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Fish swimming

This blog aims to show the research done by recipients of the Whitten Scholarship. Our first instalment is given by Isobel Ollard a PhD Student in the Aquatic Ecology Group.

Mussel memory – history lessons from freshwater bivalves

Think of a mussel and you’re probably imagining the marine Mytilus edulis, the blue mussel – either its shells washed up on a shoreline, or served up on a plate, perhaps with some tasty sauce.  However, go beyond the blue mussel and a whole world of bivalves opens up (perhaps literally!).  Perhaps the least known group of mussels are the freshwater mussels, of the family Unionida.  Living half-buried in the sediments at the bottom of lakes and rivers, they are seldom seen but globally ubiquitous, found on every inhabited continent.  What’s more, these overlooked bivalves play crucial roles in their native ecosystems.  As filter feeders, freshwater mussels (like their marine cousins) remove organic matter from the water column, helping to maintain high water clarity and preventing the kind of nutrient build-up that favours harmful algal blooms.  This allows enough light penetration to sustain bottom-rooting plants, which themselves support healthy and diverse invertebrate communities.  Mussels can filter water at rates of four litres an hour, and with densities reaching as high as 100 individuals per square metre in some lakes, the impact on the water column is tremendous.  Alongside this, the shells of mussels provide invertebrates with shelter from currents and refuge from predators, while the nutrients they excrete support a rich community of sediment-dwellers. 

IUCN Red List

Despite these important contributions to ecosystem functioning, freshwater mussels are among the most threatened taxa on earth, with 40% of all species classed as ‘threatened’ or ‘near threatened’ by the IUCN Red List.  Threats include river pollution, climate change, dredging and damming, which alters the flow of water and nutrients and impedes the passage of fish, which are used as hosts by hitchhiking mussel larvae.  Like all freshwater species, mussels are vulnerable not just to local habitat disturbance, but also upstream activities such as sewage and pollutant release and fertiliser or pesticide runoff, making their habitat protection a complex task requiring cooperation between multiple stakeholders across landscapes.  Mussels also face severe threats from invasive species, including other freshwater bivalves.  Among the most successful non-native species are zebra mussels, Dreissena polymorpha, which have spread from their native Ponto-Caspian region across Europe and North America.  With extremely fast rates of growth and reproduction zebra mussels colonise water bodies rapidly.  Unlike unionid mussels, they produce byssal threads, strong fibres which allow them to anchor securely to hard surfaces such as the concrete walls of canals and reservoirs – and to other mussel shells.  Unionids can therefore become infested with zebra mussels, which compete with them for food particles and can physically impede valve opening.

To understand the effects of these threats and the changing health of mussel populations, we need baseline data on populations in the past.  Without this, we risk ‘shifting baseline syndrome’, where successive generations accept their own environmental conditions as normal, gradually lowering the accepted threshold for ecological degradation and causing a lack of awareness of the true extent of biodiversity loss.  But acquiring this baseline data is difficult.  The further back we look, the less anthropogenically altered ecosystems are, but the less information is available.  This problem is particularly severe for non-charismatic, hard-to-see species like mussels where population changes go almost totally unnoticed by human observers. 

Early Surveys on the Thames

One of the earliest truly quantitative scientific records for mussels dates from a study conducted by Christina Negus, a scientist from the University of Reading, in 1966.  She surveyed mussels in a reach of the River Thames, recording the population densities of different species within different areas of the reach, as well as making extensive measurements of the growth rates, biomass and productivity of the population.  This study offers valuable data to investigate how populations have changed in the half century that has elapsed since – a period in which pollutant and sewage outflows have been reduced by successive pieces of environmental legislation and awareness of the need to protect river ecosystems has grown, but other threats, notably the spread of invasive species, have rapidly worsened.  Fifty-five years after Christina Negus’ original study, and having consulted with Christina herself on her methods and memories of the site, we returned to the same spot to replicate her surveys.  Our findings were sobering.  Across all species, mussel densities had declined by an average of 80%.  The rarest species, the depressed river mussel Pseudanodonta complanata, had disappeared entirely, leaving only a few shells.  And when we looked at the growth rates of the mussels, we found that all species were growing more slowly than they did in 1966.  Tellingly, we found a thriving new population of zebra mussels at the site, many growing on the shells of native unionids.  Although it is difficult to disentangle the causes of this dramatic decline, it is likely that the arrival of zebra mussels has played a significant role.  At the same time, the reduction in nutrients engendered by increased treatment of sewage prior to release may have reduced food availability for the mussels, contributing to their lower growth rates.  This raises the intriguing – and unresolved – question of whether these reduced growth rates are linked to smaller population sizes, perhaps offering an early warning sign of population collapse; or whether they in fact represent a return to a more ‘natural’ rate of growth, less affected by anthropogenically elevated nutrient availability.

Bronze Age Mussels

To address this question, we need to look further into the past, to look for baseline data on mussel populations before the onset of significant human habitat-altering activities.  Here our methods of data extraction need to get a bit more creative, but bivalves offer a handy tool.  Bivalve shells grow throughout their lifetime and they lay down a dark band each year so that the shell displays concentric annual rings, similar to the rings in a cross-section of tree trunk.  Like tree rings, these shell lines can offer information about the growth rates of the mussel and the conditions in which it grew.  This means that shells themselves – which are often well preserved in sediments - can provide a record of historical populations and habitats.  To make use of this, we studied bivalve shells recently excavated from an archaeological site on the River Nene, centred around a Bronze Age settlement known as Must Farm.  We measured the annual growth rings of the best-preserved specimens and calculated their growth rates, as well as year-to-year variability in growth.  We compared this to growth patterns in live mussels we collected from the present-day course of the river.  Intriguingly, we found that the growth rate of the Bronze Age mussels was much lower than it is today, supporting the suggestion that recent human activities might be causing mussels to grow larger than they did in the past.  We also found some evidence that mussels today show more variable year-to-year growth, with some years very good and some very bad, compared to the more constant growth patterns found from the Bronze Age specimens.  Again, this points to the impact of human disturbance in these ecosystems.

Continued monitoring

Putting these puzzle pieces together, with evidence from millennial to decadal timescales, it is clear that humans are having a significantly disruptive and damaging impact on freshwater mussel populations.  We cannot afford to lose these species, which help to maintain healthy and diverse ecosystems.  Continuing to monitor populations to build a clearer picture of how, when and where the declines are most severe will be crucial, and this information must be acted on, combining measures targeted at mussels themselves, such as captive breeding programmes with the goal of restocking depleted or extirpated populations, with measures to protect ecosystems more widely.  We must not let mussels become another species existing only in memories and muse.