Commonly known as the ‘coral beach’, the popular white beach at Ardban is actually made from the calcareous remains of species of red algae, not true coral at all. Maerl is a collective term for several species of calcified red seaweed; Phymatolithon calcareum,Lithothamnion glaciale and Lithothamnion corallioides. The former two species occur in Scotland but the latter appears to be a south-western species with no Scottish records to date. It grows as unattached nodules on the seabed and can form extensive beds in favourable conditions. It is slow growing, but over long periods its dead calcareous skeleton can accumulate into deep deposits, overlain by a thin layer of pink living maerl.

Maerl grows in relatively exposed places near sealoch entrances as well as in tidal narrows or the straits and sounds between islands. Live maerl has been found at depths of 40m, but beds are typically much shallower, usually above 20m and extending up to the low tide level.

Maerl beds are an important habitat for a wide variety of marine animals and plants, which live amongst or are attached to its branches, or burrow in the coarse gravel of dead maerl beneath the top living layer. Larvae and young stages of commercially important fish and shellfish are known to live amongst the branches, while a rich community of bivalves, burrowing urchins, sea cucumbers and worms inhabit the gravel.

Maerl can be used as an effective soil conditioner on acidic ground, and although on a small scale this isn’t a problem large scale dredging is very destructive to this fragile habitat. Scallop dredging, heavy anchors and mooring chains are also likely to cause considerable damage to living maerl beds.