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Vertical or overhanging infralittoral rock subject to considerable wave-surge, especially in the middle or back of caves but also in gullies and tunnels, and dominated by dense sheets of the ascidian Dendrodoa grossularia, together with variable quantities of the sponge Clathrina coriacea. At some sites D. grossularia forms continuous sheets, with few other species present. Other sponges such as Amphilectus fucorum, Pachymatisma johnstonia, Leucosolenia botryoides, Sycon ciliatum and Halichondria panicea regularly occur in this biotope, though generally at low abundance. Other ascidians, especially Polyclinum aurantium, Diplosoma spp. and other didemnids may also occur, though only P. aurantium is ever as abundant as D. grossularia. Being characteristically found in the middle or towards the backs of the caves mean that there is generally insufficient light to support any foliose seaweeds, although encrusting coralline algae are not uncommon. More scoured areas may also contain the anemone Urticina felina, whilst Cylista elegans is often present in low numbers. Mobile fauna are often limited to the starfish Asterias rubens and Henricia spp., the brittlestar Ophiopholis aculeata and crabs Cancer pagurus and Necora puber. The barnacle Balanus crenatus can occur, usually in low densities.