Freshwater Pearl Mussel

The freshwater pearl mussel is a large filter-feeding bivalve that is listed as critically endangered on the IUCN red list (Moorkens, 2011). A bivalve is a type of mollusc with a body that is almost completely enclosed between a pair of shells. For most of its life, it is a filter feeder and large quantities of water are pumped through the animal’s siphons, food particles are trapped and passed to the mussel’s mouth. The freshwater pearl mussel burrows to two-thirds of its shell depth, and is almost sessile in nature; some individuals never move after they reach maturity. The species is particularly notable in that individuals can grow to very large sizes relative to the other freshwater molluscs, building up thick, calcareous valves, in rivers that are oligotrophic, with low levels of calcium. Shell building is consequently very slow and individuals often live to over a hundred years of age.

Lifecycle

Freshwater pearl mussels have a complex reproductive cycle with separate male and female animals.  Reproduction occurs when sperm are released into the open water via the male’s exhalent siphon in June, and are carried to the eggs via the female’s inhalant siphon.  Following fertilisation within the female’s brood chambers, the eggs develop into a larval stage known as glochidia.  Glochidia are temporarily brooded in the female’s gills from June each year, and are then released into the open water in high numbers in an event lasting one to two days between July and September.  The glochidia must then be inhaled and attach to the gills of their intermediary host within 24 hours, i.e. Salmo salar or Salmo trutta.  The glochidia that survive on the fish develop into juvenile mussels.  They fall off in early summer (normally June) and bury themselves into gravel on the river bed for approximately five years, until large enough to withstand the flow of open water, moving stones and fish predation.  High levels of mortality are associated with each stage of the life cycle, however, the juvenile stage living in the river bed is the most sensitive stage.  Freshwater pearl mussels mature between seven and 15 years of age, and can have a prolonged fertile period lasting well into old age.

Habitat

Freshwater pearl mussels in Ireland are restricted to oligotrophic, acid to neutral waters of rivers flowing over granite or sandstone bedrock, often downstream of oligotrophic lakes. The species requires stable cobble and gravel substrates, with very little fine material. Where fine sediment is absent from the river bed, free water exchange occurs between the open river and the water within the gravel substrate (the interstitial zone). This facilitates continuous, high oxygen levels essential for juvenile survival.