S. spinulosa is very commonly reported to be found in solitary form (e.g. George & Warwick, 1985; Hayward & Ryland, 1990). A number of published records of local marine fauna refer only to the widespread presence of individuals, or at least fail to mention dense aggregations, reefs or accretions (eg around the Plymouth area, MBA, 1957; the Isle of Man, Bruce et al., 1963; and North East England, Garwood, 1982).

Wilson (1971) regarded it surprising that in the Plymouth area S. spinulosa is "almost entirely solitary", occurring commonly on stones and shells, while in the North Sea it is "frequently colonial"; he carried out settlement experiments which "lead one to expect them [colonies]".

The following is pers. comm. information from Rohan Holt (MNCR): Sabellaria spinulosa appears to be almost ubiquitous on much of the Welsh coast, at least in the north which tends to be more silted or sediment influenced. It occurs from the sublittoral fringe to the circalittoral, with many records from 15-30 m, and seems to be capable of growing on a variety of substrata, including kelp holdfasts, rock and less consolidated sediments such as stony sand or gravel. In some parts of West Wales it grew almost to the exclusion of everything else, and frequently formed sheets up to 2 or 3 cm thick. More often, north west of Anglesey and in other tide swept sites near sediment plains, it forms an underlying thin crust often covered by ascidians and the erect bryozoan Flustra foliacea. Thick crusts, sometimes extensive, were found off Northumberland and North Yorkshire by MNCR surveys.

Attrill (pers comm) found large aggregations of S. spinulosa, up to approximately 20 cm in diameter, while beam trawling on sand in the outer Thames estuary. In other parts of the Thames estuary it formed the more usual sheets or crusts, though these appeared to be thin and not particularly extensive.

Many records of S. spinulosa might just about qualify as biogenic reef communities on the basis of their strong alteration of habitat, but it seems that this species rarely forms substantial raised areas. Two of the most likely areas in which it may do so are those reported by George & Warwick (1985) in the Severn Estuary, for which detailed descriptions are lacking, and the mouth of the Wash, where true reefs protruding up to 60 cm above the surrounding seabed and extending more or less continuously for hundreds of metres have been seen on underwater video (Foster-Smith, in prep). The latter were described as being somewhat similar to reefs of S. alveolata though the arrangement of the tubes is far more irregular.

The only other clear report of truly massive, reef like structures for S. spinulosa is that of Linke (1951) in the southern North Sea where he found reefs up to 60 cm thick, 8 m wide and 60 m long (see chapter IV for more details).

There are also few details of density of animals; however, in the Bristol Channel George & Warwick (1985) reported a mean of over 4,000 individuals m^-2; off Lundy an average of 6,800 m^-2 was found on the wreck of the MV Robert (Hiscock & Rostron unpubl.); and in the south east of England up to 1,600 per individual grab sample has been recorded, although crusts in the Thames estuary contained only up to 228 m^-2 (both Attrill, pers. comm.).

The worms and tubes themselves are rather smaller than those of S. alveolata, and structured, ‘honeycomb’ like arrangements have never been reported for S. spinulosa.

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