S. alveolata creates tubes of coarse sand grains cemented together, dense aggregations of which may be regarded as reefs. Details of tube structure and form are given in Wilson (1971) for Cornish colonies and in Vovelle (1965) for French communities. Wilson (1971) observed that tubes can vary in colour depending on the source of sand grains; shell fragments are taken up in preference to mineral grains though the latter are used where shell fragments are not present. The tubes in Cornwall were found to be up to 20 cm in length and up to around 5 mm in diameter at the external opening. Each tube has an additional cup shaped ‘porch’ area at the outward end which is up to 10 mm ‘deep’ and 15 mm across. The head and tentacles protrude into this area when submerged. Single tubes are rarely straight and may be strongly recurved, but aggregated tubes are often less so.Cunningham et al. (1984), described three forms of aggregations within Britain, based largely on the work of Gruet (1982). These were:

• extensive sheets in which the tubes overlap and lie at an acute angle to the substratum (‘placages’ sensu Gruet, 1982). These occur on low lying, even beaches with more or less homogeneous substratum;
• hummocks in which the tubes radiate out from the initial settlement point (‘champignons’, sensu Gruet, 1982). These are found on more heterogeneous substrata such as large boulders and rock outcrops on sand or shingle. Colonies of this type such as those described by Wilson (1971) in Cornwall can be a metre or two across, though it is sometimes unclear to what extent this overlaps with the third form (below);
• ‘reefs’, formed of more extensive areas of hummocks fused together, in which strange shapes can result from a combination of further growth, new larval settlement and erosion. There is clearly a likelihood of overlap with the hummock form (above). Reefs of up to around 50 cm thickness have regularly been reported (Wilson, 1971; Gruet, 1982; Cunningham et al., 1984).

For the purposes of this report, all three of the above are considered to fit with the definition of biogenic reef communities, though in the case of the first two there could clearly be some instances in which the designation might not be appropriate.

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