Mussel bound

Freshwater mussels are the cornerstone of river ecology. But some, such as the aptly named depressed mussel, are under threat. Simon Hadlington reports on work to boost their numbers, and a potential spin-off for water companies

For most people, mussels are something you order in a restaurant or buy at the supermarket. Occasionally, we see mussels in the wild, attached by their beards to rocks or timber piles at the seaside. But for David Aldridge, the most interesting mussels are to be found at the bottom of our rivers. These are freshwater mussels, of which there are six species in Britain.

One, the depressed river mussel (yes, that's its name), Pseudanodonta complanata, is disappearing rapidly, and is on a list of the most threatened species in the UK drawn up as part of the Government's Biodiversity Action Plan. Dr Aldridge and his colleagues at Cambridge University are identifying the factors that have led to the demise of this mussel, and are devising conservation strategies to save it. The group has successfully reared mussel larvae in the laboratory, raising the possibility of re-establishing the mussel in suitable habitats.

"Freshwater mussels are much more widespread than people realise," says Dr Aldridge. "If you plunged your hand into the mud at the bottom of most rivers, the chances are you would come up with a handful of mussels." Some species are relatively small - about the size of a 50p piece - while the largest, the swan mussel (in ancient times, people believed that they grew into swans) can grow to around 30cm in length. "Mussels are a keystone fauna in river ecology," explains Dr Aldridge. "They feed by filtering the water, removing suspended particles such as algae. It has been estimated that an adult mussel might be able to filter as much as 40l of water a day."

This natural filtration helps to clear the water, allowing light to penetrate deeper, which in turn enables plant life to flourish. This results in healthier populations of aquatic insects, the staple diet of most fish.

Five species of mussel live in lowland rivers in Britain, and one, the pearl mussel, in upland waters. The pearl mussel is also a threatened species, and there is some evidence that the swan mussel might be declining. Several years ago, reports came from mainland Europe that the depressed mussel appeared to be in decline. To determine the status of the depressed mussel in the UK, Dr Aldridge examined records kept by the Conchological Society over the past 100 years. "We revisited each site recorded and found that, in about 30 per cent of sites, the depressed mussel no longer existed."

One reason why the depressed mussel is important is that Britain is a heartland for the species. "Many of the rare species in the UK are on the edge of their natural range, so you wouldn't necessarily expect high populations," says Dr Aldridge. "But the depressed mussel has historically had very healthy numbers here, so is important not only for our regional biodiversity, but internationally, too."

The depressed mussel is typically around 7cm long and slightly flattened in profile, hence its name. It has a deep olive-green sheen and a large fleshy foot that protrudes from the shell into the sediment, anchoring it to the bottom of the river. Unusually, the depressed mussel tends to occur in highly localised populations, usually within the lowest 20km of a river system, and they are concentrated in small, densely populated "hotspots".

The Cambridge researchers have identified two main threats to the depressed mussel: the way rivers are managed for flood defence, and pollution. During their survey, the researchers were examining spoil by the side of a river in the Somerset levels. The spoil had been dredged from the bottom of the river as part of the routine management of the river for flood defence, when silt is removed to maintain the flow of the water. "There were a lot of mussels in the spoil, and 90 per cent of them were depressed mussels," says Dr Aldridge. "Unfortunately, the dredging virtually wiped out the mussels at that point and it has since failed to recover. It is clear that a single dredging can destroy a local population."

The Cambridge team is now working with the Environment Agency to try to devise new regimes for both silt-dredging and weed-cutting that might be more "mussel friendly". Weed is similarly removed from rivers to improve the flow. This can be done by scooping the weed out with a large bucket, or by cutting it with a sub-surface trimmer. "We've shown that the weed buckets can remove a fifth of a mussel colony each year," says Dr Aldridge. "Weed-cutters have far less impact."

Back in the laboratory, the scientists have successfully raised larvae of the depressed mussel. "We are the first in the world to breed this mussel in captivity," says Dr Aldridge. "We want to develop a propagation programme to re-establish colonies that have disappeared."

In the meantime, one of Dr Aldridge's research students, Anna McIvor, is investigating how the remarkable filtering properties of mussels might be harnessed for cleaning water. A big problem for water companies is the growth of algae in reservoirs. When the water is treated for drinking, the first stage is to filter it through sand. Algae can clog up the filters. "I've been working with a company looking at the feasibility of using mussels to filter out algae and other particles," says McIvor. "We placed two types of freshwater mussel, duck and painter mussels, in cages in a tank and drew water through it. We got a significant reduction in algae. More work is needed to determine whether this is a feasible first step to treat drinking-water, but companies are desperate for a solution, and the one I worked with seemed very interested."