Wild Harris

Freshwater Pearl Mussel

Pearl mussels like beautiful clean rivers and so it is no surprise that they thrive in Harris.  Sadly, they are very rare and in great danger of becoming extinct in the next 25 years. Only 150 rivers in the whole world contain healthy populations of pearl mussels and half of these rivers are in Scotland. This means that we have a big responsibility to look after pearl mussels and make sure that they do not disappear.

freshwaterpearlmussel9280LCampbell_1798304371.jpgFreshwater Pearl Mussel    (Photo: Laurie Campbell)

Special Features (Adaptations)

Pearl mussels can grow to a length of 120mm and they live for a very long time. The oldest common mussel you could find on the beach is 24 years old but the oldest freshwater pearl mussel found was more than 120 years old.

From the name you can guess that some freshwater mussels contain a pearl. In fact most pearl mussels do not contain pearls and less than one in every hundred mussels has a pearl inside. The pearls are beautiful but also dangerous for the mussel. Because they are rare and precious, freshwater pearls are worth lots of money and have been hunted for thousands of years. Unfortunately pearl hunters must kill the mussels to find the pearl. Pearl mussels cannot hide or escape from hunters who kill all of the big mussels that they find leaving no adult ones behind to make a new population. It is now illegal to touch or harm pearl mussels in any way.

freshwaterpearlmussel2615 L.Campbell.jpgShells discarded by illegal pearl fishers (Photo: Laurie Campbell)

Life cycle and habitat

Adult pearl mussels live slightly buried in clean gravelly burns and rivers. To eat they pump water through a pipe in their shells and sieve out tiny pieces of food. One mussel can sieve 50 litres of water a day. When they are 12 years old mussels become sexually mature. Female mussels expel clouds of tiny larvae into the water.  The vast majority of larvae die because they are swept into lochs or out to sea, but the lucky ones are breathed in by young salmon and trout. These larvae snap their shells shut onto the inside of the fishes gills. They are so small that they do not annoy the fish too much even though they stick on to them for a whole year.

Eventually the larvae drop off the fish and fall to the river bed. Hopefully they land on gravel where they could spend the next 100 years. Mussels can move slightly if they are annoyed but they move very slowly. Some pearl mussel larvae land on sand or muddy river beds, these larvae die because the mud sticks to their gills and stops them from breathing.

The importance of Freshwater Pearl Mussels to Harris

Our grandparents remember when pearl mussels were found in lots of rivers in the Outer Hebrides. Now pearl mussels have disappeared from most of our rivers burns.

The main reasons for this are:

1) For more than 2000 years, since before Roman times, pearl mussels have been hunted for their pearls.

2) Pearl mussels need clean burns and are easily killed by pollution like sewage, poisonous chemicals and even mud.

3) Pearl mussels can only move very slowly and are hard to see. This means that the poor mussel is easily squashed by people working in rivers or dries out if we put dams across their rivers.

4) Pearl mussels need healthy numbers of young salmon and trout to carry them to their homes. In lots of places salmon and trout numbers have fallen (see the salmon sheet for reasons why).

In Harris we are lucky, most of our burns and rivers are very clean and still have trout in them, but pearl mussels are still disappearing.

Why?

Because pearl mussel hunters still kill lots of mussels leaving behind only shells.

Our pearl mussels are hanging on for their lives. We must stop people from hunting the last ones or these cool old dudes could be lost from Harris forever.

Links

http://www.snh.org.uk/pdfs/education/musselfactsheet.pdf

http://www.snh.org.uk/pdfs/education/poster/musselposter.pdf

http://www.arkive.org/freshwater-pearl-mussel/margaritifera-margaritifera

by Katherine Ross