Freshwater Pearl Mussel
Habitat and Distribution
The freshwater pearl mussel has very specialised habitat requirements which are found in many upland rivers in Scotland. Pearl mussels live partially or totally buried in coarse sand or fine gravel, often around boulders and other large rocks that help to stabilise the river bed.
Pearl mussels also require fast-flowing water that is free from pollution. But, despite having a shell that is made from calcium, they are only found in water that is low in this mineral. Hence their original distribution in Scotland reflects that of non-calcareous rock. They were once found throughout Scotland except for the Tweed catchment, parts of the Central belt and the sandstone area of Caithness. But now, due to a number of threats, pearl mussel distribution has been reduced to a few remnant populations in Southern Scotland and some more abundant populations in the Highlands.
Pearl mussels are known to be sensitive to pollution with lower oxygen levels, slight increases in nutrient levels, silt and heavy metals all causing damage. As mussels often live buried in the river bed and require clean flowing water, anything that clogs up the spaces in the sediment can be extremely damaging. This is particularly true for juveniles that most often live entirely buried in the river bed and they can be the most vulnerable to enriched or silty conditions.
In the few populations where pearl mussels are still abundant, it is thought they can play an important role in helping to cleanse the river water. An adult can filter up to 50 litres of water every day and therefore a very large population will help to maintain the high water quality needed by the resident fish species and other animals and plants living in the river.