Freshwater pearl mussel (Margaritifera margaritifera)

GenusMargaritifera (1)
SizeLength: 12 – 15 cm (2)

Classified as Endangered (EN) on the IUCN Red List. Subspecies: Margaritifera margaritifera durrovensis is classified as Critically Endangered (CR) on the IUCN Red List (1).

One of the longest-living invertebrates in existence (2), the freshwater pearl mussel has, like all bivalve molluscs, a shell consisting of two parts that are hinged together, which can be closed to protect the animal’s soft body within (3). The shell is large, heavy and elongated (4) (5), typically yellowish-brown in colour when young and becoming darker with age (2). Older parts of the shell often appear corroded, an identifying feature of this mussel species (6). The inner surface of the shell is pearl white, sometimes tinged with attractive iridescent colours (4). Like all molluscs, the freshwater pearl mussel has a muscular ‘foot’ (3); this very large, white foot enables the mussel to move slowly and bury itself within the bottom substrate of its freshwater habitat (4) (5).

The freshwater pearl mussel can be found on both sides of the Atlantic (4), from the Arctic and temperate regions of western Russia, through Europe to north-eastern North America (2).

Clean, fast-flowing streams and rivers are required for the freshwater pearl mussel (4) (2), where it lives buried or partly buried in fine gravel and coarse sand (2), generally in water at depths between 0.5 and 2 metres, but sometimes at greater depths (4). Clean gravel and sand is essential, particularly for juvenile freshwater pearl mussels, for if the stream or river bottom becomes clogged with silt, they cannot obtain oxygen and will die (4). Also essential is the presence of a healthy population of salmonids, a group of fish including salmon and trout, on which the freshwater pearl mussel relies for part of its life cycle (4).

Capable of living for over an incredible 200 years (7), the freshwater pearl mussel begins life as a tiny larva, measuring just 0.6 to 0.7 millimetres long, which is ejected into the water from an adult mussel in a mass of one to four million other larvae. This remarkable event takes place over just one to two days, sometime between July and September (2). The larvae, known as glochidia, resemble tiny mussels, but their minute shells are held open until they snap shut on a suitable host. The host of freshwater pearl mussel larvae are juvenile fish from the salmonid family, which includes the Atlantic salmon and sea trout (2). The chances of a larva encountering a suitable fish is very low (6), and thus nearly all are swept away and die; only a few are inhaled by an Atlantic salmon or sea trout, where they snap shut onto the fish’s gills (2).

Attached to the gills of a fish, the glochidia live and grow in this oxygen-rich environment until the following May or June, when they drop off. The juvenile must land on clean gravely or sandy substrates if it is to successfully grow (2). Attached to the substrate, juvenile freshwater pearl mussels typically burrow themselves completely into the sand or gravel, while adults are generally found with a third of their shell exposed (2). Should they become dislodged, freshwater pearl mussels can rebury themselves, and are also capable of moving slowly across sandy sediments, using their large, muscular foot (2).

The freshwater pearl mussel grows extremely slowly (6), inhaling water through exposed siphons, and filtering out tiny organic particles on which it feeds (2). It is thought that in areas where this species was once abundant, this filter feeding acted to clarify the water, benefiting other species which inhabited the rivers and streams (2). Maturity is reached at an age of 10 to 15 years (2), followed by a reproductive period of over 75 years in which about 200 million larvae can be produced (6). In early summer each year, around June and July, male freshwater pearl mussels release sperm into the water, where they are inhaled by female mussels. Inside the female, the fertilised eggs develop in a pouch on the gills for several weeks, until temperature or other environmental cue triggers the female to release the larvae into the surrounding water (2).

Once the most abundant bivalve mollusc in ancient rivers around the world, numbers of the freshwater pearl mussel are now declining in all countries and this species is nearly extinct in many areas (4). The causes of this decline are not fully understood, but alteration and degradation of its freshwater habitat undoubtedly plays a central role (4). The negative impacts humans have on rivers and streams come from a wide range of activities such as river regulation, drainage, sewage disposal, dredging, and water pollution, including the introduction of excess nutrients (4). Anything that affects the abundance of the fish hosts will also affect the freshwater pearl mussel; for example, the introduction of exotic fish species, such as the rainbow trout, reduces the number of native fish hosts (4). Introduced species are also directly affecting the freshwater pearl mussel; the invasion of the zebra mussel (Dreissena polymorpha), which has been spread to new locations by being transported on the bottom of boats or in ballast waters, has impacted freshwater pearl mussel populations in all countries it has invaded (4).

The freshwater pearl mussel, which is completely protected in most European countries (9), has been the focus of a significant amount of conservation efforts (8). Measures have included the transfer of adult mussels to areas where it had gone extinct (8) (10), the culture of juvenile mussels, and the release of juvenile trout, which have been infected with glochidia, into small rivers (10), but mainly the freshwater pearl mussel has benefited from habitat restoration projects in some areas (10). Due to the essential role salmonid fish play in the life of the freshwater pearl mussel, the conservation of salmon and trout is also central in the survival of this endangered freshwater mussel (4).

For further information on the conservation of freshwater habitats and biodiversity see:

Information authenticated (01/09/09) by Dr Mark Young of Aberdeen University

  1. IUCN Red List (December, 2009)