Reefs in Strangford Lough vary from tide-swept bedrock and large boulders in the main channel of the Narrows, through sand-scoured bedrock and boulders at either end of the channel, to more sheltered bedrock and boulders in the main central portion of the Lough and in parts of the intertidal. Beds of horse mussels (Modiolus modiolus) used to form extensive biogenic reefs within the central portion of the Lough.
In the Strangford Narrows, rock surfaces are entirely clothed in colourful soft coral, sponges (some reaching huge sizes), ascidians and sea- anemones. Large boulders on the bed of the Narrows are subject to strong tidal streams.These boulders are clothed with encrusting sponges, hydroids, and sea anemones. Coarse sand scours rock surfaces at the sides and either end of the Narrows. Here, the characteristic species is the bryozoan Flustra foliacea.
Glaciated or sea-worn bedrock outcrops are found at many locations. Massive boulders occur on the shore and form rocky islands known as ‘pladdies’. The fauna and flora associated with these outcrops are dependent on the rock type, their position and the prevailing conditions.
Horse mussel beds form one of the richest communities in the Lough, with about 200 associated species growing and sheltering on the hard surface of live mussels and dead mussel shells. Full development of the community depends on the very sheltered, plankton-rich waters of extremely low turbidity found in the central to northern area. M. modiolus rarely occurs in such still waters. Many mobile suspension-feeders also occur, particularly the scallop Chlamys varia, and brittlestars in the central to south-western area where water movement is slightly greater.
The 2007 condition assessment found that this feature was in unfavourable condition because of the reduction in the extent of the Horse Mussel beds.