Distribution Overview of Biogenic Reefs
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
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
(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