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Young Freshwater Pearl Mussels Caught on Camera

Microscopic image of Glochidia fish parasite

The ancient and elusive freshwater pearl mussel (Margaritifera margaritifera) is one of the most critically endangered molluscs in the world.

Highly protected under national and international legislation, the UK has a variety of projects that aim to reverse the ongoing decline of this species.

As part of a UK-wide project to safeguard the species Natural Resources Wales has developed a freshwater pearl mussel restoration programme.

Freshwater pearl mussels are one of the longest living invertebrates and can survive up to a 100 years old. They have a complex lifecycle and recently our team of biologists at Cynrig Fish Culture Unit have caught on camera one of the most fascinating moments in the cycle.

During the summer months the female mussel releases millions of larvae, known as glochidia.  Once the larvae enter the water they literally hitch a ride on salmon or trout clamping onto gill tissue, remaining attached over the winter months.

Incredibly, a fish can be inoculated with hundreds of glochidia at any one time without showing any signs of stress!

We recently dissected and examined a 1 year old brown trout from our rearing tanks and were pleased to find that the fish was inoculated with a positive and healthy number of glochidia, with an average of up to 500 per fish.

Using specialist software our microscope has accurately measured the size of the larvae, measuring on average 300 microns or 1/3 of a millimetre.

In early summer the next phase of the lifecycle occurs when the glochidia drop off the gills and settle into sand or gravel which helps protect the young mussels and allow them to grow into adulthood.

During this stage our team will catch the larvae in plankton nets and place the young mussels into purpose built troughs and tubs, feeding the young mussels on a diet of sieved detritus and concentrated algae.

Once the stock has reached a level of maturity the pearl mussels will be released into good water quality areas and close to known pearl mussel beds in the Wye catchment. 

The molluscs play a central role within our ecosystem, filtering up to 50 litres of water a day and it is programmes such as ours  that will ensure the survival of our ‘ecosystem engineers’ who benefit the ecological health of our waters.

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