Freshwater Pearl Mussel Ark Project

In 2007, a partnership project between the FBA, Natural England and the Environment Agency was set up to create an ‘ark' facility at Windermere to conserve dwindling populations of the freshwater pearl mussel Margaritifera margaritifera from English rivers, and to develop a captive breeding and reintroduction programme. Over the past four years, the FBA has received populations of mussels from nine rivers across England. The aim is to captive breed from these adults and reintroduce juveniles into their native rivers. Captive rearing activities have been successful over the past four years and we have reared juveniles to over three years old (right). The overall project aims are to:

1) Create an ark facility to maintain rescued populations from rivers;
2) Develop a captive breeding programme and;
3) Develop a reintroduction strategy for these populations.

About the species

The freshwater pearl mussel Margaritifera margaritifera is classified as Endangered on the IUCN Red List of Endangered Species due to its unprecedented, worldwide decline during the latter part of the 20th Century. Many factors have contributed to the decline including pearl fishing, pollution, siltation, and declines in host fish populations.

There are still several million individuals living in the wild with English and Welsh populations making up over 500,000 of these. Scotland is an international stronghold with an estimated 12 million pearl mussels in the cleaner rivers, but most of these occur in one river. Pollution and siltation cause particular problems for pearl mussels. Despite adults reproducing successfully, there continues to be a near total loss of juveniles annually due to anoxic conditions in river gravels. The result is aging populations with the youngest individuals in some rivers being over 40 years old.

M. margaritifera have a very interesting and complex life cycle which requires a host fish for their larvae (glochidia).

Life History

The freshwater pearl mussel has a very long life-span, commonly reaching ages of over 120 years! The species lives buried in clean gravel in oligotrophic rivers and feeds by taking water in through the inhalant siphon and sifting out food-material before expelling the water through the exhalant siphon.

M. margaritiferais normally dioecious (separate sexes), with males releasing sperm into the water column in June – July. Sperm is inhaled by the females and fertilises eggs which are in a specialised sac next to the gills. Depending upon water temperature, larvae (known as glochidia) are released into the water column between July and September. A single female can release up to 4 million glochidia pear year, each measuring 60-70µm in length. Glochidia require a salmonid fish host (Atlantic salmon, Salmo salar or brown trout, S. trutta in the UK) for the next stage in their development. Glochidia are inhaled by the host and as water passes over the fish gills, the glochidia snap shut onto the gill filaments. Glochidia become encysted within the gill tissue and grow there until the following spring when they drop off the fish in May or early June (around 400µm in size). Juveniles must land in clean, well oxygenated gravel substrates where they will burrow into the interstices.

Mussels reach sexual maturity between 10 and 15 years of age, when the shell length exceeds 6.5cm. It is thought adult mussels do not have an upper age limit for reproductive activity and continue to reproduce until they die.

For more information on the Freshwater Pearl Mussel Ark Project at the FBA please contact Louise Lavictoire (llavictoire@fba.org.uk) or Roger Sweeting (rsweeting@fba.org.uk).