Sea cave communities vary considerably depending on the many different factors, how long and how often they are submerged and the amount they are exposed to wave-surge.
Caves are typically colonised by encrusting animal species, but may also support shade-tolerant seaweeds near their entrances. Physical conditions, wave surge, scour and shade, change rapidly from cave entrance to the inner parts of a cave, and this often leads to an obvious change in the communities present.
A high proportion of caves are in the intertidal zone or in shallow water. 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 barnacles Balanus crenatus, cushion sponges, encrusting bryozoans and colonial ascidians.
Caves that occur in deeper water are subject to less water movement from the surrounding sea, and silt may accumulate on the cave floor. These caves, particularly where they are small, provide shelter for crabs, lobsters Homarus gammarus, crawfish Palinurus elephas, and fish such as leopard-spotted goby Thorogobius ephippiatus.
Due to the fact that they are very difficult to access, very little is known about the seacaves and associated communities on Lundy. There is no definitive audit of how many seacaves there are, but we have some understanding of the population trends and condition of some rarer sea cave species. It is also well documented that some of the intertidal caves on Lundy are used by Grey Seals, Halichoerus grypus, as pupping sites.