All biogenic reefs have a limited distribution within the UK and Europe. The most important known areas of biogenic reefs in the UK are listed in Table 3 with brief descriptions or comments. The locations of these plus relevant biogenic reef MNCR biotopes from Table 2 are given in Figure 2.

S. alveolata reefs are a southern phenomenon reaching their northerly limit within Britain. Extensive reefs are commonly reported on the French, Spanish and Portuguese Atlantic coasts (Almaca, 1990; Anadon, 1981; Gruet, 1981; Gruet, 1989), and extend as far south as Morocco (Gruet, 1982 cited in Cunningham et al., 1994; Hawkins, pers. obs.). They also occur in the Mediterranean. Within Britain, the range is essentially southern and western (Figure 2a), with reefs not reliably found east of Lyme Regis in the English Channel (Hawkins, pers. obs.), although there are dubious reports from the Isle of Wight (Gubbay, 1988), and none reliably reported north of the Northern Solway Firth (Cunningham et al., 1984). Within Britain and France at least they are usually intertidal, though they have also been reported from the subtidal, sometimes extensively. They are generally limited to areas of hard substratum, including cobble, adjacent to sand and with moderate to considerable exposure to waves (see chapter III). Cunningham et al. (1984) recommended eleven sites for SSSI or SSI status (listed in Table 3) and essentially these remain among the most interesting known intertidal reefs, although those in the Barn Scar and Dubmill Point areas of Cumbria are probably equally valuable. English Nature considered S. alveolata reefs on the Cumbrian coast to be of national importance (English Nature, 1993). However, it is now known that there are also very extensive and rich subtidal reefs in the Severn Estuary which may also be of national importance (see linked table).

Published information on world-wide distribution was not found, but in the North East Atlantic S. spinulosa has a widespread distribution, which encompasses the whole of the British Isles, including Shetland (MNCR database; Hayward & Ryland 1990) and the Mediterranean Sea (Bhaud & Gruet, 1984), although it is limited to areas with very high levels of suspended sediment.

Areas where dense aggregations have been reported include the Thames Estuary (Attrill, 1996), Dublin Bay (Walker & Rees, 1980), off Lough Foyle, N. Ireland (Erwin et al., 1990), Burrow Head, North Irish Sea (Earll, 1992; Covey in prep), Morecambe Bay (Irving et al., 1996), Hilbre Island, Dee Estuary (McIntosh, 1922 cited in George & Warwick, 1985), Gower

Figure 2a

Figure 2b

Figure 2c

Figure 2d

Figure 2e

Figure 2f

(Hiscock, 1979), North and West Wales (R Holt pers. comm.; Hiscock, 1984), the Bristol Channel (George & Warwick, 1985), the Solent (Environment Agency South West pers. comm.), Seven Sisters, Sussex (Wood & Jones, 1986), the Wash (Dipper et al., 1989; Foster-Smith et al., 1997; National Rivers Authority, 1994; Warren, 1973), Northumberland and North Yorkshire (Connor et al., 1996 and R Holt, pers. comm.), St Andrews, Scotland (McIntosh, 1922 cited in George & Warwick, 1985), and several locations in the southern North Sea (e.g. Riesen & Reise, 1982; Linke, 1951; Dorjes, 1992). With the exception of Linke (1951), none of the above references seem to describe very massive reefs of the sort created intertidally by Sabellaria alveolata, but ‘crusts’ or ‘sheets’ of variable thickness and rarely more than a few cm thick. These may just about fit the criteria for biogenic reefs, at least in some cases, although lack of clear descriptions is a problem in many cases. There are examples of what seem to be true, well developed reefs in the mouth of the Wash (see Table 3) (Foster-Smith et al., in prep February 1998) and possibly in the Bristol Channel (George & Warwick, 1985), though the physical nature of the latter is not well described. S. spinulosa reefs and crusts appear to have a somewhat southerly distribution (figure) although this may reflect the distribution of suitable habitat, such as perhaps turbidity, rather than temperature.

M. modiolus is a northern species which occurs in both the Atlantic and Pacific Oceans. In the north east Atlantic it occurs from at least the Bay of Biscay north (though occasional specimens may occur as far south as North West Africa) to northern Norway and the White Sea, and occurs off Iceland and the Faeroes (Brown, 1984; Hayward & Ryland, 1990; Roberts, 1975; Schweinitz & Lutz, 1976; Tebble, 1966). Within Britain it is more frequent in northern and western areas (Figure 2c); extensive dense beds do not seem to occur south of the Severn Estuary on the west coast or the Humber Estuary on the east coast. Numerous descriptions of Modiolus communities mention clumps of Modiolus, often scattered on muddy or muddy-gravel sediments (see chapter III), and in many cases it seems unlikely that these would fit the above description of ‘biogenic reefs’. Beds which are probably dense enough or discrete enough to fit the definition of biogenic reefs have been reported from: the south east of the Isle of Man (Jones, 1951); north east of the Isle of Man (Holt et al., unpublished); several parts of Strangford Lough (Magorrian et al., 1995); off the Lleyn Peninsula (Rees, pers. obs.), Lochs Creran, Eil and Leven (Howson et al., 1994), off the Ards Peninsula, relatively small areas in Lochs Duich, Long and Alsh (SNH unpublished information), and the Shetland Voes, and probably occur elsewhere, particular in the Irish Sea and the west of Scotland.

The taxonomy of the genus Mytilus has been the subject of some debate in recent years, and there is often confusion between three ‘species’ in particular: Mytilus edulis, M. trossulus and M. galloprovincialis. The status of the latter has been particularly debated; it was once widely thought to be a separate species but is now regarded by many as a form of M. edulis (see e.g. McDonald et al., 1991; Gosling, 1992a; Seed, 1992 for summaries). Nevertheless, M. edulis seems to have a broad distribution; it is circumpolar, and reported as far south as Japan and China in the Pacific and to North Carolina in the Atlantic, while in Europe the northern limit is within the Arctic circle, and it is found as far south as the Mediterranean (Clay, 1967). It forms dense aggregations which might fit the definition of biogenic reefs over much of this geographical range.

Mytilus edulis can be abundant all over the UK in intertidal and sometimes subtidal habitats, ranging from fully saline to highly estuarine, and again over much of this range it is capable of forming dense beds, by embyssment to one another and to underlying substrata, which could justifiably be called biogenic reefs (Figure 2d). In the south west of England it frequently forms hybrids with the more southerly distributed Mytilus galloprovincialis, but in more sheltered and lower salinity areas, where true reefs tend to occur, M. edulis is much the more dominant form (Gardner, 1994; Gardner, 1996). Hybridisation also occurs on Scottish and Irish coasts, but here there is less intermixing of genes between populations of the two forms than in the south west of England (Gosling, 1992a).

A number of important Mytilus reef sites are given in the linked table, but this list is by no means exhaustive.

This species supposedly has a world-wide distribution, but there is a great deal of taxonomic confusion and it is presently thought that the species found within Britain is limited to the north east Atlantic and the Mediterranean. Furthermore, there is a strong possibility that within the Mediterranean it is actually part of a complex of two or three species (ten Hove, pers. comm.). Dr. T. Pillai of the Natural History Museum, London is presently reviewing the genus. S. vermicularis has nevertheless been found throughout Britain from the Shetland to the Channel Islands, particularly on the southern and western coasts (Hayward & Ryland, 1990), but reefs within Britain are reported only from Loch Creran and Linne Mhuirich (at the top end of Loch Sween) (Earll, 1982; Howson et al., 1994). Surveys during 1993 and 1994 failed to find any living reefs in Linne Mhuirich (Moore, 1996). The only other reports found are from two sites in Ireland: Ardbear Lough, Galway; (Bosence, 1973; Bosence, 1979), and Killarey Harbour, Galway (Connor et al., 1997); and ‘the Mediterranean’ (ten Hove, 1979) where it may not necessarily be the same species. Moreover, the Mediterranean aggregations are "at a smaller scale than in Irish waters" (Zibrowius, pers. comm.). McIntosh (1923) noted that examples of S. vermicularis from the south of England, including Falmouth and Exmouth, were often in massive groups of aggregated tubes attached to Pecten, oysters and other bivalves. Allen (1915), describing the polychaete fauna of Plymouth and South Devon reported that "large masses of this species were obtained by a diver somewhere in the Hamoaze and brought to the laboratory", but there were no other similar reports from this area according to the MBA (1957). The MNCR database includes one reference to abundant S. vermicularis in Loch Glendhoo, north-west Scotland, which comprises dense aggregations on bedrock at the base of a steep cliff. The distribution of reefs in Britain and Ireland is shown in the linked figure.

A Perspective on Biogenic Reefs and SACs

References