Worm reefs in the Atlantic littoral zone

The sedentary polychaete Sabellaria alveolata (honeycomb worm) is a sessile, tube-dwelling species which builds its tubes from sand and shell fragments held together with biological cement. Colonies form on fixed and stable substrates where there is a plentiful supply of sediment on the mid to lower shore. The colonies can form relatively quickly and may take the form of sheets, hummocks and reefs as well as evolving from globular formations into reef platforms. In the Bay of Mont Saint-Michel, France, they create irregularly shaped, patchy banks that cover approximately 225ha. These are considered to be the largest reef formation in Europe with worm densities estimated to reach up to 60,000 individuals/m² and reef structures more than 2m thick. At the other extreme the S.alveolata reefs at the edge of its range on the central coast of Portugal and north of the Cumbrian coast in the UK tend to be very scattered and not particularly extensive. 

S.alveolata reefs can be relatively unstable and undergo a natural cycle of development and decay. As new individuals prefer to settle on active colonies or the remains of old colonies, the age and morphology of reefs are not directly related to the age of individual worms which are typically 4-5 years with a likely maximum of 9 years. The species assemblages found on S.alveolata reefs are unique because they are composed of a mixture of species typically found on hard structures, sandy and muddy sediments as well as from subtidal, intertidal and terrestrial habitats (insect larvae).

Indicators of Quality:

The overall quality and continued occurrence of this habitat is largely dependent on the presence of S. alveolata which creates the biogenic structural complexity on which the characteristic associated communities depend. The density and the maintenance of a viable population of this species is a key indicator of habitat quality, together with the visual evidence of presence or absence of physical damage. Scientists working on the Mont-Saint-Michel S. alveolata reefs defined the vitality status of the reef by integrating the physical characteristic of the reef and its dynamics. This took into account the degree of fragmentation of reef features, cover by species which are known to degrade, smother and break up areas of S.alveolata reef (the oyster Crassostrea gigas and the mussel Mytilus galloprovinciallis) and the prevalence of different structural characteristics within the reef formations. In Morecambe Bay (UK), the health of the reefs was determined with reference to the percentage of newly settled worms, those with crisp apertures, to those with worn apertures and dead worms. A generic and universally applicable health metric has still to be developed.  

Synthesis of Red List assessment

Confidence in the assessment

Pressures and threats