The Cardigan Bay SAC supports both rocky reefs (where animal and plant communities develop on rock or stable boulders and cobbles), and ‘biogenic reefs’ (where the reef structure is created by the animals themselves). Reefs in the Bay consist largely of boulder, cobble and pebble, but along the beaches and just offshore there are occasional areas of bedrock.
Biogenic reefs of the honeycomb worm Sabellaria alveolata are common in the intertidal and shallow subtidal environment, particularly in the northeast of the site. A large section of stony reef extends north and northeast from Cemaes Head and Cardigan Island. Another reef section extends out from Ynys-Lochtyn running virtually unbroken to the edge of the SAC in Aberarth (see map below). A total of 48 reef biotopes have been recorded (inter-tidally and sub-tidally) within the SAC.