Marine Biodiversity Data Portal - NI

Correspondence with existing habitats

UK BAP broad habitat: Littoral rock

May be component part of Annex I habitats

Description: This habitat is found from the mid-shore down to the extreme lower shore, and encompasses areas of boulders (greater than 256 mm diameter) that support a diverse underboulder community. The underboulder habitat, along with fissures, crevices and any interstitial spaces between adjacent boulders, form a series of microhabitats that add greatly to the biodiversity of a shore.

This habitat can occur on a variety of substrata (including bedrock, mixed rock and sediment or mud), but there needs to be a sufficient gap on the underside of the boulder to support an underboulder community. The richest underboulder communities are often found where there is running seawater (for instance, from pools or lagoons emptying after the tide has fallen). Boulders with a limited underboulder community are not included here, as may occur for example where boulders are embedded in sediment, in low salinity conditions, and where boulders experience high levels of mobility and scour.

Underboulder habitat provides an environment of shade, moisture and shelter. The undersides of boulders can therefore sustain a diverse collection of animals needing these conditions to survive. Underboulder communities are generally dominated by an encrusting fauna of bryozoans, sponges, ascidians, barnacles, chitons and calcareous tube worms. Crustaceans such as the hairy porcelain crab Porcellana platycheles, the long-clawed porcelain crab Pisidia longicornis, other small crabs and squat lobsters shelter on the undersides of boulders together with scale worms, sea cucumbers, and brittle stars. Herbivores include the top shells, periwinkles, the cushion star Asterina gibbosa and the green sea urchin Psammechinus miliaris. Underboulder habitat plays an important role in the life cycle of marine animals, for example the undersurfaces are an important refuge for the eggs of fish, dog whelks and sea slugs. The sheltered gaps between and under the boulders provide security for mobile species such as larger crabs and fish, and also the juveniles of many more species.

Whilst boulders are widespread around the UK coast, only a component of these support a diverse underboulder community. In Northern Ireland good examples can be found at Ballyhenry Island and Portballintrae.

Associated biotopes:

• LR.MLR.BF.Fser.Bo – Fucus serratus and under-boulder fauna on exposed to moderately exposed lower eulittoral boulders
• IR.MIR.KR.Ldig.Bo – Laminaria digitata and under-boulder fauna on sublittoral fringe boulders

• EUNIS code: A1.2142 and A3.2112

  Both these biotopes correlate directly with this habitat. However this underboulder habitat can also be associated with other biotopes which occur on boulders on the mid to lower shore.

Priority species: Boreochiton ruber, Tonicella marmorea

Threats:

• The level of threat this habitat experiences is influenced by ease of access and proximity to dense populations. Consequently, in certain parts of the UK such as Scotland, it is not considered to be under threat.
• Boulder turning for winkles: This is where winkles are collected for human consumption, from around and underneath the boulders. Again, boulders may not be replaced in their original position. In Northern Ireland this activity has increased in recent years.
• Public shore visits: One of the recreational activities to be had on a shore is to turn boulders to see what lives beneath. Chronically elevated levels of recreational disturbance is generally only associated with popular tourist destinations with easy shore access.
• Building of cairns: A recent activity is the building of cairns using boulders that are found on the shore, the boulders are not replaced in their original position.
• Water quality: De-oxygenation of underboulders and consequent death of underboulder fauna may be caused by anthropogenic organic input (e.g. sewage and agricultural inputs), and also as a consequence of eutrophication. Note that de-oxygenation may also occur naturally from rotting seaweed drift.
• Non-native species: Non-native species such the carpet ascidian Didemnum vexillum overgrow native sessile invertebrates, this can be clearly seen at Paddy’s point, Strangford Lough. The non-native red ripple bryozoan Watersipora subatra has replaced native underboulder communities in several parts of the UK and could threaten this habitat in Northern Ireland.