Natural England - Subtidal biogenic reefs on sediment

Subtidal biogenic reefs on sediment

Underwater reefs, made from the hard parts of living things.

Reef forming Ross worms © JNCC/Elizabeth Wood

Reef forming Ross worms -
© JNCC/Elizabeth Wood

A biogenic reef is a reef made from the hard parts of living things: tropical coral reefs are one of the most recognisable examples. Below the tides in temperate UK waters, there are reefs made by tubeworms and mussels.

Honeycomb and Ross worms build tubes from sand and shell fragments. They are usually found individually but sometimes their tubes form crusts or low reefs up to several metres across.

Both blue and horse mussels are found in underwater beds, in living and dead mussel shells are bound up with sand and mud.

Worm and mussel reefs are found in a range of seascapes from exposed open coasts to estuaries, and marine inlets. They may be found on sand, mud or gravel, and in areas where river input makes the seawater less saline.
These living reefs are important as they provide a stable home for other marine life in an otherwise featureless seabed. As a result, a larger number of plants and animals are found on the reefs than on the surrounding soft sediment.

Worm reefs are most seriously affected by changes in the sand supply, which may result from the construction of coastal defences, by shrimp trawling, and to a lesser extent, by aggregate extraction. Mussel reefs have been seriously damaged by scallop dredging.

Brief description of European distribution

In Britain, horse mussel reefs do not occur south of the Humber and Severn Estuaries. Ross worm reefs are found from north of the Shetland Isles to the Mediterranean Sea.

Conservation status / need  

Horse mussel beds are OSPAR Threatened and/or Declining Species and Habitats

UKBAP Priority Habitats

Reefs are listed under Annex I of the Habitats Directive, although not specifically as ‘biogenic reefs’.

Further information sources

Sublittoral sedimentexternal link (JNCC)

Sublittoral biogenic reefs on sedimentexternal link (JNCC)

Horse Mussel bedsexternal link (PDF) (MarLIN) 

Biogenic reefsexternal link (UK Marine SACs Project)

Interesting fact 

It is estimated that an area of mussel bed the size of a tennis court (400,000 mussels) can filter the equivalent of four Olympic-sized swimming pools of seawater in just one day.