Country diary: Mussels and salmon in a most fragile harmony

‘In contrast to the black-as-bogwater exterior, the mussel shell’s interior is a shimmering nacreous-white.’ Photograph: Mary Montague

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.