard parts of living things.

Intertidal biogenic reef large © Keith Hiscock

Intertidal biogenic reefs © Keith Hiscock

A biogenic reef is a reef made from the hard parts of living things. There are two types of biogenic reefs found between the tides in England – Sabellaria reefs, created by the tubes of the honeycomb worm, and those created by mussels. There is competition for space between the two types of animals, as the presence of blue mussels can stop the worm reefs forming.

Honeycomb worms build tubes from sand and shell fragments. They are found on the lower part of the seashore attached to rocks, boulders or cobbles but they also need a sand supply for tube-building. They are, therefore, found on more exposed shores where there is sufficient water movement to bring new sand from nearby. These worms may form large reefs up to several metres across and a metre deep.

Adult mussels can be found in high densities on the mid to lower shore on beaches of mixed mud, sand and gravel. The sticky, byssus thread ‘beards’ of the mussels bind the live and dead shells together with mud and sand to form the reef.

These living reefs are important as they provide a home for seaweeds and for animals including barnacles, dogwhelks, winkles, and small crabs. The shape of the reefs may also create rock pools, which would not form otherwise on the soft shore, and so provide an additional refuge for animals until the tide returns.

Honeycomb reefs are most at seriously affected by changes to their supply of sand.

Brief description of European distribution

In Britain, biogenic reefs are most abundant on the south-west and west coasts with isolated records from the south east and east coasts. There are also isolated records in Ireland. The northern limit is the Outer Hebrides. Honeycomb worm reefs are at their northernmost limit in Britain, and can be found as far south as Morocco and in the Mediterranean.