Freshwater Pearl Mussel

About

Freshwater Pearl Mussels (FWPMs) live in very clean rivers with sandy and rocky substrate. They like to be partly buried in coarse sand and fine gravel, but don’t take kindly to being smothered in fine silt! They don’t become sexually mature until they are 10 – 15 years old. When spawning takes place in early summer, the young live inside the females’ shells for a few weeks. Then, in late summer all the females release the young into the water at the same time. There can be up to 4 million larvae released from each female, most of which will be swept away. To survive, the larvae must quickly find a young trout or salmon to latch onto. They attach to the gills of the fish where they can live safely until the following spring when they drop off and start to live in the substrate.

Because they are so long lived, very sensitive to disturbance, and the vast majority of the young die; this species is very vulnerable to extinction. They are disappearing everywhere throughout their range and Scotland is one of their last strongholds. The decline is thought to be as high as 95 or even 100% in some areas, and the species is in great danger of becoming extinct in the foreseeable future if things don’t improve. To make matters much worse, the FWPM has been exploited for centuries by people hoping to make a fast buck from their pearls, even though only a tiny proportion actually have a pearl inside them!

Here at the Trust, we have been lucky enough to do some FWPM survey work in partnership with Scottish Natural Heritage. Our staff have been trained in FWPM survey and are licensed to go and look to see how they are distributed in our rivers. We can’t release any information on where we have found the species, but we have been very pleasantly surprised that they are doing better than we thought they were.

How to identify

The FWPM is a large bivalve mussel which means there are two sides to the shell connected by a hinge which can open and close. There are three types of large mussel found in our local freshwaters – duck, swan and pearl mussel, and they can look quite similar. The FWPM shell is dark in colour, almost black, and tends to be wider than the others. If in doubt, do not disturb any mussel in case it is a pearl mussel.

Did you know ?

FWPM may have been a factor in precipitating the first roman invasion of Britain in 55BC! 

When present in a large population, these filter feeding mollusks can help to clean a river. One mussel can filter 50l of water a day, improving the habitat for fish and other species. A healthy population of freshwater pearl mussels is a good indicator of a healthy river. 

How you can help? 

If you are out and about in your local river and you see a mussel that you think may be a pearl mussel, we would love to hear about it. It’s important not to touch living pearl mussels (in fact, it is an offence to do so!), but more often than not, it would only be the dead shell that is found, whether that be in amongst marginal debris, or even on a pile of feeding remains that an otter may have left on the bank. We would find a photo, or the dead shell itself useful to help confirm the species. We will always keep information on FWPM locations confidential.

You should call Police Scotland on 101 if you see any suspicious activity in or near a river that may contain freshwater pearl mussels.