The Marine Nature Conservation Review (MNCR) biotope classification provides a hierarchical framework for differentiating and classifying the shallow-water benthic habitats and biological communities of the British Isles (Connor et al., 1997). The basic unit of classification is the Biotope, a recognizeable Community of conspicuous species occurring in a Habitat, defined according to parameters of the physical environment such as substratum type or degree of wave exposure. Groups of biotopes with similar overall character, suitable for local mapping where biotopes consistently occur together and are relatively restricted in their extent, are termed Biotope complexes. For the purposes of this report, the ‘Sea pens and burrowing megafauna’ biotope complex is taken to include all biotopes containing either sea pens or burrowing megafauna as characterizing species (as opposed to considering only those biotopes in which both groups are present). This inclusive definition has the advantage of including situations where, for example, biotopes occur along a gradient of water depth or sediment type, with biological communities changing as a result of the differing environmental requirements of the characterizing species. The relevant biotopes from the MNCR classification are summarised below. Full descriptions are given in Connor et al. (1997).

1. MNCR Code CMU.SpMeg Sea pens and burrowing megafauna in circalittoral soft mud

2. MNCR Code CMU.SpMeg.Fun Sea pens, including Funiculina quadrangularis, and burrowing megafauna in undisturbed circalittoral mud

These are the typical deep mud biotopes of the Scottish sea lochs, characterized by the sea pens Virgularia mirabilis and Pennatula phosphorea, and the megafaunal burrowers Nephrops norvegicus, Callianassa subterranea, Calocaris macandreae, Maxmuelleria lankesteri and Lesueurigobius friesii. The biotope coded as CMU.SpMeg has been recorded in most of the Scottish sea lochs (Howson et al., 1994) and in the Shetland voes (Howson, 1988). It has also been observed by towed camera surveys in the north-eastern Irish Sea (Hughes & Atkinson, 1997) and in the deep offshore waters of the North Sea (Dyer et al., 1982). These offshore examples of the biotope cover extensive areas and form the major Nephrops fishing grounds. The biotope coded CMU.SpMeg.Fun is a variant recorded in the deeper basins of some of the Scottish sea lochs, characterised by forests of the larger sea pen Funiculina quadrangularis.

3. MNCR Code CMU.BriAchi Brissopsis lyrifera and Amphiura chiajei in circalittoral mud

This deep, offshore mud biotope is characterized by the urchin Brissopsis lyrifera and the brittlestar Amphiura chiajei. The megafaunal burrowers Nephrops norvegicus and Calocaris macandreae may also be present. Connor et al. (1997) recognize this biotope from the northern Irish Sea off the coast of Cumbria. Its status requires clarification, and it may turn out to be identical with CMU.SpMeg, with the apparent differences resulting from contrasting survey techniques (D. W. Connor, personal communication).

4. MNCR Code CMS.AfilEcor Amphiura filiformis and Echinocardium cordatum in circalittoral clean or slightly muddy sand

This biotope exists in conditions of slightly greater wave exposure, and consequently coarser-grained sediments than are typical of CMU.BriAchi. An infaunal brittlestar and urchin are again the main characterizing species, but the community also includes Callianassa subterranea. The sea pen Virgularia mirabilis may also occur, but not in large numbers. The biotope is widespread around the British Isles, being recorded from a number of Scottish sea lochs, from the northern Irish Sea, the central and southern North Sea and the Isles of Scilly (references given in Connor et al., 1997).

5. MNCR Code CMS.VirOph Virgularia mirabilis and Ophiura spp. on circalittoral sandy or shelly mud

6. MNCR Code CMS.VirOph.Has Virgularia mirabilis and Ophiura spp. with hydroids and ascidians on circalittoral sandy or shelly mud with shells or stones

The sea pen Virgularia mirabilis may occur in moderate numbers on sandy or shelly substrata such as occur in many sea lochs, usually at shallower depths than the finer muds supporting the CMU.SpMeg biotope. The brittlestars Ophiura spp. are the other major characterizing species. The variant CMS.VirOph.HAs is distinguished by the greater numbers of small stones and shells on the sediment surface, which provide a substratum for attached hydroids, ascidians and other epifauna. Both biotopes are recognized in the Scottish sea lochs (Connor et al., 1997).

7. MNCR Code IMU.PhiVir Philine aperta and Virgularia mirabilis in soft stable infralittoral mud

Virgularia mirabilis may also occur at high densities on fine-grained and physically very stable muds, typically in shallow water (to 12 - 15 m). The opisthobranch gastropod Philine aperta is usually very common. Burrowing megafauna are generally rare or absent. In the UK this biotope is almost confined to the most sheltered basins of certain sea lochs, with one example known from Portland Harbour in southern England. Further south it can be recognized in the Gulf of Gascony and the Mediterranean (Connor et al., 1997).

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