Biodiversity & Conservation

Maerl - Phymatolithon calcareum

Phymatolithon calcareum

Image Keith Hiscock - Phymatolithon calcareum forming a maerl bed. Loch Carron, West Scotland. Image width ca 20 cm.
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Distribution map

Phymatolithon calcareum recorded (dark blue bullet) and expected (light blue bullet) distribution in Britain and Ireland (see below)

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  • EC_Habitats
  • UK_BAP
Taxonomy icon Taxonomy Taxon English term
Phylum Rhodophyta Red seaweeds
Class Florideophyceae
Authority (Pallas) Adey et McKibbin, 1970
Recent synonyms None
Map icon Recorded Distribution in Britain and Ireland Recorded around the Shetland Orkney Islands and along the east coast of Scotland, south coast of England with isolated records at Bideford Bay, Pembrokeshire and Caernarfon Bay. Abundant around the west coasts of Ireland and Scotland
Habitat information icon Habitat information Typically found in less than 20 m depth on sand, mud or gravel substrata in areas that are protected from strong wave action but have moderate to high water flow. The crustose form is very rare in the British Isles. Usually found as unattached plants forming beds of coralline algal gravel (maerl) in the sublittoral and occasionally lower littoral. Typically found together with Lithothamnion corallioides in the southern British Isles or Lithothamnion glaciale in the northern British Isles.
Text page icon Description A fragile, coralline alga often confused with Lithothamnion corallioides. Its form is very variable but commonly resembles stag's horns of irregular diameter. Older specimens become somewhat erect with nodular branches and are reminiscent of red 'coral'. Unattached plants may reach about 7 cm in diameter with branches up to 6 mm in diameter and mauvish brown in colour. The surface can be smooth or flaky.
Identifying features
  • Thallus comprising either attached crustose plant or unattached branching systems.
  • Unattached plants varying from single branches to subglobular or flattened branching systems, sparsely to densely branched.
  • Branches sometimes confluent, slender to robust, terete to flattened, to 6 mm diameter with apices rounded to flattened.
  • Surface mainly rather smooth or with white flaky areas.
Additional information icon Additional information Maerl is a generic name for certain coralline algae that grow unattached on the sea bed. No crustose plants of Phymatolithon calcareum have been recorded from the British Isles.

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This review can be cited as follows:

Angus Jackson 2007. Phymatolithon calcareum. Maerl. Marine Life Information Network: Biology and Sensitivity Key Information Sub-programme [on-line]. Plymouth: Marine Biological Association of the United Kingdom. [cited 03/09/2010]. Available from: <http://www.marlin.ac.uk/speciesinformation.php?speciesID=4121>