Maerl Phymatolithon calcareum

Maerl is a generic term for purple-pink hard coralline algae that form twig-like shapes (thalli) as unattached ‘carpets’ on the seabed, known as ‘maerl beds’. Phymatolithon calcareum is a fragile, branched species of maerl that usually resembles a stag’s horns. On open coasts exposed to waves, maerl grows as flattened discs.


As a type of ‘coralline’ algae, maerl deposits lime (calcium carbonate) in its cell walls as it grows, creating a hard, brittle skeleton. It grows at a very slow rate, around 1-2 mm per year. Maerl is typically found in waters less than 20m depth on sand, mud or gravel substrata, in areas generally protected from strong wave action. Maerl beds are highly variable. They can range from a thin layer of living maerl on top of a thick deposit of dead maerl, to a deposit of completely dead maerl. Aggregations of living unattached maerl are often called ‘rhodoliths’.

These habitats support a variety of algae and other species including bivalves, urchins, sea cucumbers, brittlestars and anemones. Young scallops in particular seek out living maerl beds as nursery areas.


  • Diameter: 7cm (Phymatolithon calcareum)
  • Lifespan: individual maerl thalli (‘shoots/twigs’) may live for more than 100 years; extensive maerl beds may be thousands of years old.


Maerl beds are listed as a Priority Marine Feature in Scotland and are protected in eleven locations by a network of Marine Protected Areas. Maerl is very sensitive to sedimentation, (which stifles the algae and blocks out light, preventing photosynthesis) and to changes in pH, temperature, wave action, dredging and trawling. If global emissions continue on their current trajectory, and sea water temperatures continue to rise, an 84% decline in maerl bed distribution around Scotland is projected.


Maerl beds are found along the entire west coast of Britain, but the vast majority are in Scotland where they are widespread along the west coast, the Hebrides and the Northern Isles. Together, they represent about 30% of all maerl beds in north-west Europe. Best seen at some of the protected maerl bed habitats such as, off the coast of south Arran, Wester Ross, Sound of Barra (site of the most extensive maerl bed in Scotland), Loch Carron, Wyre and Rousay Sounds (Orkney) and Fetlar to Haroldswick (Shetland). South Skye and the coast of Lochalsh from Broadford to Plockton have some of the best and most biodiverse maerl beds.

When to see

January to December


  • The white beaches of Western Scotland are actually made of washed up maerl that have been crushed by the waves and bleached in sunlight.
  • Maerl beds provide a carbon-storing habitat processing carbon from the atmosphere and helping mitigate the impacts of climate change.
  • Three species are most abundant in the UK and their relative composition varies with latitude, salinity and wave exposure. Phymatolithon calcareum is typically found together with Lithothamnion corallioides in the southern British Isles or Lithothamnion glaciale in the north.
  • Maerl is sometimes referred to as “the trees of the sea” because, like trees, they form annual rings which record their growth. Cutting them open gives a unique record of sea temperatures and chemistry, stretching back long before instrument records began.

Common name


Species name

Phymatolithon calcareum

IUCN Red List status

Listed as a Priority Marine Feature in Scotland and protected in Marine Protected Areas

When to see in Scotland

January to December

Where to see in Scotland

Scottish Wildlife Trust snorkel trails explore the coasts of Scotland, and coastal reserves such as Hill of White Hamars and Isle and Eigg.

Stay up to date with the Scottish Wildlife Trust by subscribing to our mailing list Subscribe now

Back to top