St. Martin's Sedimentary Shore

Also within

Cornwall County Isles Of Scilly Scenic Area

Condition (Natural England)

Climate exposure

This is about exposure to a nationwide pressure this habitat type carries — not a verdict on this site's own management.

Geology & landscape

Rock Type: Granite

Landscape processes: Coastal, Volcanic / Igneous

Description

Description The Isles of Scilly are situated nearly 40 kilometres south west of Cornwall, remote from most major sources of disturbance by man. There is almost no freshwater input with associated water borne pollution and suspended fine sediment. As a consequence there is a range of marine wildlife that is exceptionally undisturbed. Stable oceanic conditions and a wide variation in degree of exposure to wave and current action throughout Scilly produce a high diversity of marine habitats. The mild climate results in an abundance of Mediterranean Atlantic species. For these reasons the sediment habitats and their marine wildlife are considered to be nationally and perhaps internationally important. The intertidal sediment flats are subject to fully saline conditions, shelter from strong wave action and a variety of tidal currents. The quartz sediments derived from the granite that forms Scilly are coarse grained, which enable a number of species, which are more usually found offshore in coarse shell or gravel deposits, to occur in the intertidal zone. St Martin's flats form the largest area of sand exposed at mean low water within the Isles of Scilly, comprising a two kilometres section of shore line on the island's west coast. It is influenced by complex local tidal and current patterns, leading to sediment sorting, periodic reshaping of the shore and a mixture of habitat types and associated communities. St Martin's flats are a moderately exposed sandy shore with bivalve molluscs, burrowing heart urchins and marine worms polychaetes. The intertidal zone comprises a coarse to medium sand beach with boulders grading through to cobbles and some areas of rocky reef. The shore here grades into equally undisturbed and diverse marine communities into the sublittoral fringe lowest part of the shore usually below mean low water mark and sublittoral. St Martin's is of particular interest as the relatively diverse wildlife found within the lower shores includes species, such as the burrowing heart urchin Echinocardium cordatum, that are not usually found to extend above mean low water. The surface dwelling animals include amphipod crustaceans such as sandhoppers Talitrus saltator in the well drained upper shore with strandline seaweeds and lower down the shore Haustorius arenarius, together with gastropod snails such as the common necklace shell Euspira nitida and the netted dog whelk Tritia reticulata. Some areas are characterised by burrowing heart urchins, such as Echinocardium cordatum, with fewer numbers of other heart urchin species, usually below mean low water, including the very small urchin Echinocyamus pusillus. There is a rich range of bivalve molluscs, with notable species including the tellin Angulus tenuis, the rayed artemis Dosinia exoleta and the razorshell Ensis arcuatus. Other species within the sediment include the lug worm Arenicola marina on the mid and lower shore, the sand mason worm Lanice conchilega and smaller numbers of other polychaete worms. Highly mobile species within the sediment are dominated by polychaete worms and include Malacoceros fuliginosus, Glycera sp. Nephtys spp. and Travisia forbesii. Other mobile species include the sea cucumber Leptosynapta inhaerens and the worm like hemichordate Glossobalanus sarniensis. Rocky habitats, including boulders and intertidal reefs, occur in association with the above sediment shore areas and support a variety of important species and communities including those under boulders. These are included within the area of the SSSI where they form an integral part of the shore or are important in delineating the lateral extent of the shore.

Species mentioned