Seapens and burrowing megafauna in circalittoral soft mud
|Within Vocab||Marine Habitat Classification for Britain and Ireland Version 97.06|
|Definition||Plains of fine mud at depths greater than about 15 m may be heavily bioturbated by burrowing megafauna; burrows and mounds may form a prominent feature of the sediment surface with conspicuous populations of seapens, typically Virgularia mirabilis and Pennatula phosphorea. These soft mud habitats occur extensively throughout the more sheltered basins of sealochs and voes and are present in quite shallow depths (as little as 15 m) in these areas probably because they are very sheltered from wave action. This biotope also seems to occur in deep offshore waters in the North Sea, where densities of Nephrops norvegicus may reach 68 per 10 m-2 (see Dyer et al. 1982, 1983), and the Irish Sea. The burrowing crustaceans present may include Nephrops norvegicus, Calocaris macandreae or Callianassa subterranea. The former of these species is the only one frequently recorded from surface observations, whilst grab sampling may fail to sample any of these species. Indeed, some forms of sampling may fail to indicate seapens as characterising. The crab Goneplax rhomboides may sometimes be recorded, again rarely, in this habitat. Large mounds formed by the echiuran Maxmuelleria lankesteri are also present in some sealoch sites. It is unclear from the data examined whether differences in the balance of species composition from site to site represent additional biotopes within this assemblage. Pachycerianthus multiplicatus is quite specific to this habitat and is scarce in Great Britain (Plaza & Sanderson 1997). The ubiquitous epibenthic scavengers Asterias rubens, Pagurus bernhardus and Liocarcinus depurator are present in low numbers. The brittlestars Ophiura albida and Ophiura ophiura are sometimes present, but are much more common in slightly coarser sediments. In the deeper fiordic lochs which are protected by an entrance sill, the tall seapen Funiculina quadrangularis may also be present (CMU.SpMeg.Fun). The brittlestars Amphiura chiajei and Amphiura filiformis may be present in large numbers, although there may be some sites, where these species are absent. The infauna may contain significant populations of the polychaetes Pholoe spp., Glycera spp., Nephtys spp., spionids, Pectinaria belgica and Terebellides stroemi, the bivalves Nucula sulcata, Corbula gibba and Thyasira flexuosa and the echinoderm Brissopsis lyrifera , although the latter may not be frequently found in remote samples. Overall, CMU.SpMeg is closely allied to CMU.BriAchi and COS.ForThy and shows strong similarities in infaunal species composition. It may differ from these biotopes as a result of a lack of disturbance or linkage to productive overlying waters (?). IMU.PhiVir is superficially similar to CMU.SpMeg but is found in shallower, less thermally stable waters and lacks the large burrowing species.|
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