Subtidal biogenic reefs on sediment

Underwater reefs made from the hard parts of living things

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

For the official habitat definition please see the documents listed below.


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.

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


Conservation Status

  • This habitat may contain components that are UK BAP Priority Habitats (BAP habitats are now Habitats of Principal Importance/Priority Habitatsand OSPAR habitats. Horse mussel beds, blue mussel beds and cold-water coral reefs are all OSPAR threatened and/or declining habitats and UK BAP Priority habitats, and ross worm reefs and honeycomb worm reefs are UK BAP Priority habitats.  
  • Reefs are listed under Annex I of the Habitats Directive, although not specifically as ‘biogenic reefs’.


Official habitat definition

EUNIS habitat A5.6 Sublittoral biogenic reefs


Further information

JNCC Marine Habitat Classification

JNCC - UK BAP Priority Species and Habitats

UK Biodiversity Action Plan; Priority Habitat Descriptions. BRIG (ed. Ant Maddock) 2008 (updated July 2010)

Case Reports for the OSPAR List of threatened and/or declining species and habitats

JNCC EUNIS habitat correlations table