Maerl - A Rocky Seaweed

The beautiful, shining white 'coral' beaches of western Scotland are made, not of coral, but of dead fragments of a strange hard seaweed called maerl, crushed by the waves and bleached by the sun. Living maerl is a beautiful purple-pink, and forms spiky underwater 'carpets' on the seabed

Why is maerl important?

Maerl beds provide vital shelter for a wide range of marine creatures. Experiments have shown that young scallops in particular have a strong preference for living maerl beds as nursery areas. Protecting maerl beds therefore helps to sustain scallop fishing, important commercially in western Scotland. It is ironic, therefore, that scallop dredging has been shown to cause significant damage both to maerl beds - by breaking up and burying the thin layer of living maerl - and to their associated species. Maerl is fragile and slow-growing, and can also be damaged by heavy anchors and mooring chains.

Protecting living maerl beds is vital to the wise management of Scotland seas. A Habitat Action Plan   for maerl beds is being taken forward by the Scottish Government as part of the Scottish Biodiversity Strategy.

What is maerl?

Maerl is an unlikely seaweed - an unusual unattached form of a group of red seaweeds called 'coralline' algae. These seaweeds deposit lime in their cell walls as they grow, giving them a hard, brittle skeleton. Coralline algae are more familiar as hard, pink coatings lining rock pools on the seashore. Maerl is an unattached version, which grows as small branched nodules on sandy seabeds. There are two common species of maerl in Scotland: Phymatolithon calcareum is widespread, and Lithothamnion glaciale is more northern in its range, although it is not easy to tell them apart. On open coasts exposed to some waves, maerl grows as flattened discs, but where there is less wave action, it often forms dense, branched spiny nodules up to 10cm in diameter.

Maerl beds

In many areas of the west coast, and in sealoch narrows, extensive beds of the beautiful purple-pink living maerl develop, together with old skeletons of dead white maerl, which builds up into a deep gravel beneath. These beds are an important habitat for a wide variety of smaller marine plants and animals, which live amongst their branches. They are particularly good places for juvenile animals to hide from predators. Many bivalves, urchins, sea cucumbers, anemones and worms burrow in the maerl gravel beneath the living bed

In some places where wave movement periodically stirs the seabed, maerl beds develop into a 'ridge and furrow' formation, with the coarser material - live maerl, pebbles and dead mollusc shells - lying in the furrows, and maerl gravel forming the ridges between. Seaweeds often grow on the shells and pebbles in the furrows, making the whole seabed look like a ploughed field with a seaweed 'crop' planted in the furrows. Some characteristic and rare seaweeds grow in this habitat.

The value of maerl

Because maerl is rich in lime, it has been collected in the past, and used as a soil conditioner, or as a filter for acid drinking water. Commercial-scale extraction has been proposed (but never taken forward) from Barra in the Western Isles, and experimental dredging has been carried out in Wyre Sound on Orkney.

Today, we are recognising increasingly that the true value of maerl is as a living community, for the many animals that shelter amongst it, and the commercially valuable species it helps support