Habitat account - Rocky habitats and caves


8330 Submerged or partially submerged sea caves 

Background to selection

Distribution of SACs/SCIs/cSACs with habitat 8330 Submerged or partially submerged sea caves.  Click image for enlarged map.
Distribution of SACs/SCIs/cSACs with habitat 8330 Submerged or partially submerged sea caves. Click image for enlarged map.

Description and ecological characteristics

 

This Annex I type includes submerged sea caves and also partially submerged caves which are only exposed to the sea at high tide. Caves vary in size, from only a few metres to more extensive systems, which may extend hundreds of metres into the rock. There may be tunnels or caverns with one or more entrances, in which vertical and overhanging rock faces provide the principal marine habitat. They are typically associated with 1170 Reefs.

 

Sea cave communities vary considerably depending on the structure and extent of the cave system, their degree of submergence and of exposure to sand scour and wave-surge, and their geology. Caves are typically colonised by encrusting animal species but may also support shade-tolerant seaweeds near their entrances. Physical conditions, such as inclination, wave surge, scour and shade, change rapidly from cave entrance to the inner parts of a cave, and this often leads to a marked gradation in the communities present.

 

A high proportion of caves are in the intertidal 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 communities of mussels Mytilus edulis, barnacles Balanus crenatus, cushion sponges, encrusting bryozoans and colonial ascidians, depending on the degree of water movement and scour at particular points in the cave system.

 

Caves that occur in deeper water are subject to less water movement from the surrounding sea, and silt may accumulate on the cave floor. The sponges Dercitus bucklandi and Thymosia guernei, the soft coral Parerythropodium corallioides, solitary ascidians, bryozoans and sessile larvae of jellyfish are characteristic of deeper cave systems. 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.

 

The type of rock in which the cave is formed has an important influence on its shape and qualities as a substrate for plants and animals. In chalk caves in south-east England bands of microscopic algae occur, including Chrysophyceae and Pilinia maritima, that are highly specific to this habitat type.

European status and distribution

 

Although sea caves are distributed throughout Europe where there are rocky coastlines, they are a relatively scarce habitat. The UK has the most varied and extensive sea caves on the Atlantic coast of Europe, and also holds a high proportion of the total extent of coastal chalk, a comparatively rare habitat in Europe.

UK status and distribution Click to view UK distribution of this habitat

 

Submerged or partially submerged sea caves are widely distributed in inshore waters, but no examples are currently known offshore (between 12 and 200 nautical miles from the coast).

Site selection rationale

 

Sites have been selected to encompass the range of structural, geological and ecological variation of sea caves and to cover their geographic range in the UK. Selection was confined to well-developed cave systems, with extensive areas of vertical and overhanging rock, and those that extend deeply into the rock, which are likely to support a wider range and higher diversity of plants and animals. Sites in which there are well-developed gradations in community composition from the cave entrance to the inner parts of a cave have been favoured. Because of the European importance of chalk caves, emphasis has been given to this sub-type within the SAC series.


Site accounts

Berwickshire and North Northumberland Coast Eastern Scotland, Extra-Regio, Northumberland and Tyne and Wear
This is an extensive and diverse stretch of coastline in north-east England and south-east Scotland. Caves occur throughout the site in both the intertidal and the subtidal zones in a range of different hard rock exposures. There are examples of partially submerged caves in the cliffs north of Berwick and in the limestone at Howick (south of Craster), and there are submerged sea caves, tunnels and arches in the volcanic rock of the Farne Islands and around StAbbs Head. Caves occur in association with 1170 Reefs, in both the intertidal and the subtidal zones. Depending on the depth of the cave and its morphology, the site supports a range of distinct biological communities.
Flamborough Head East Yorkshire and Northern Lincolnshire, North Yorkshire
There are larger numbers and a wider range of cave habitats at Flamborough than at any other chalk site in Britain. This site, on the east coast of England, represents caves of the North Sea coast cut into soft rock exposures and is important for its specialised cave algal communities, which contain abundant Hildenbrandia rubra, Pseudendoclonium submarinum, Sphacelaria nana and Waerniella lucifuga. There are more than 200 caves within the site, particularly around the headland and on the north-facing cliffs. Some of these caves are partially submerged at all stages of the tide, others dry out at low tide, and some lie above the high water mark but are heavily influenced by wave splash and salt spray from the sea. The largest caves are known to extend for more than 50m from their entrance on the coast.
Lyme Bay and Torbay Extra-Regio
A large number of infralittoral sea caves have been identified within Torbay and the surrounding coastline from Mackerel Cove in the north, to Sharkham Point in the south. Examples of the classical wave-eroded sea caves are found at all the sites. They occur in several different rock types, and at levels from above the high water mark of spring tides down to permanently flooded caves lying in the infralittoral zone.
Papa Stour Extra-Regio, Highlands and Islands
Papa Stour has excellent examples of caves, tunnels and arches occurring in cold northerly waters. In very exposed sea conditions the caves support rich communities that illustrate the effects of surge, scour and changes in light conditions. The cave walls have extensive faunal turfs, and among the more unusual species present is the northern anemone Phellia gausapata. The rare, surge-tolerant alga Schmitzia hiscockiana is found on boulders in cave entrances. Further diversity is due to the presence of sheltered gullies and tunnels where the community zonation is influenced by tidal flows.
Rathlin Island Northern Ireland
Rathlin, situated off the north coast of Northern Ireland, includes well-developed examples of both partially submerged and submerged caves and overhangs in limestone and basalt in a strong tidal stream. Submerged caves occur mainly at depths ranging from 20 to over 100m. The site has a rich assemblage of sponges and hydroids. Species found include sponges such as Stryphnus ponderosus and Dercitus bucklandi, and the anemones Sagartia elegans, Parazoanthus axinellae and P. anguicomus, which are frequent. The site is used by cave-breeding 1364 Grey seal Halichoerus grypus.
Skerries and Causeway Extra-Regio
Habitat occurrence account not yet available.
South Wight Maritime Extra-Regio, Hampshire and Isle of Wight
The southern shore of the Isle of Wight, off the coast of southern England, includes a number of either submerged or partially submerged sea caves. The exposure of the south coast of the island to high wave energy has allowed the erosion of the Cretaceous calcareous hard cliffs to form sea caves. Examples of this habitat can be found from the Needles along the south-west coast of the Island to Watcombe Bay, and also in Culver Cliff on the south-east coast of the Island. This site also contains the only known location of subtidal chalk caves in the UK. The large littoral caves in the chalk cliffs are of ecological importance, with many hosting rare algal species, which are restricted to this type of habitat. The fauna of these sea caves includes a range of mollusc species such as limpets Patella spp. and the horseshoe worm Phoronis hippocrepia.
St Kilda Extra-Regio, Highlands and Islands
The StKilda archipelago is a westerly outlier of the Outer Hebrides and supports one of the most extensive sea cave systems in the UK. Throughout the island group basalt and dolerite dykes have eroded to form caves and tunnels above and below the water. The communities these support are diverse and reflect the degree of surge to which they are exposed. In shallow water in the extremes of surge the cave walls are blanketed only by the sponge Myxilla incrustans. With a reduction in surge, species such as the northern anemone Phellia gausapata are common, and thin encrusting sponges, bryozoans and the anemones Corynactis viridis and Sagartia elegans are abundant. Microhabitats in the deeper caves show a wave exposure gradient, with species usually found in more sheltered conditions, such as the fan-worm Sabella pavonina and the burrowing anemone Cerianthus lloydii, present in the inner regions. Rarely recorded nocturnal species have also been found in the inner caves, most notably the crab Bathynectes longipes and the anemone Arachnanthus sarsi.
Thanet Coast Kent
Thanet Coast provides the second most extensive representation of chalk caves in the UK on the extreme south-east coast of England. The site is bordered by about 23km of chalk cliffs with many caves and stack and arch formations. Partially submerged caves around Thanet vary considerably in depth, height and aspect and hence in the algal communities present. Some caves extend for up to 30m into the cliffs and reach 6-10m in height, although many are much smaller. They support very specialised algal and lichen communities containing species such as Pseudendoclonium submarinum and Lyngbya spp., some of which were first described from Thanet and have never been recorded elsewhere.

SACs/SCIs/cSACs where this Annex I habitat is a qualifying feature, but not a primary reason for site selection

Cardigan Bay/ Bae Ceredigion Extra-Regio, West Wales and The Valleys
Limestone Coast of South West Wales/ Arfordir Calchfaen de Orllewin Cymru East Wales, West Wales and The Valleys
Lundy Devon
Mousa Extra-Regio, Highlands and Islands
North Rona Extra-Regio, Highlands and Islands
Pembrokeshire Marine/ Sir Benfro Forol Extra-Regio, West Wales and The Valleys
Pen Llyn a`r Sarnau/ Lleyn Peninsula and the Sarnau East Wales, Extra-Regio, West Wales and The Valleys
Y Fenai a Bae Conwy/ Menai Strait and Conwy Bay Extra-Regio, West Wales and The Valleys
 

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