Another Life: 10m larvae, 2 survivors, then 100 years protecting our rivers

The first November squalls of the new wet rain give notice of the Atlantic winter monsoon that now attends our share of climate change. The hills of the west will gush white water from a thousand gothic orifices, rivers will seethe through bushes and roll boulders under bridges, silt will be sucked out from clear-felled, boggy forests and cutaway peatland, fertilizers flow from fields and drains and conifer plantations.

All to raise yet more concern about a mollusc termed – even in such rainfall – an "umbrella species", since conservation of Margaritifera margaritifera, the freshwater pearl mussel, shields the future of pretty well everything else alive in our last, relatively clean, rivers. Just how pristine and self-governing these waters need to be has taken decades of research to discover – decades of dramatic declines of the mussel across most of the northern hemisphere.

Substantial populations able to recruit young mussels are now found on fewer than 50 rivers in Canada, north-west Russia and north-east Scandinavia, with a handful of sites in central Europe. Scotland’s mussels, packed into a few Highland rivers, could fade out within 25 years. Last month, Wales set about rescuing its only stretch of mussels in one river in Snowdonia; England has another, with more than 350,000 mussels, all of them ageing, in rivers that let few young survive.

By one estimate, Ireland has some 12 million, in 135 clusters, an important share of the European population. Many are in south Kerry – more than 2,750,000 adults in each of the Caragh and Blackwater catchments, neither river in favourable condition. This autumn sees the launch of a €5 million restoration project for the National Parks and Wildlife Service, half- funded by the EU.

An adult Margaritifera is a sooty black fistful, perhaps 14cm long, lined with lustrous nacre in silver, violet, pink and blue. For most of the time it sits firmly upright in sand or gravel, filtering food through its frilly siphon. Evolving happily in rivers with the least available calcium, its shell grows very slowly, ridge by ridge. This offers a phenomenal life-span for an invertebrate – well over 100 years.

Its infant survival depends on equally phenomenal chance.

In early autumn, the female mussels are triggered to release – all at the same time, like tropical corals – clouds of larvae called glochidia, at almost 10 million per animal. The larva is a microscopic mussel, well able to close its pair of shells.

Whirling downstream on the way to the sea, each glochidium has a matter of hours in which to be breathed into the gills of a swimming salmon or trout and snap tight on one of the filaments. Of some 10 million larvae, perhaps 40 find a host. And of those, perhaps two will hang on long enough – a couple of weeks – absorbing food from the gill tissue, to grow into viable, but still minute, young mussels.

Then they fall off the fish and burrow into sand or gravel, where they live for some five years, feeding on bacteria. It takes this long for them to grow big enough to withstand the tug of water in flash floods and the shifting of stones at the surface of the river bed. For this they must have a flow of well-oxygenated water, free of silt, chemicals, rotting vegetation or high acidity and maintaining a maximum depth and optimum rate of flow – a most exacting ecological regime.

Historically, the first big toll on Margaritifera was the trade in pearls, made when a mussel seals in glowing nacre not a grain of sand but an invasive, parasitic fluke. This happens in perhaps one in 100 mussels, so that huge numbers may be sacrificed for a single saleable pearl, their shells left heaped on the bank. Outlawed across both islands, this illegal pillage persists, so that the exact whereabouts of mussel colonies is kept as secret as possible – even from devious inquiries under the Freedom of Information Act.

Arterial river drainage in the 20th century wrought brutal river havoc before its ecological damage was appreciated. Today, the OPW continues its work, but knowing its impacts and putting some of the boulders back. Coillte, too, tries to buffer mussel rivers from the impact of silt, phosphate fertilizer and piles of decaying brash with drainage traps and wider bank margins.

Pioneers in saving Margaritifera are Donegal County Council, in partnership with the Northern Ireland Environment Agency. With as much as 3.5 tonnes of silt a year eroding into rivers from a square kilometre of grazed land, they are working on better livestock management. At Derryveigh, on Coillte land, trials of selective felling may offer the alternative to clear- felling. In Co Tyrone, too, new methods will check the flow of silt from forestry operations – this a millennium on from the time when freshwater pearls from Tyrone were sent to adorn His Grace, the Archbishop of Canterbury, in 1094.