A summary of the main known occurrences of biogenic reefs or possible biogenic reef biotopes, concentrating on cSACs and pSACs plus important or well developed areas elsewhere. This table relates only to the five species considered in detail in this report (Sabellaria alveolata, S. spinulosa, Modiolus modiolus, Mytilus edulis and Serpula vermicularis). Distribution in relation to cSACs and pSACs is given as far as possible.

Sabellaria alveolata

  • eleven areas recommended for SSSI status by Cunningham et al. (1984)

The terms sheets, hummocks and reefs are as defined by Cunningham et al. (1984) (i )

Distribution within demonstration UK Marine cSACs

Cardigan bay cSAC

  • Aberporth - sheets and hummocks (Cunningham et al., 1984).

Lleyn Peninsula and the Sarnau cSAC

  • Shell Island, Mochras - S. alveolata sheets cementing boulders and cobble, with fucoids.
  • Llwyngwril and Barmouth area - sheets.

Less well developed reefs at Criccieth and on the north side of the Lleyn Peninsula.

All the above based on information provided by CCW plus Cunningham et al. (1984).

Morecambe Bay cSAC

Newly discovered small areas off Morecambe frontage which have replaced Mytilus community on boulder scar, plus areas on boulder/cobble around Piel Flats and smaller areas on the West side of Walney Island (C Lumb; J Andrews; B Green, pers. Comm.).

Solway Firth cSAC

There are numerous extensive areas on the Cumbrian coast but the majority are outside the cSAC. There is a very extensive area of S. alveolata sheets and hummocks on boulder scar at Dubmill point which is on the border of the cSAC and much of this area is likely to be included within the cSAC. A few areas of S. alveolata are known from the Dumfriesshire coast but no extensive ones likely within the cSAC.

Distribution within other UK Marine cSACs and pSACs

Severn Estuary pSAC

Very extensive sublittoral S. alveolata reefs, with some S. spinulosa, on tide swept hard substrata in turbid waters - covering the greater area of the subtidal - ‘among the finest and most notable in Britain, with particularly rich associated communities’ (CCW). Also some intertidal areas though not very notable according to Cunningham et al. (1984) and unclear if they would qualify as biogenic reefs.

Drigg Coast cSAC

Drigg coast cSAC includes well developed S. alveolata reefs at Barn Scar and Kookarah. Some nearby to the south eg Tarn Bay/Selker area are probably better examples but are not covered.

Distribution within other areas

South west England

Numerous areas though there is some suggestion of reduction in range in this area, and conspicuously absent from much of West Cornwall. Notable areas listed by Cunningham et al. include:

  • Duckpool, N. Cornwall (hummocks);
  • Dawlish, S. Devon (hummocks);
  • Millook, Cornwall (sheets and hummocks).


A number of areas identified by Cunningham et al. (1984) including some in Cardigan Bay in the Aberystwyth area, and some on the Gower peninsula, plus:

  • Dunraven bay (unusual reefs and hummocks).

Cumbrian Coast

As well as those areas mentioned above under Morecambe Bay, Drigg Coast and Solway Firth cSACs, there are many well developed and extensive reef areas on boulder scars on open coasts between the Duddon estuary and St Bees Head, including:

  • Tarn Bay (reefs and hummocks);
  • Annaside Bank (hummocks).

There are also numerous areas, though probably generally with lower cover, between St Bees Head and Dubmill Point at the entrance to the Solway estuary, particularly in Allonby Bay, including;

  • Crosscanonby.

Northern Ireland

Reefs are reported from the Down Coast at Rossglass (Crisp, 1964) and Glassdrummand Port (Wilkinson et al., 1988). At Glasdrummand they are known to extend into the subtidal.

South west Scotland

  • Auchenmaig Bay, in Luce Bay Galloway (hummocks).

Sabellaria spinulosa

There are numerous areas, particularly off North and West Wales but also elsewhere, where crusts of S. spinulosa occur but these appear to be largely thin, poorly developed annual features (see text). Only those which seem more likely to represent true biogenic reefs are listed here.

Distribution within demonstration UK Marine cSACs

Berwickshire & North Northumberland Coast cSAC

S. spinulosa dominated biotopes reported in this area by MNCR but not clear whether they qualify as biogenic reefs nor whether they are actually within the cSAC boundary.

The Wash and North Norfolk Coast cSAC

Well developed examples have recently been found in the mouth of the Wash within the cSAC boundary (Patterson, pers. comm.). Work is in progress to define these areas more clearly, but they appear to be continuations of areas found outside the cSAC boundary, see below (Foster-Smith, pers. comm.). Sites rich in S. spinulosa have previously been found within the Wash (National Rivers Authority, 1994) though these examples may not have constituted reefs.

Lleyn Peninsula and the Sarnau cSAC

There are well developed crusts of S spinulosa within the cSAC Lleyn Peninsula and the Sarnau but it is debatable whether they qualify as biogenic reefs (E I Rees, pers. obs.).

Morecambe Bay

None known, though reportedly there were areas which were lost due to the activities of prawn fishing in the approach channels to the Bay (Mistakidis, 1956; Taylor & Parker, 1993).

Distribution within other UK Marine cSACs and pSACs

None known



Distribution within other areas

Bristol Channel

Well developed reefs with associated fauna of relatively slow growing invertebrates reported in the Bristol Channel (George & Warwick, 1985).

The Wash

Well developed examples have been found well offshore from the Lincolnshire coast, outside the boundary of the Wash and N. Norfolk Coast cSAC. Good video footage shows well developed reef areas protruding up to 60 cm above the surrounding seabed. Extent of coverage is not yet known though video footage covering c 300 m showed almost continuous cover of reefs continuing beyond the videod area (Foster-Smith et al., in prep; Foster-Smith, pers. Comm.).


There are well developed reefs off the north coast of Anglesey (E I Rees, pers. obs.).

North Yorkshire & Durham Coasts

S. spinulosa dominated biotopes reported in this area by MNCR but not clear whether they qualify as biogenic reefs

Modiolus modiolus

Distribution within demonstration UK Marine cSACs

Berwickshire & North Northumberland Coast cSAC

Dense beds reported in the vicinity of the Farne Islands but these are not mentioned in draft information on important communities prepared by SNH - extent and density of these is unclear (Helen Davis, English Nature, pers. comm.).

Lleyn Peninsula and the Sarnau cSAC

Clumps of Modiolus have long been known from the North side of the Lleyn Peninsula. In 1993-95 acoustic surveys showed that they form true reefs raised over a metre above the seabed. The BIOMAR team carried out RoxAnn surveys in the area in 1995. A detailed acoustic survey of the area was done in 1997 by NW & N. Wales Sea Fisheries Committee for CCW as part of the cSAC demonstration project.

Strangford Lough

There are extensive areas dominated by clumps of Modiolus on a muddy bottom which probably do not qualify as reefs although they support similar communities (Magorrian et al., 1995). Much denser areas which probably do qualify as biogenic reefs exist in places, including between some of the pladdies (islands) (R Holt, M Service, pers. comm.).


Distribution within other UK Marine cSACs and pSACs

Lochs Duich, Long and Ailsh Reefs pSAC

Information provided by SNH: "in a number of places, circalittoral boulder slopes support a community typified by the squat lobster Munida rugosa and the horse mussel Modiolus modiolus which forms biogenic reefs in places."

Distribution within other areas


Numerous areas where Modiolus is important though in many cases may be clumps rather than true biogenic reefs. Busta Voe is thought to have the most extensive and highest quality Modiolus biotopes in the Voes (Alistair Davison, pers. comm.).

Off Humber estuary

Dense beds reported (Murray et al., 1980) though unclear whether they really constitute biogenic reefs.


Irish Sea

Numerous areas of coarse bottoms have been known to have clumps or patches of Modiolus, eg much of the area between Anglesey and the Isle of Man. North and north west of Anglesey there may be some more extensive and dense patches.

South east of Isle of Man

Extensive areas reported to be dominated by M. modiolus (Jones, 1951) but thought to be much reduced in density and extent in recent years, due probably to scallop dredging.

North of Isle of Man

At least 6 km2 of seabottom recently discovered where M. modiolus on tide swept gravel and shell creates numerous steep banks up to 1 m high with very rich associated fauna (Holt et al., unpublished).

West of Scotland

M. modiolus forms very dense and beds, though unclear how physically raised, in upper Loch Creran, and in deeper tide swept channels between basins in Loch Leven and Loch Eil (Howson et al., 1994). Numerous other areas where Modiolus is important, though in many these are probably scattered clumps rather than true biogenic reefs.

East coast of N. Ireland and Rep. of Ireland

Probably quite extensive dense beds off the Ards Peninsula, with small areas also reported in Carlingford Lough and at Portrush (Erwin et al., 1986). Unclear in all cases whether they constitute true biogenic reefs. On the southern tip of the Codling Bank off Co Wicklow a gravel

embedded type of Modiolus bed was located in 1997 during an Ireland -Wales INTERREG project.

Mytilus edulis

The common mussel is widespread and abundant on many parts of the UKs coasts. There are likely to be numerous, particularly estuarine areas, where Mytilus beds develop to the stage where they constitute biogenic reefs, in addition to those mentioned here.

Distribution within demonstration UK Marine cSACs

Berwickshire & North Northumberland Coast cSAC

Large mussel beds exist in the Lindisfarne and Budle Bay estuary area, probably constituting biogenic reef in places.

The Wash and North Norfolk Coast cSAC

In the Wash there are areas which qualify as biogenic reefs. There is a long history of data on stocks and recruitment fluctuations in records kept by Eastern Sea Fisheries Committee and CEFAS.

Burry Inlet

Extensive and well developed reefs form in three main areas within the ‘estuary of the three rivers’ at Laugharne Sands, Salmon Point Scar and Scott’s Bay. Relatively rich associated fauna.

Lleyn Peninsula and the Sarnau cSAC

Areas of cobble on the Sarns have shallow sublittoral beds of Mytilus but unlikely to constitute biogenic reefs.

Morecambe Bay

Mussel beds occur on a number of "skears" within the bay. On some, such as Head Skear in the Walney Channel the beds build up well developed bioherm mounds. On other more exposed skears, such as South America Skear, there are from time to time very dense spat settlements which build up sufficient mud that the mussels detach from the stones and are washed away by storms. Such beds are a very important source of seed for re-laying, for example in the Menai Strait.

Distribution within other UK Marine cSACs and pSACs

Dornoch Firth pSAC

Extensive beds which probably constitute biogenic reefs. May have a sublittoral component. (McKay, pers. comm.).

Plymouth Sound and Estuaries cSAC

Mytilus beds which may constitute reefs at the mouth of the Lynher estuary.

Pembrokeshire Islands cSAC

Dense mussels form on tideswept areas but not clear if they form true biogenic reefs (information from CCW).

Solway Firth cSAC

Dense beds/reefs, somewhat transient, occur on some scars & sand banks especially Silloth - Dubmill Point, and may extend subtidally.

Distribution within other areas

Cromarty Firth


Culbin Sands


Ythan Estuary

Very dense and extensive reefs. Well studied in relation to bird feeding.


Mytilus found sublittorally around the Tay Road Bridge though unclear whether these beds constitute reefs.

Firth of Forth


South West England and South Wales

Exe Estuary - good examples of well developed beds which form biogenic reefs. Probably also the Teign. Almost certainly many others in the general area (Moore, 1995).

Cumbrian coast

Many extensive boulder scars get heavily covered with Mytilus but probably rarely develops to true biogenic reef status.

Northern Ireland

Lough Foyle - Mytilus forms beds in sand and gravel though debatable whether it constitutes biogenic reefs.

West of Scotland

Mytilus widespread in sealochs on gravel/pebble/cobble; often described as clumped, but forms dense areas which might represent biogenic reefs in freshwater influenced areas at heads of lochs and in tidal narrows.

Serpula vermicularis

Distribution within demonstration UK Marine cSACs

None known



Distribution within other UK Marine cSACs and pSACs

None known


Distribution within other areas

Loch Sween, Scotland

Reefs reported previously (Earll, 1982), but recent surveys found only dead colonies (Moore, 1996).

Loch Creran, Scotland

Well developed reefs, particularly in the lower basin but also in the upper (Howson et al., 1994; Moore, 1996).

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