Intertidal Monitoring of the Number and Extent of Sea Caves, Pen Llyn ar Sarnau Special Area of Conservation (SAC) 2004 dataset https://portal.medin.org.uk/portal/start.php?tpc=007_NRW_DS102026 Submerged or partially submerged sea caves are included as an Annex I type within the EU Habitats Directive (92/43/EEC). These caves may vary in size and exposure, and thus the communities associated with them vary considerably depending on the structure and extent of the cave system. Sea caves generally provide habitat for colonies of encrusting animal species, but may also have shade-tolerant algal communities at their mouth. Caves on the shore and in the shallow sublittoral zone are frequently subject to conditions of strong wave surge and tend to have floors of coarse sediment, cobbles and boulders. These materials are often highly mobile and scour the cave walls. Caves that are subject to strong wave surge are characterised by communities of mussels Mytilus edulis, barnacles Balanus crenatus, cushion sponges, encrusting bryozoans and colonial ascidians, depending on the degree of water movement and scour at particular points in the cave system. The UK has the most varied and extensive inshore sea caves on the Atlantic coast of Europe. In Wales, sea caves are important within the Pen Llyn a'r Sarnau SAC and are therefore a qualifying feature, but not a primary reason for the site selection. As such there is a requirement to assess the number and extent of intertidal sea caves within this SAC. Further background information on the surveying of sea caves in Welsh SACs can be found in Bunker and Holt (2003). Sea caves are important within the Pen Llyn a'r Sarnau SAC and are a qualifying feature. As such there is a requirement to assess the number and extent of intertidal sea caves within this SAC. The specific aims of the survey were to identify and map the location of sea caves along the coastline, use digital photography to record their extent and to measure the threat from in-fill by the railway company. 20090131