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Marine Habitat Classification

Description of biotope or habitat type

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   Crepidula fornicata and Mediomastus fragilis in variable salinity infralittoral mixed sediment

Physical habitat description

Salinity Variable (18-35 ppt)
Wave exposure Very sheltered, Extremely sheltered
Tidal streams Moderately strong (1-3 kn), Weak (>1 kn), Very weak (negligible)
Substratum Mixed muddy sediment
Zone Infralittoral
Depth Band 0-5 m, 5-10 m, 10-20 m
Other Features

Biotope origin

Derived using data from Various
Faunal group Epifauna/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.SMx.SMxVS.CreMed <I>Crepidula fornicata</I> and <I>Mediomastus fragilis</I> in variable salinity infralittoral mixed sediment

  • 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.


Variable salinity mixed sediment characterised by the slipper limpet Crepidula fornicata and the polychaetes Mediomastus fragilis and Aphelochaeta marioni. Other numerically important taxa include the oligochaetes Tubificoides benedii, syllids such as Exogone naidinaand Sphaerosyllis, and other polychaetes Nephtys hombergii. Lepidonotus squamatus and Scoloplos armiger may also be common. Shell debris and cobbles are colonised by encrusting worms, the ascidians Ascidiella aspersa, Ascidiella scabra, Molgula sp. and Dendrodoa grossularia (the ascidians may not be recorded adequately by remote infaunal survey techniques).


This biotope occurs in the lower estuary where currents allow a stable environment to develop. It is associated with oyster beds and relict oyster beds (SS.SMx.IMx.Ost) in southern England and Wales. It may be found adjacent to or in conjunction with SS.SMu.SMuVS.AphTubi and SS.SMx.SMxVS.AphPol. It may be associated with SS.SMx.IMx.VcorAsquAps and possibly form a component of SS.SMu.ISaMu.CundAasp.

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
Crepidula fornicata 85 Common 81-100%
Aphelochaeta marioni 11 Common 61-80%
Crepidula fornicata 11 Abundant 81-100%
Mediomastus fragilis 8 Common 61-80%
Carcinus maenas 7 Occasional 21-40%
Tubificoides benedii 7 Common 61-80%
Hydrozoa 5 21-40%
Exogone naidina 4 Frequent 61-80%
Sphaerosyllis 4 Frequent 61-80%
Nephtys hombergii 3 Common 61-80%
Harmothoe impar 2 Common 41-60%
Lepidonotus squamatus 2 Common 61-80%
Nephtys 2 Common 41-60%
Polydora ciliata 2 Common 21-40%
Scoloplos armiger 2 Abundant 61-80%
Tubificoides swirencoides 2 Common 21-40%
Abra alba 1 Common 21-40%
Capitella capitata 1 Frequent 41-60%
Carcinus maenas 1 Abundant 41-60%
Chaetozone zetlandica 1 Frequent 21-40%
Cirriformia tentaculata 1 Common 21-40%
Melinna palmata 1 Common 41-60%
Tharyx killariensis 1 Frequent 21-40%
Tubificoides pseudogaster 1 Frequent 21-40%

Similar biotopes or habitat types

SS.SMx.IMx.CreAsAn is found in more exposed marine areas with a lower silt content and in fully marine conditions.

SS.SMx.SMxVS.CreMed may be separated from SS.SMu.SMuVS.AphTubi by the relative abundances of the slipper limpet Crepidula fornicata in addition to A. marioni.

SS.SMx.SMxVS.CreMed may be distinguished from SS.SMx.SMxVS.AphPol by the abundance of C. fornicata and its sediment characteristics. SS.SMx.SMxVS.AphPol may also be found in reduced salinity conditions.

Classification history of this biotope or habitat type

Classification version Code Habitat name
2015 (15.03) SS.SMx.SMxVS.CreMed Crepidula fornicata and Mediomastus fragilis in variable salinity infralittoral mixed sediment
2004 (04.05) SS.SMx.SMxVS.CreMed Crepidula fornicata and Mediomastus fragilis in variable salinity infralittoral mixed sediment
1997 (97.06) SS.IMX.EstMx.CreAph Crepidula fornicata and Aphelochaeta marioni in variable salinity infralittoral mixed sediment

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