The Scilly archipelago, off the south-west tip of England, encompasses extensive sublittoral sandy sediments, which, between the islands, are contiguous with the intertidal sandflats. They are important in the UK for the extent and diversity of their associated communities. In particular, their isolation and the presence of oceanic water contribute to the special nature of the site, which is characterised by shallow sandy sediments with low silt content and by the fully marine salinity. There are rich communities present on the tide-swept sandbanks in the narrow channels between the islands and in the deeper, more stable, wave-sheltered sediments. The fauna of these sediments includes tanaid crustaceans, a diversity of polychaete worms, and various echinoderms. The shallow sublittoral sediments are colonised by the most extensive and best-developed eelgrass Zostera marina beds in southern England (Hocking & Tompsett 2001). These beds have a rich associated flora and fauna of algae, hydroids, sea anemones, molluscs and fish. Fauna with warm-water affinities include the trumpet anemone Anthopleura ballii.
The Isles of Scilly archipelago supports extensive areas of undisturbed intertidal sandflats in the extreme south-west of the UK. The islands are particularly important for exceptionally rich communities occurring in coarse sediments, including clean sand, a substrate that is usually poor in species. Although sheltered, the sediments include little mud because the surrounding seas have a low suspended sediment concentration, resulting from the islands’ isolation and the presence of oceanic water. The sandflats exposed at low tide between the northern islands are of international marine nature conservation importance, owing to their extent and diversity and the presence of species rarely found elsewhere in the intertidal. The lower shore sandflats are particularly notable, for they include the fringes of the most extensive and diverse beds of eelgrass Zostera marina known in southern England (Hocking & Tompsett 2001), with an unusually species-rich associated biota, including various seaweeds and fish and rich sediment communities of anemones, polychaete worms, bivalve molluscs and burrowing echinoderms. These include many species restricted to the sublittoral elsewhere in the UK. Many southern species are present, often in large numbers, including some, such as the hermit crab Cestopagurus timidus and the spiny cockle Acanthocardia aculeata, that are recorded only rarely in the UK.
The Isles of Scilly are surrounded by reefs and rocky islets, some only extending into the shallow sublittoral, others extending well beyond 50 m depth. The location of the islands, exposed to the full force of the Atlantic, leads to the development of extremely exposed communities on west-facing reefs, whilst on the east-facing coast, more sheltered and silted reefs occur. The south-westerly position of the islands leads to a range of warm-water species being present, including sunset cup-coral Leptopsammia pruvoti, pink sea-fans Eunicella verrucosa, and Weymouth carpet-coralHoplangia durotrix.