Pearls beyond price

The autumn downpours produced some memorable floods in the west. Mountains gushed white water from a thousand gothic orifices; rivers seethed through the bushes on their banks. Our own little hill-stream, leaping and swerving, pounded the bridge with flying rocks in its pell-mell rush to the sea. From every estuary, a plume of peat-silt curved into the waves.

You would think, watching the churning that every scrap of aquatic life must be smashed and swept away. The impact of the bigger floods can indeed be catastrophic: algae and other plants stripped from their rocks; water-insects whirled off to feed fish they never thought to meet.

Floods are a natural hazard for fast-flowing ecosystems, and a month or two will usually see a stream recolonised by most of its regular species. Even individual fish reappear at their old stations.

But for years now, the floods of the west have not been natural. Hill grasses and heather that once absorbed the rain and released it slowly have been grazed away by sheep. Drainage for forestry, turf-cutting and farming rushes water off the land. Even the rain itself seems to be falling in torrents that are anything but "soft", in patterns shaped by man-made global warming.

The scouring of the rivers plays havoc with their banks and beds and thus with the life of one of Ireland's most remarkable and precious animals, already at the mercy of quite phenomenal odds.

Margaritifera margaritifera is the freshwater pearl mussel, its clam-shaped shell a sooty black fistful, up to 14 cm long, lined with lustrous nacre in silver, violet, pink and blue. For most of the time, it sits in the riverbed, buried two-thirds deep in the gravel and filtering food through its siphon. Because it lives in rivers with the least available calcium, its shell grows very slowly and its life is longer than our own - well over 100 years.

In August and September, the female mussel releases a cloud of larvae called glochidia into the water - an average of 9.8 million per animal. Each larva is a microscopic mussel, able to close its pair of shells.

Whirling downstream towards the sea, each glochodium has a matter of hours in which to be breathed into the gills of a 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 develop into viable young mussels six times their original size.

Then they fall off the fish and bury into the gravel, where they live for about five years. It takes this long for them to grow big enough to withstand the tug of water and the movement of stones at the surface of the river-bed: unlike the marine mussel, Margaritifera has no byssal threads to hold it in place.

The mussel entered our rivers after the Ice Age, since when, in a near-symbiotic relationship, it has paid for its brief lodgings on the salmon and trout by helping to keep the waters clear: in one Russian river, mussels filter 90 per cent of the volume in low-water years. Over the centuries, the mussel populations have even adapted their shape to the flow of particular rivers.

Across mainland Europe, the past century has seen a wholesale decline in Margaritifera, so that it is now strictly protected under EU directives. In Ireland, the decline has been slower, but only eight of our rivers now have pearl-mussel populations that are reproducing themselves.

The mussel makes a pearl, not by coating a grain of sand, but to wrap up an intruding parasitic fluke. Pearls from Irish rivers have been valued for 1,000 years or more, and were still being heavily fished in Victorian and Edwardian times. By 1911 the naturalist Arthur Stelfox was noting the decline in many rivers "perhaps owing to depredations of the pearl-searchers and their wanton destruction."

Pearl fishing is now totally outlawed in Ireland. But the searching continues, notably by Scottish and northern Irish fishers who have exhausted the rivers in their own regions, at least to the point where the prospect of finding a pearl is now remote.

The chances are one in 100 shells, which makes the bigger Irish populations of mussels a prime target. Given the colossal odds against the larvae being inhaled by a passing salmonid, the minimum population of mussels likely to maintain itself is 500 reproducing individuals within a half-kilometre of river. Just a couple of days of reckless ripping open of shells can doom a whole population to slow but inevitable extinction.

For the past few decades, however, the more important threats to Margaritifera have been less brutally direct. They are detailed in a study of the mussel's biology and present situation in Ireland, written by Dr Evelyn Moorkens and published by Duchas as Number 8 in their Irish Wildlife Manuals.*

Its distribution map shows the pearl mussel still widespread in Ireland, notably in the granite and sandstone rivers of Donegal, Connacht and the south-west. Margaritifera is an excellent barometer of water quality, since it must have clean, well-oxygenated water, right down into the gravel, and low levels of minerals and nutrients.

Run-off from slurry and fertiliser produce conditions in which adult mussels clam up and stop feeding. Algal growth clogs up the river-bed and starve the juvenile mussels of oxygen. One spill of sheep-dip can poison an entire population of thousands of mussels. Forestry and drainage schemes have smothered river-beds in peat silt. Eroding and collapsing banks can add another layer, and floods pouring off the overgrazed bogs yet another.

Any Margaritifera that reaches a century in modern Ireland is, indeed, a pearl beyond price.

* Contact Dr Ferdia Marnell at Duchas, 7 Ely Place, Dublin 2.