Maerl is a generic term for red hard coralline algae. It has a hard chalky skeleton that grows as small rounded nodules or short branched twig-like shapes (thalli). In high abundance, maerl can form unattached ‘carpets’ on the sea bed, known as ‘maerl beds’ that act as important habitat for other species. Phymatolithon calcareum is a fragile, branched species of maerl that usually resembles a stag’s horns.

Maerl beds are a Priority Marine Feature (PMF) in Scotland. PMFs are habitats and species that are considered to be marine nature conservation priorities in Scottish waters.

Behaviour

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.

Size

Diameter: 7cm (Phymatolithon calcareum)

Status

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.

Distribution

Scotland has approximately 30% of the maerl beds in north-west Europe and most of the beds in the UK.

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

All year.

Facts

• Individual maerl thalli (the twigs) may live for more than 100 years; extensive maerl beds may be thousands of years old!
• 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.