Guardian River Dee’s pearl mussels get a helping hand – or gill      They were hunted to near-extinction. Now a £3.5m project aims to let the Scottish molluscs flourish again. The sun shines on clear river water running through a valley in the Cairngorms, bringing the stones on the river bed into colourful focus. Here and there are dark shadows, half-buried clusters of dull black shells, lined and gouged by decades of shifting water and gravel: the pearl mussels of the river Dee. The pearl mussel is one of the most ancient invertebrates on the planet, a freshwater shellfish that has helped to shape the history of Britain. One pearl mussel in every 1,000 or 5,000 – no one can be sure – lives up to its name and contains a lustred treasure. Julius Caesar’s biographer, Suetonius, cites a desire for pearls as one of the reasons why the Romans invaded Britain in 55BC. But pearl hunting, pollution and habitat destruction have almost led to the mussel’s extinction. It has been illegal to pearl fish and to sell or buy a Scottish pearl since 1998, but their numbers remain perilously low. This week or the next, some of the remaining female mussels along all 86 miles of the Dee will spawn – all at once. On just one day a year, they release millions of tiny larvae, or glochidia, into the water.

The baby mussels face the most extraordinary odds – they have to latch on to the gills of a passing fish to survive. Roughly one in four million make it, fewer if the salmon and trout are in short supply. Which is why a remarkable project is under way to attempt to transform the Dee, restoring its natural course to improve the prospects of the pearl mussel.

On the Slugain, a tributary of the Dee near Braemar, a giant digger has arrived, part of the £3.5m Pearls in Peril habitat restoration project being undertaken by Scottish National Heritage (SNH). The plan is to break down artificial banks and barriers that over decades have been built to force rivers off farmland or away from housing. These constructions funnel the water into channels that flow faster and fill up more quickly – a flood risk for towns and roads downstream – and no longer have the meandering shallows, cascades and pools that suit the mussels and the fish they rely upon. Click here to read more.