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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.

   Piddocks with a sparse associated fauna in sublittoral very soft chalk or clay

Physical habitat description

Salinity Full (30-35 ppt)
Wave exposure Moderately exposed
Tidal streams Moderately strong (1-3 kn)
Substratum Bedrock
Zone Circalittoral
Depth Band 5-10 m, 10-20 m
Other Features Soft rock: chalk or clay. Often with vertical faces.

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 CR.MCR.SfR.Pid Piddocks with a sparse associated fauna in sublittoral very soft chalk or clay

  • 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.


This biotope occurs on circalittoral soft rock, such as soft chalk or clay, most often in moderately exposed tide-swept conditions. As soft chalk and firm clay are often too soft for sessile filter-feeding animals to attach and thrive in large numbers, an extremely impoverished epifauna results on upward-facing surfaces, although vertical faces may be somewhat richer. The rock is sufficiently soft to be bored by bivalves. Species vary with location, but Pholas dactylus is the most widespread borer and may be abundant. Other species present may include the sponges Dysidea fragilis and Suberites carnosus and the polychaete Bispira volutacornis. Foliose red algae may be present on the harder, more stable areas of rock. Mobile fauna often include the crabs Necora puber and Cancer pagurus.


Subtidal chalk reefs or clay outcrops, mostly known from south-east England.

Temporal variation

Not known.

Characterising species

Taxon Relative importance of taxon for defining this community (%) Typical abundance - SACFOR scale % of core records where taxon was recorded
Pholas dactylus 83 Common 81-100%
Rhodophyta 4 Occasional 21-40%
Bispira volutacornis 2 Occasional 21-40%
Suberites carnosus 2 Frequent 21-40%
Necora puber 1 Rare 21-40%

Similar biotopes or habitat types

This biotope is found on sublittoral fringe soft rock on moderately exposed coasts. It is characterised by boring bivalves and the kelp Laminaria digitata.

Classification history of this biotope or habitat type

Classification version Code
1997 (97.06) CR.MCR.SfR.Pid

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