Intertidal Mud

Muddy seashores

Muddy seashores © Paul Kay

Mudflats, which are covered by the sea at high tide and exposed as the tide goes out, are found in sheltered, coastal inlets, such as harbours and estuaries.  They are created because the quiet water allows fine silts and clays to settle from the water onto the seabed.  Rotted vegetation, transported to the sea by rivers and streams, is also part of the mudflat mix.

Mudflats might seem unpleasant, inhospitable areas of waste ground to us but they are teeming with life.  The mud is rich in nutrients and is anything but barren. 

The surface of the mudflats is home to shore crabs, shrimps and sea snails and a few seaweeds may grow on any piece of hard substrate – a pebble or an empty shell. However, it is the life within the mud that is amazing.  Mudflats are home to vast numbers of worms, such as ragworms and lugworms, as well as cockles and other bivalves (with their paired, hinged shells).

Mudflats are hugely important, as they are the larders of the seashore world, providing food for breeding and wintering waterfowl and wading birds.  These probe the mud for worms or cockles, as do the fish that use the areas when the tide comes in.

Many of our estuaries have European and international conservation designations, mainly to protect the birds, which are attracted there by the plentiful food supply within the mud.

Threats to our mudflats include bait-digging, reclamation, pollution and sea level rise.

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


European distribution

Intertidal mudflats are widespread on the Atlantic coasts of Europe, particularly in the large estuaries feeding into the North Sea.


Conservation status/need

Intertidal Mud Fact

  • This is a UK BAP Priority Habitat (BAP habitats are now Habitats of Principal Importance/Priority Habitats).
  • OSPAR List of Threatened and/or Declining Species and Habitats (Region II – North Sea and Region III – Celtic Sea)
  • Annex I habitat under the Habitats Directive
  • Protected under the Birds Directive
  • An important feature in estuary Sites of Special Scientific Interest, under the UK Wildlife and Countryside Act 1981


Official habitat definition

EUNIS habitat A2.3 Littoral mud

UK Biodiversity Action Plan; Priority Habitat Descriptions. BRIG (ed. Ant Maddock) 2008 (updated December 2011)

Descriptions of habitats on the OSPAR list of threatened and/or declining species and habitats (OSPAR agreement 2008/07).


Further information

JNCC - UK BAP Priority Species and Habitats

OSPAR Commission – Background Document for Intertidal Mudflats

Biodiversity Action Reporting System

JNCC Marine Habitat Classification

JNCC EUNIS habitat correlations table