Molluscs

Molluscs

Freshwater Pearl Mussel

The freshwater pearl mussel is found on the North Tyne and River Rede. These make up one of only two remaining significant populations in England. A breeding programme for the freshwater pearl mussel is currently being implemented at the Kielder hatchery to try to address the decline in the species.

The freshwater pearl mussel is a bivalve mollusc that lives in fast-flowing, nutrient-poor rivers with clean sandy and stony bottoms. They are filter feeders, extracting fine organic particles from the water. Their shell is oval and elongated and is dark brown or blackish in colour. They can grow up to 15cm long and have a natural life span of up to 80 to 100 years, making them one of the longest-lived known invertebrates.

Freshwater pearl mussels have a very interesting life cycle. During reproduction, females inhale male sperm from the water, producing tiny bivalve larvae called ‘glochidia’ from the fertilised eggs. Each female can shed about 3 million of these in late summer. The larvae lodge on the gills of young salmon and trout, making the presence of good fish populations essential to the mussels’ survival. It is estimated that only 0.1% survive this stage of the life cycle.

In spring the juvenile mussels fall off their host gills, needing a suitable location in clean sand or gravel to bury themselves. There is further massive mortality at this stage as juvenile mussels are eaten by fish. Those that are successful can grow quite rapidly in the right conditions, reaching 2cm in 4-5 years. Adult mussels are known to be eaten by otters and birds. The freshwater pearl mussel is a rare, globally threatened species that had been lost from all but seven rivers in England. Many of the UK populations may not have produced young since the 2nd World War, creating a fragile, aging population.