It’s breeding season at the Freshwater Biological Association’s Species Recovery Centre in the Lake District

Words, videos & photos by Yasmin AliEskandari, Pearl Mussel Project Officer

It’s that time of year here at the Freshwater Biological Association’s Species Recovery Centre – breeding season!

The Species Recovery Centre is a bespoke facility nestled on the shores of Windermere in the Lake District where the Freshwater Biological Association has been conducting research since 1929.

In 2007, it established a Freshwater Pearl Mussel Ark where it is captive breeding the critically endangered Freshwater Pearl Mussel (Margaritefera Margaritifera).

How freshwater pearl mussels procreate

Freshwater pearl mussels can live to be over 100 years old and start to breed from around 12 years of age (when they reach about 6.5 cm in length), so captive breeding of this species is a slow but rewarding process.

Around June, the male mussels release their sperm, which are inhaled by the female to fertilise her eggs, and the developing larvae (glochidia) brood within the female gills.

Glochidia is developed

When the glochidia are ready, the female mussel will release them into the water column – as many as 1 to 4 million glochidia are released, but only a small percentage may survive.  At this stage, the glochidia cannot obtain their own nutrients so they need to attach (encyst) on to the gills of the right salmonid host – some mussel populations prefer salmon and some prefer trout although it’s unclear exactly why!

The glochidia ‘snap’ shut on the fish gills and a protective cyst forms around each one – this is where the term ‘encystment’ comes from. We can test whether any released glochidia are truly viable by taking a small sample, introducing some salt water and observing through a microscope – if the glochidia snap shut, then we know they are ready to find a suitable host.

The encystement process

Once attached, the glochidia will stay encysted on the fish gill for around nine months to undergo metamorphosis. When they’re fully formed, the glochidia (which has transformed into a juvenile mussel) will drop off (excyst) from the host fish gill. In the wild, these juvenile mussels would require pristine river gravel substrate to bury themselves in, where they’d remain for around five years.

The encysted gills of a host salmonid

At this early age, the juvenile mussel uses tiny hairs called cilia on the surface of their foot, mantle and gills to direct water currents into its shell. Food particles (mainly algae and bacteria at this stage) are filtered out and directed towards the mouth. As the juvenile mussel grows, the gills develop to be the primary filter pump, powerful enough to filter 50 litres of river water per mussel per day!

Breeding in captivity

Here at the Freshwater Biological Association’s Species Recovery Centre, we use plankton nets and bottles to collect water containing newly excysted mussels. We then take samples of the water to our laboratory where we use microscopes and pipettes to gently extract the tiny (think grain-of-salt sized!) juvenile mussels and transfer them to one of our aquarium systems.

We’ll be using these same methods to select and breed the right donor population of freshwater pearl mussels for the LIFE R4ever Kent project – we’ll keep you updated on our progress!

Learn more about the critically endangered freshwater pearl mussel and the rest of our ‘Big Four’ on the LIFE R4ever Kent project here. 

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