Freshwater pearl mussel
Relationsship and features of typical shells (taxonomy)
Shell very thick and heavy, elongate, oval and shaped like a "kidney" [compare with the Thick shelled river mussel (U. crassus)]. Shell reaching a total length of 10 - 15 cm and a height of 5 - 7,5 cm. The colour is dark brown-black, to blue and black. Hinge-plate with anterior lateral and cardinal teeth, one in the left valve and two in the right valve. In larger specimens the umbo is mostly lost by corrosion. These injuries are often of yellow colour caused by keratin. To observe ! There is a possible risk of confusion between the Thick shelled river mussel and the Freshwater pearl mussel. The differences in the shape of the hinge-plate is a distinct feature when separating the two species.
The species inhabits clear brooks, streams and rivers, poor in lime. It prefers a bottom with gravel and stones. Here the population densities can become high. The fertilised eggs develop to glochidia during approximately a five week period in the female mussels gill chamber. A female Freshwater pearl mussel produces in average 3 -5 million glochidia during the breeding season ! Host fishes are salmonids, i.e. Brown trout and Salmon. The glochidia stays on the gills of the host fish for a period of 8 - 10 months. The following interstitial period reaches up to several years - not much is known about this, probably very ecologically sensitive period. It has been shown that approximately one out of hundred million larvae (1:100 000 000) develop into a full grown mussel. The Freshwater pearl mussel has disappeared from about half of its previous habitats during the last century and have also been extinct in many places in Sweden. The extensive pearl fishery in former days is a great cause of the decline. Other reasons for the ongoing decline are many; loss of suitable habitats, i. e. destroying of suitable bottom substrate, pollution, acidification, and loss of host fish are the main threats to the species. The Freshwater pearl mussel is a protected species in Sweden. It is also Red listed as a "Vulnerable" (VU) species. Destroying of suitable bottom substrate are probably negative during the interstitial period of the life cycle and is thereby a serious threat. Loss of juvenile mussels in the populations is an early warning that the reproduction is out of function. This is the current situation in many of the populations in the south of Sweden, while the northern part of the Country still have some normally reproducing populations. The shell is, due to the mussels long life span, functioning as an important environmental archive. Even the Freshwater pearl mussels History of Culture is important.
Rare in Sweden. The Freshwater pearl mussel was originally distributed from Scania in the very south to Lappland in the very north. The species shows nowadays important gaps in the distribution, especially to the south and the east of the Country, where farming grounds and limestone rocks are abundant.
Photo: Håkan Holmberg
Scientific basis: Ted von Proschwitz