Intertidal underboulder communities

The marine life living under boulders on the seashore

Intertidal underboulder communities © Keith HiscockTo marine life, the area underneath boulders is a different environment from the rest of the shore.  Under boulders, it is damp and shaded, which provides a refuge on an otherwise hostile beach. A ‘boulder’ in this sense does not have to be as big as you might imagine – anything larger than about 25cm across is included in the definition. 

Important communities of plants and animals only begin to develop where there is a sufficient gap on the underside of the boulders.  Such communities can occur anywhere from the mid-shore down to extreme low water, but the richest examples are often found where there is running seawater (for instance, from pools or lagoons emptying after the tide has fallen).

Sea mats, sponges, sea squirts, barnacles, coat-of-mail shells, tube worms and tufts of pink coralline seaweed encrust the undersurfaces.  Sea slugs, such as the sea-lemon, feed on encrusting sponges and the European cowrie eats sea squirts.  Brittlestars, porcelain crabs (which are actually lobsters) and squat lobsters also cling to the undersides of boulders. The undersurfaces are an important refuge for the eggs of fish, dog whelks and sea slugs. Crabs and fish hide amongst the boulders, and they may be a particularly important shelter for young edible lobsters.

Threats to these habitats include boulder turning when hunting for soft ‘peeler’ crabs and winkles, and by people exploring the shore – the failure to return overturned rocks to their original position can be devastating for the community living beneath it.

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


European distribution

Not all boulder seascapes have rich, varied communities of animals living beneath them.  These are nonetheless found all around the UK and often occur in areas of strong tidal currents. Important underboulder communities have been found along the coast of Northumberland, near Plymouth, in the Isles of Scilly,  the Menai Strait and at the entrances to sea lochs.


Conservation status/need

Intertidal underboulder communities fact

This is a UK BAP Priority Habitat (BAP habitats are now Habitats of Principal Importance/Priority Habitats).


Official definition

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


Further information

Marine Life Information Network - Underboulder communities

JNCC - UK BAP Priority Species and Habitats

JNCC EUNIS habitat correlations table