S. vermicularis usually occurs as individuals encrusted on hard surfaces. A tendency to form aggregations is widely reported (e.g. Hayward & Ryland, 1990; Nelson-Smith, 1967; Zibrowius, 1973) but true reefs have an extremely limited distribution as outlined below. On reefs in Ardbear Lough individual worm tubes had an outside mean diameter of 5.2 mm, and a mean length of 120 mm (up to 180 mm), which is slightly larger than in worms from the open sea (Bosence, 1979). Initial growth is encrusting but after that the worm grows away from the substratum in a sinuous fashion, sometimes intertwining, and reefs develop by continued additions of further worms onto the old. Pieces of reef may fall off into the surrounding sediment where they may continue to grow. In Loch Creran individual reefs are reported to reach up to around 75 cm, in height and 1 m across, but adjacent reefs may coalesce to form larger reefs up to 3 m across (Moore, 1996). Bosence (1979) described reefs up to 2 m in height and 1 m across from Ardbear Lough but implied that aggregated reefs could extend for several hundred metres, although the continuity of such areas was not indicated.

The structure usually appears to be quite open, providing lots of cryptic surfaces and spaces for other organisms. This appears to be related to the regular spacing of the tube apertures of worms at 10-15 mm apart in order to avoid interference of the expanded branchial crowns (diameter 15 mm) during feeding (Bosence, 1979).

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