Marine Biodiversity Data Portal - NI

Correspondence with existing habitats

UK BAP broad habitat: Littoral sediment

UK BAP broad habitat: Sublittoral sediment

May be component part of Annex I habitats

Description: Currently this habitat in NI is represented by littoral clay exposures, which are soft enough to allow them to be bored by the piddock Pholas dactylus, for example north of Sketrick Island, Strangford Lough.

Peat and clay exposures with either existing or historical evidence of piddock activity are unusual communities of limited extent, adding to the biodiversity interest where they occur. These unique and fragile habitats are irreplaceable, arising from former lake bed sediments and ancient forested peatland (or ‘submerged forests’). Depending on erosion at the site, both clay and peat can occur together or independently of each other.

On the surface of the clay, there may be small clumps of the mussel Mytilus edulis, together with barnacles and the winkle Littorina littorea. The polychaete worms Polydora spp. and Hediste diversicolor can sometimes be present within the clay. When the piddocks have died, their holes provide a micro-habitat for species such as small crabs and anemones. There is little information on the extent, depth and communities associated with subtidal examples.

This habitat is distributed along the north and south coasts of Wales, and the south and east coasts of England. Clay exposures with piddocks are also found in Cumbria. Little is known about UK distribution of subtidal peat and clay exposures, but they are likely to occur in the vicinity of intertidal occurrences.

Associated biotopes:

• LR.MLR.MusF.MytPid – Mytilus edulis and piddocks on eulittoral firm clay
• CR.MCR.SfR.Pid – Piddocks with a sparse associated fauna in sublittoral very soft chalk or clay

• EUNIS code: A1.223, A4.231

Threats:

• Coastal development: Physical damage arising from development of infrastructure, trenching and cable/pipe-laying.
• Coastal protection: Coastal defence works can affect peat and clay habitats, both directly and indirectly, through habitat loss and also alteration of sediment regimes.
• Dredging activity: Maintenance and capital dredging operations may result in direct habitat removal or indirectly through changes in sediment and hydrological regimes.
• Mussel fisheries: Both peat and clay habitats are vulnerable to physical disturbance and smothering arising from dredge, mussel lay and mussel collection operations associated with commercial mussel fisheries.
• Non-natives: There is no evidence to suggest that native piddocks have been displaced in the UK, but in Belgium and The Netherlands, the non-native American piddock Petricolaria pholadiformis, has almost completely displaced the native piddock, Barnea candida. Petricolaria pholadiformis is now widespread in the southern half of Britain.
• Bait collection: In some areas piddocks are harvested as fishing bait, which results in physical damage to the habitat.
• Collection of shellfish: The collection of winkles and oyster is known to occur within the vicinity of this habitat in Strangford Lough which can result in physical damage to this habitat.
• Climate change: Both clay and peat habitats are sensitive to increases in wave exposure, which can increase the rate of erosion. Elevated wave exposure may result from changes to tidal heights and increased storm events which may be linked to the effects of climate change.
• Eutrophication: Increased nutrients from sources such as agricultural run off and sewage discharges can lead to eutrophication and the development of anoxic conditions which are fatal to piddocks.

Action plan and proposed objectives:

Maintain extent and quality of existing peat and clay exposures with Piddocks in Northern Ireland, subject to best available information.