The ‘Sea pens and burrowing megafauna’ biotope complex is a community type found in subtidal particulate substrata, ranging from muddy sands with admixtures of shell and gravel to fine, clay-dominated muds. It is typically found in areas of full salinity, highly or completely sheltered from wave exposure, and with weak or extremely weak tidal currents. These biotopes do not correspond exactly to any category defined in the Habitats Directive Annex I classification. They could be included within the broad category of ‘Sandbanks covered by sea water at all times’, although this Annex I feature is more typically associated with coarser (sandier) sediment habitats. In geographical terms, particular examples of the biotope complex can be found in ‘Large shallow inlets and bays’.

The biotope complex is widespread around the British Isles but is best-known from the north and west of the UK, particularly the north-eastern Irish Sea (Hughes & Atkinson, 1997) and the Scottish sea lochs (Howson et al., 1994). The designation of the biotope complex refers to a taxonomic grouping, ‘sea pens’ - three species of colonial anthozoan cnidarians - and a functional category, ‘burrowing megafauna’ - a taxomomically-diverse assemblage of crustaceans, worms and fish whose common feature is their construction of large and conspicuous burrows in the sea bed. Although grouped together for current purposes, and often occurring in the same habitat, it is important to note that sea pens and burrowing megafauna are functionally quite dissimilar and not invariably associated with each other. The focus on large animals which are either highly visible (sea pens) or which produce conspicuous traces on the sea floor (burrowing megafauna) should also not obscure the fact that these habitats typically support a rich fauna of smaller, less conspicuous animals living buried within the sediments, including numerous species of nematodes, polychaete worms, bivalves and crustaceans.

The habitat characteristics and community ecology of the ‘Sea pens and burrowing megafauna’ biotope complex will be reviewed in detail in Chapters III and IV. The principal species used in the definition of this biotope complex are identified and briefly described below.

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