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Marine Habitat Classification

Description of biotope or habitat type

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   Seapens and burrowing megafauna in circalittoral fine mud

Physical habitat description

Salinity Full (30-35 ppt), Variable (18-35 ppt)
Wave exposure Moderately exposed, Sheltered, Very sheltered, Extremely sheltered
Tidal streams Weak (>1 kn), Very weak (negligible)
Substratum Mud
Zone Circalittoral
Depth Band 10-20 m, 20-30 m, 30-50 m
Other Features

Download comparative physical and biological data. The comparative tables enable a rapid comparison of the species composition and principal physical characteristics between a given set of biotopes.

Distribution of habitat SS.SMu.CFiMu.SpnMeg Seapens and burrowing megafauna in circalittoral fine mud

  • Records used to define the biotope (core records)
  • Other records assigned to this biotope, marked as 'certain'
  • Other records assigned to this biotope, marked as 'uncertain'
  • Predicted extent of the level 3 (for sublittoral rock & deep sea) or 4 (for sublittoral sediment) habitat

Point data based on records in the UK Marine Recorder Snapshot. Predicted habitat extent is from UKSeaMap.


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. The burrowing crustacea present typically include Nephrops norvegicus, which is frequently recorded from surface observations although grab sampling may fail to sample this species. Indeed, some forms of sampling may also fail to indicate seapens as characterising species. 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 anemone Cerianthus lloydii and the ubiquitous epibenthic scavengers Asterias rubens, Pagurus bernhardus and Liocarcinus depurator are present in low numbers in this biotope whilst the brittlestars Ophiura albida and Ophiura ophiura are sometimes present, but are much more common in slightly coarser sediments. Low numbers of the anemone Pachycerianthus multiplicatus may also be found, and this species, which is scarce in the UK, appears to be restricted to this habitat (Plaza & Sanderson 1997). 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.


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.

Temporal variation

No temporal data available.

Characterising species

Taxon Relative importance of taxon for defining this community (%) Typical abundance - SACFOR scale % of core records where taxon was recorded
Hydractinia echinata 1 Occasional 21-40 %
Virgularia mirabilis 20 Frequent 61-80 %
Pennatula phosphorea 6 Frequent 41-60 %
Cerianthus lloydii 13 Occasional 61-80 %
Chaetopterus variopedatus 2 Occasional 21-40 %
Nephrops norvegicus 18 Frequent 61-80 %
Pagurus bernhardus 5 Occasional 41-60 %
Liocarcinus depurator 8 Occasional 41-60 %
Turritella communis 3 Frequent 21-40 %
Asterias rubens 8 Occasional 61-80 %
Amphiura chiajei 2 Common 21-40 %

Similar biotopes or habitat types

PhiVir is superficially similar to SpnMeg but is found in shallower, less thermally stable waters and lacks the large burrowing species and the sea pen Pennatula phosphorea.

In the deeper fjordic lochs which are protected by an entrance sill, the tall seapen Funiculina quadrangularis may also be present and distinguishes SpnMeg.Fun from SpnMeg.

SpnMeg is closely allied to BriAchi and may show strong similarities in infaunal species composition. However, epifaunally they differ in the abundance of characterising species such as Pennatula phosphorea and Amphiura spp.

MegMax is found in similar habitats but has reduced numbers of seapens and a wider range of megafauna.

Classification history of this biotope or habitat type

Classification version Code
97.06 CMU.SpMeg
96.7 CMU.SpNep

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