Intertidal sand flats and mud flats are sediment habitats, which are uncovered by the tide. There are vast areas of intertidal sand and mud flats within the European marine site, including Budle Bay, Fenham Flats, Holy Island Sands, Cheswick and Goswick Sands.

Mud flats and sheltered sand flats tend to be exposed to low energy water movement. Higher levels of organic matter accumulate in these habitats, providing a rich source of food for the animals living both in and on the sediment. Consequently, sheltered mud and sand flats support a wide variety of marine life. A number of marine worms are common in these habitats, for example the ragworm (Nereis virens) prefers muddy sands to clean sands. Small crustacean amphipods (eg. Corophium volutator) burrow into the mud, along with bivalves such as Macoma balthica and Angulus tenuis.

The muddy sediment areas around Lindisfarne also support the largest beds of narrow-leaved (Zostera angustifolia) and dwarf eelgrass (Zostera noltii) on the east coast of England. These beds and their associated communities of plants and animals are becoming increasingly rare and are an important foraging area for birds such as wigeon and light-bellied brent geese.

Cleaner, sheltered sands at Low Newton-by-the-sea support a rare intertidal population of the heart urchin (Echinocardium cordatum). This animal burrows in the sand, but is normally only found subtidally. Razor clams (Ensis ensis) can be found in vertical burrows in the sand, into which they descend rapidly when disturbed.

Areas of clean sand normally occur in locations with high-energy water movement. The communities within this habitat tend to be less diverse than sheltered locations due to the mobility of the sediments. Robust marine worms such as Scolelepis squamata or the lugworm (Arenicola marina) may exist in these conditions and some small crustacean amphipods.

Mussel beds are present in certain sediment areas where small pebbles and shells provide some initial anchorage. Once established, these beds provide a sub-habitat for a host of other small plants and animals, which in turn are a rich food source for larger marine predators such as fish, birds, crustacea such as the shore crab (Carcinus maenas) and echinoderms such as the common starfish (Asterias rubens).

The distribution of sand and mud flats within the Berwickshire & North Northumberland Coast European marine site can be seen in the Map Gallery

Pictures of some of the animals that live in intertidal sand and mud flats can be seen in Sights and Scenes