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Marine Habitat Classification

Description of biotope or habitat type

To understand more about what this page is describing, see How to use the classification. See also How to cite.

   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 Burrows

Biotope origin

Derived using data from Video/still
Faunal group Infauna

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 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 stroemii, the bivalves Nucula sulcata, Varicorbula gibba and Thyasira flexuosa, and the echinoderm Brissopsis lyrifera. The biotope may include a range of the component fauna, potentially including burrowing megafauna and seapens, burrowing megafauna without seapens or seapens without burrowing megafauna depending on local environmental conditions. This biotope is closely related to the biotope SS.SMu.CFiMu.MegMax and SS.SMu.CFiMu.SpnMeg.Fun and may be have infaunal communities similar to the biotopes SS.SMu.CFiMu.BlyrAchi or SS.SMu.IFiMu.PhiVir, depending on environmental factors.


These soft mud habitats occur extensively throughout the more sheltered basins of sea lochs 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. Seapens may be absent in biotope variants impacted by abrasion pressures.

Temporal variation

No temporal data available.

Characterising species

Taxon Relative importance of taxon for defining this community (%) % of core records where taxon was recorded Relative frequency %
Pennatula phosphorea 60 1-20% 9.8
Funiculina quadrangularis 10 1-20% 4.9
Virgularia mirabilis 5 1-20% 6.5
Nephrops norvegicus 5 1-20% 6.5
Cerianthus lloydii 4 1-20% 4.7
Liocarcinus depurator 3 1-20% 3.6
Asterias rubens 3 1-20% 3.2
Amphiura filiformis 1 1-20% 3.1
Pagurus bernhardus 1 1-20% 2.6
Turritellinella tricarinata 1 1-20% 2.5
Seraphsidae 1 1-20% 1.8
Pachycerianthus multiplicatus 1 1-20% 1.6

Similar biotopes or habitat types

SS.SMu.IFiMu.PhiVir is superficially similar to SS.SMu.CFiMu.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 SS.SMu.CFiMu.SpnMeg.Fun from SS.SMu.CFiMu.SpnMeg.

SS.SMu.CFiMu.SpnMeg is closely allied to SS.SMu.CFiMu.BlyrAchi 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.

SS.SMu.CFiMu.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 Habitat name
2015 (15.03) SS.SMu.CFiMu.SpnMeg Seapens and burrowing megafauna in circalittoral fine mud
2004 (04.05) SS.SMu.CFiMu.SpnMeg Seapens and burrowing megafauna in circalittoral fine mud
1997 (97.06) SS.CMU.SpMeg Seapens and burrowing megafauna in circalittoral soft mud
1996 (6.95) CMU.SpNep

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