Not only have very few of us ever seen one - it's a fair bet that nearly all of us don’t even know that they exist. But the life and story of the freshwater pearl mussel - a large, dark-coloured mollusc with an iridescent inner shell -  is an utterly extraordinary one. These river dwellers are, without doubt, one of Ireland’s most remarkable and rare species.

Here are just a few bare facts. The fossil record shows that pearl mussels have existed for hundreds of millions of years, since the time of dinosaurs. Not only are they the oldest animals in Ireland, but they’re also our longest living invertebrate - there are pearl mussels alive today that were born in the 1890s. 

These mussels don’t think about sex until they’re about 7-years-old. When it happens, it’s a complicated - if remote - business.  The males will release millions of sperm into the water. If a female is lodged nearby, she will inhale the sperm into her body where her eggs are ready and waiting to be fertilised.  She then releases up to four million miniature mussels into the open water.  

The chances of these babies surviving are slim, and 99.9% of them will die.  Those who manage to live on, then face the next challenge: they must find a salmon or trout in the river and hitch a lift on their gills.  Given the decline in our salmon and trout populations, this is no easy task, but young mussels who manage to do this will remain on the oxygen-rich gills - unbeknownst to the fish - for nine months.

When they drop off the fish, they bury themselves in the river bed away from the fast-flowing water. They grow slowly over five years and when they’re strong enough to withstand the flow, they burrow a muscular foot into the riverbed and emerge upwards and begin to feed.  

The daily life of a mussel in a river resembles the clichéd old man at the bar: they sit in one place, and drink, drink, drink. Over a 24 hour period, mussels will suck in over 100 pints of river water and pass it through a siphon - think of a kind of sieve - which catches minuscule particles of food for them to eat.

"They might look like they’re doing nothing, just sitting there on the river bed for the whole of their lives, but they’re actually filtering massive amounts of water," says scientist and mussel expert Dr. Mary Catherine Gallagher. "They’re cleaning our water, basically,."

But if the water is clogged with silt, or polluted with nutrients such as nitrogen and phosphorous, then the pearl mussel will eventually choke and die. These are some of the pressures that have led to the fact that the pearl mussel is teetering on the edge of extinction; in the last 100 years, nearly 90% of them have died out across Europe.  

Water pollution, dredging of rivers, siltation and the tragic decline of salmon and trout have all caused a rapid slump in mussel numbers in Ireland, where nearly half of all European mussels now live. The Munster Blackwater, for example, was once thought to have 30,000 pearl mussels; today, the species is barely clinging on. 

The good news is that farmers along the western coast of Ireland, from Kerry to Donegal, have come together to work with scientists to try to rescue and restore this charismatic species. They’re involved in a publicly-funded project that pays them to manage their land in ways that enhances water quality.  

Farmers can choose what they do, how they do it, and when they do it - in return, their fields are scored from zero to ten.  The higher the score, the better the quality, and the larger the annual payment. It’s hoped that as farmers’ fields score higher over time, the water quality in their local rivers will continue to improve, ensuring that the freshwater pearl mussel will thrive into the future.

Colm Gavin is a young farmer in the Delphi Valley in Mayo who keeps Mayo blackface sheep on his hills. His farm borders one of the most important rivers for pearl mussels in the country, and he’s one of 450 farmers in this scheme. Colm remembers seeing pearl mussels when he was a young boy.

"When we were younger we would have been down at the river and seen them," he says. "One of our biggest surprises was that they weren’t in every river -  we were aware of them because they were always there, but we weren’t aware there was a problem with them elsewhere."

Together with his father Martin, Colm has changed the way he farms to reduce the amount of silt and run-off that goes into the river.  A series of traps make sure that the silt in the water is filtered off before it reaches the pristine river at the bottom of the field.

Actions like this are essential if river quality is to be maintained at a level that the pearl mussel needs to survive. But this project hasn’t just benefitted the species that live in the river - it has also revolutionised the way some farmers think - and feel - about their land.  

"This field would have been classed as having no agricultural value," says Martin, pointing to a field that leads to the river below. "And now it’s the field that scores the highest value for the pearl mussel because it has so much variety of vegetation," he adds proudly. 

Colm has also noticed that the project has led to a positive feeling between the farmers in the Delphi Valley. They now have a 'pearl mussel’ WhatsApp group, and are focused on getting their field scores as high as possible.

"The one thing this project has done is that it’s brought us farmers that are in these mountain areas together, that we mightn’t have had dealing with previously. Our farming network needs that," he says.

Scientists are very hopeful that the sterling work of these farmers will secure a positive future for the pearl mussel. "We were following the policies of increasing sheep numbers - that was definitely having a seriously negative affect on the environment that we appreciate now," says Martin.  

But since’ he has started this project, he has realised the nature-filled farm he wants to leave for future generations. "Now, my focus is to leave that legacy behind me the same as I inherited when I came here as a young fella," he says. 

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