Mussel Memory: Repeating a 65 year old study

To the aquatic realm now, and mussel numbers are a good indicator of the health of a river. And Cambridge University zoologist Isobel Ollard set out to replicate a study from over half a century ago on a stretch of the River Thames near Reading to see what had changed. Alarmingly, mussel numbers were down 95%, with some species disappearing completely…

Isobel - Muscles are one of the most threatened animal groups globally. But in the UK we really have very little information about how they're doing. And this study, which we replicated, was carried out in 1964 and it was one of the first properly quantitative surveys of muscle populations anywhere in the world. And it found, for example, that freshwater muscles contribute over 90% of the biomass in the river Thames, in the riverbed. So we wanted to return to that site and use the same survey methods, find out what's changed in the intervening period.

James - And other than that headline stat of 95% population decrease, what were the other key findings of your study in comparison to that one 50 years ago?

Isobel - So we found that one species, the depressed river mussel, has disappeared completely from the site. So although we found a few shells, we didn't find any live muscles of that species. And two other species have declined really sharply. So the duck mussel is down 99% and the Painter's mussel is down 97%. And those are both species, which are typically seen as common. We also found that individual sizes in the muscles have changed. So they're growing more slowly now than they were in 1964 and they're reaching sizes which are about 10 to 35% smaller than they were nearly 60 years ago.

James - All sounds quite worrying.

Isobel - Absolutely, yeah. And really for two reasons. So the first being that mussels are often viewed as indicator species about the health of an ecosystem. So if mussels are declining, that's kind of bad news, which suggests that the habitat quality more generally is probably deteriorating. And the second reason is that mussels are really important in the ecosystem. So they're described as ecosystem engineers because of the roles that they play in underpinning the ecosystem more widely. So they're filter feeders. They take in algae and other organic particles from the water column and help to keep the water clear and a single muscle can filter up to 40 liters of water in a day. So you can imagine the amount of water filtration that's going on across the river and we know that having a high density of muscles is associated with high riverbed biodiversity.

James - So what's the cause? What's behind this dramatic decline?

Isobel - One clear reason that we found is the fact that this site's been invaded by zebra mussels, which are a non-native species to the UK, which are now found in fairly high densities in the river Thames. And they weren't present at that site in 1964. And they're a really problematic invasive species so they can settle on hard surfaces, including on the shells of native muscles. And that means not only do they compete for food with those muscles, but they can also smother them and ultimately stop them from opening their shells at all. There's probably also been wider ecosystem degradation, so things like pollution. We also looked at the level of nutrients in the river and we found that the level of particularly phosphate has declined a lot in the intervening period and that's probably due to tighter controls around sewage releases. And while that's good for the ecosystem generally because less nutrients means less chance of developing harmful algal blooms, that could be potentially bad news for the muscles in that it means less food availability for the mussels and we think that might be contributing to their slower growth rates.

James - So what do you propose we do about this? Is there anything we can do about this?

Isobel - So I think one of the really key things that comes out of this is that we need more monitoring, especially for species which we think are common. We really have no idea how widespread this picture might be across the UK. In order to kind of pin down the causes and design better interventions and to conserve these species, we need to do more monitoring more widely and across more species.