This programme looks at life in rivers through one of the creatures that lives there - the freshwater mussel Margaritifera. This plain dark bivalve, a larger cousin of the marine edible mussel, has a very unusual life. It has the largest, heaviest shell of all of Ireland's molluscs (or snails), but it mostly lives in the rivers with the lowest levels of calcium. It does this by growing very slowly; this species can live for over 100 years.
A typical Margaritifera river is one of very low nutrients of any sort. To live for a hundred years, these mussels are adapted to filter feeding very small quantities of food and thus growing very slowly. They live in harmony with two of Ireland's other special animals, salmon and trout. The mussels filter the river water for the fish, and the fish provide their gills for the mussel larvae to latch on to for their first winter, an adaptation that stops the larvae in these fast-flowing rivers from flowing down into the sea!
The salmon and sea trout that live in these mussel rivers both go to sea for part of their lives. These fish are adapted go to sea to get the nutrients they need to grow to adulthood, as the rivers should be too low in nutrients to allow for this level of growth. Thus the clean wild rivers were in harmony with the species that could live there, with very different life strategies.
What has happened to our rivers? Margaritifera is in serious decline, endangered throughout the world. Ireland was until recently a stronghold for mussels, with large reproducing populations particularly in rivers in the remote areas of the west of Ireland. Now every population we have is threatened with extinction, due to lack of survival of juvenile mussels. Although adult mussels live for a long time, without producing a new generation, extinction is inevitable. The poorer the water quality, the faster the mussels die out. Juveniles, and in some cases adults, are killed by silt building up on the river bed, and high nutrient levels leading to algal blooms, both of which prevent oxygen getting to the mussels.
Ireland is proud of its clean rivers, but the quality of the Margaritifera rivers has declined to a level that mussels cannot tolerate. Very few can be considered to be grossly polluted, and some are labelled as unpolluted under Irish quality standards. However, compared with how they were in the recent past, they have sadly lowered in quality. This leaves conservationists trying hard to find a means of managing a catchment in a way that will prevent nutrients and silt from getting into the river and trying to captive breed the most depleted populations until river standards improve.
Dr Evelyn Moorkens