Not many people are aware of them and yet others hunt them. All it takes is a second to force open a shell and in most cases find no treasure. Even so, that swift act of theft results in the death of yet another freshwater pearl mussel.
Margaritifera margaritifera and Margaritifera durrovenis are large bi-valve or two-shelled mollusc. They live burrowed in the sediments of Irish rivers. They are also the oldest animals in Ireland, some fossils of this family date back to some 150 million years ago to the time of the dinosaurs.
They grow very slowly, and live up to the age of 130. They can grow up to 8 cm in length and are smaller than their pearlless fellow freshwater bivalve molluscs, the swan mussel and duck mussel, both of which average about 14 cm, although swam mussels can grow up to 20 cm. The heavy shell of the freshwater pearl mussel is composed of calcium products, yet the ordinary pearl mussel, Margaritifera margaritifera, live in rivers with very little calcium, often in water acidic enough to dissolve the oldest parts of their shell. Its even rarer relation, M. m durrovenis is found in harder, more lime-rich waters.
Man has been aware of them since the 11th century and there are references to them in a document dated 1049. With the arrival in European markets of the more sophisticated oriental pearls, the Irish lost interest in their native varieties as they were inferior to the eastern newcomers. Freshwater pearls are of poor quality, lacking the luminescence expected of fine pearls. It was always a gamble. Sir Robert Redding, when writing about pearl fishing in the North of Ireland in Transactions of the Royal Society in 1688, pointed out that only about one shell in 100 has a pearl and that only one pearl in 100 is "tolerably clear".
Many factors - such as pollution, habitat destruction, removal of bank side vegetation and increased peat siltation arising from run-off caused by overgrazing of uplands peat lands by sheep - have led to the decline of the pearl mussel. Dirty, silted water chokes the mussel, which feeds by sucking. It has a pair of siphons pumping water in and out, in effect sieving food particles.
The biggest threat is posed by mussels being hunted for their largely worthless pearls. It is a cruel irony, but a very real danger. Ireland and Scotland are possibly the only countries left in Europe in which the species is still well represented. Indeed, the Margaritifera durrovensis, first discovered in 1926 and one of the few animal species endemic to Ireland, is facing extinction. It is now found only along a 10 kilometre stretch of the main channel of the River Nore. There may be fewer than 2,000 individuals. Most rivers have no young mussels under the age of 15 years. Irish pearl mussels are protected by law, but legal protection is not enough.
Such is the concern for the future of our pearl mussels that it has become ill-advised to point to the places, mainly along our west coast, where they may be found - in case it alerts poachers. Dr Noel Kirby of Duchas, director of Connemara National Park and responsible for the western region, has a particular interest in their survival and is on the alert for illegal hunting. "We are asking people to be more aware of the pearl mussels, to acknowledge them as part of our heritage. They are protected and should not, must not be interferred with." As he points out, "it's very simple. If a shell is already open, the mussel has either died or been killed by being forced open. If the shell is closed tight and has to be forced open, then by forcing open the shell, you are killing the mussel."
Survival is more complicated. Not only are mussels dependent on their river environment and the continuance of clean, well-oxygenated water flowing over sandstone or granite, they also rely on the presence of salmon and trout. The fish unwittingly have a role in an unconventional reproduction process. Female pearl mussels are not sexually mature before the age of seven years.
Sexual intercourse between mussels is a distant affair. Males release sperm into the river water. Nearby females then draw it in through their siphons. The eggs are fertilised, nurtured and eventually hatched along the female's gills. These very small shelled larvae or glochidia are then released to swim up and attach themselves to the gills of passing salmon or trout. The fish are oblivious to all this. After some months, the young mussels leave the fish, burying themselves in the gravel of the river bed until capable of withstanding the force of the flowing water above.
Interestingly, the story of this singular life cycle was not unravelled until 1862. There is of course an obvious link between the absence of young mussels and the decline of salmon and brown trout. Still, it is fascinating to consider not only the heritage represented by these ancient animals but also their dramatic battle for life in an increasingly endangered environment.
