I am holding the empty bivalve shell of a freshwater pearl mussel. A mussel of this size – almost the length of my hand – lived for about 100 years before its flesh dissolved, most likely in the stomach of an otter or mink. The shell was abandoned on the riverbank.

I turn it over. In contrast to the black-as-bogwater exterior, the interior is a shimmering nacreous-white. Formerly common enough to be harvested by local pearl hunters, this species is now globally endangered. Nevertheless, among the rolling hills of the Sperrins mountain range, vital populations survive in the Owenkillew and Owenreagh rivers. Mussels feed by filtering particles out of the current, cleaning the water, thereby providing an important service to the river’s ecosystem.

Salmon returning here to spawn depend on clean, well-oxygenated water, and juvenile salmon remain in freshwater habitats until they are mature enough to migrate to the sea. The freshwater pearl mussel depends on these juveniles. Over the summer, female mussels release clouds of larvae into the water. By chance encounters, some larvae get to attach to a salmon’s gills, becoming enclosed by the tissue to absorb nutrients from it. Call this parasitism or payback, but it’s the way an infant mussel is both sustained and safely ferried around the river system. It’s also how the mussel species is perpetuated. The following spring, the young mussel drops off its host and burrows into a gravel bed. From here, all being well, it will grow to adulthood, slowly emerging from the substrate as its shell thickens.

‘The Owenkillew River is under threat from the kind of covetous gaze that once plundered freshwater pearl mussel populations.’ Photograph: Mary Montague

But all is not well. The Owenkillew River is under threat from the kind of covetous gaze that once plundered freshwater pearl mussel populations. As I wrote last April, Dalradian, a gold-mining company, is awaiting permission to begin full-scale operations. Campaigners have long argued that, should this happen, potentially toxic effluent could be discharged into the nearby Pollanroe Burn, eventually reaching both the Owenreagh River and the Owenkillew. As filter feeders, mussels are especially vulnerable, but salmon may also be at risk.

I look up from the shell in my hand to the gleaming darkness of the living water, and pray that all of the river’s spawn gets to grow into the future.