There are few parasites that quicken the pulse of conservationists, and fewer still that are highlighted as flagship species, but that is the fate of the freshwater pearl mussel on English and Welsh rivers and streams.
What are freshwater pearl mussel?
The freshwater pearl mussel is a mollusc, who as adults sit on the river bed
Why are freshwater pearl mussels important?
When adults freshwater pearl mussels filter our rivers through their gills, improving water quality for other species, like eels and otters.
Freshwater pearl mussel life cycle
This is in part because of their extraordinary life-cycle: vast numbers of tiny larvae, known as glochidia, are released by the mature female into the stream. They must then be inhaled by young salmon or trout and fix onto their gills - this is the parasitic stage - where a cyst forms and they spend about nine months in this rich, oxygenated environment before dropping off into the river gravels and beginning their journey to adulthood.
And what a journey this is, in terms of time if not distance. To begin to understand the life of the freshwater pearl mussel – and its plight - we need to tune our minds to a different temporal beat. The mussel can live to be 120 years old or more, does not become sexually mature until about 15 years old, and does not emerge from the river gravels to filter feed in their beds for about five years, after dropping off the gills of its host.
The life-cycle stages of a freshwater pearl mussel
Clean river water and plenty of time is required for reproduction
1. FERTILISATION
The males release sperm to fertilise the females’ eggs, which develop into glochidia
2. RELEASE
Millions of glochidia are released by the female in the summer
3. ENCYSTMENT
99.99 per cent of the glochidia are swept away but a few stick to the gills of host salmon or trout
4. SETTLEMENT
After about 9 months, juvenile mussels – just 0.4mm – will drop off the gills and settle in gravel and sand upstream
5. JUVENILES
These tiny mussels will remain buried in the river bed for at least five years
6. ADULT MUSSELS
After 12-15 years, the mussels enter sexual maturity. The adults can survive up to 120 years filter-feeding on fine organic matter in the river water
Freshwater pearl mussel's pearl
The mature bivalve mollusc, in its gnarly shell, looks as if it has sprung life from the centre of a cleaved stone. The name ‘pearl mussel’ hints at the jewel that may lie within and, though pearl fishing is now illegal, it is this prospect of value, historically sought out by poachers, that is in part responsible for its precipitous decline.
Shaun Davies is a conservation volunteer and angler on Cumbria’s River Irt. He has lived in the area for 40 years and remembers seeing evidence of hunting for pearls.
“Piles of broken shells, heaped up on the bank,” he told me. The old landlord of the local pub used to talk of how people would come down and camp, then pay for beer with the curious black pearls of the region. The mussels were persecuted.”
What is the pearl made of?
A pearl is actually composed of calcium carbonate’s crystal forms, mainly argonite, deposited in fine, concentric layers. The spherical pearls that were most sought after by poachers are very rare in nature, occurring in about 0.01 per cent of pearls collected. During the Middle Ages, pearls were much sought after by nobility to decorate their garments and by the church to adorn their ornaments. T
he dense mussel beds in northern Europe were their main source. Naturally occurring pearls from the freshwater pearl mussel were the most highly valued. Today, attempting to collect pearls and/or damaging freshwater pearl mussels is illegal. The freshwater pearl mussel remains protected under the Wildlife & Countryside Act of 1981.
Why have freshwater pearl mussels declined?
The main reason for the mussel's decline, though, is its rather exacting requirements: : it needs the very best water quality and river habitat to successfully complete its life-cycle. Modern agricultural practices and river modifications have conspired against it to such an extent that it is now classed as critically endangered by the IUCN and its populations are continually decreasing.
It is the same story throughout Europe. Over the past 100 years there has been more than a 90 per cent decline in populations. While the situation is a little better in Scotland, the Ehen in Cumbria is the only river in England where freshwater pearl mussels breed successfully. If they are present in our rivers at all, they are usually about 70 years old.
They have the potential to breed, but water quality and habitat, particularly the silting up of the clean gravels that are so vital to their development, often prevent them from doing so.
There is a further ‘wow’ factor when you think that these surviving mussels first dropped from the gills of their hosts in about 1950 - before modern, intensive agriculture and its destructive consequences really took off.
Freshwater pearl mussel conservation
There is now a concerted conservation effort to save the freshwater pearl mussel. Given its dwindling and ageing populations, its longevity, and the slow transition between its different life stages, not to mention the need to restore its river habitat, there is some urgency about the task. Saving the freshwater pearl mussel is like a race against a very slowly ticking clock.
The Irt is one of the rivers selected to focus research on re-establishing the species. Nearby, the Freshwater Biological Association (FBA), in collaboration with Natural England and the Environment Agency, established an appropriately named Ark in 2007, to house broodstock from seven surviving mussel populations in English rivers and hopefully breed from them. The idea was that the Ark would act as an insurance against any catastrophe that might wipe out the species.
While the life-cycle of the freshwater pearl mussel was broadly understood, the detail wasn’t. The learning curve has been steep and the process largely one of trial and error. And given the long timescales involved, it has also been very slow.
Louise Lavictoire is Head of Science at the FBA and has been involved with the Ark since its inception. Lavictoire has been on the sharp end of
the learning experience when it comes to captive breeding. For instance, at first it was thought that either salmon or trout would serve as the host. In practice, this turns out not to be the case: some populations require trout, and some salmon, though others can attach to both species.
“It was a lot of presumption in some respects,” says Lavictoire, when describing the project’s beginnings. “We knew that salmon and trout were hosts, and we knew about the life-cycle, but we didn’t know how ‘picky’ the mussels were!”
The precise reason for their ‘pickiness’ isn’t fully understood, though it is probably something to do with the host’s immune response when the glochidia first attach. These are parasites, remember.
“We can only guess that [the reason] is the coevolution of the mussels in the specific rivers with fish that have been there - or have predominated - over the evolutionary history of the population,” says Lavictoire.
When you consider that salmonids (ie salmon and trout) can also be specifically adapted to a single river, it shows quite how complex these local ecosystems can be.
Lessons have been learned as to the best ways to keep the captive mussels. Again, the process has been slow. “In the beginning, we didn’t know how susceptible the juveniles were going to be to disturbance,” says Lavictoire, “because if you are a juvenile wild mussel and you drop off into, hopefully, clean substrate in the river, you might be there for the next 130 years. So we presumed they would be very sensitive to disturbance.”
Consequently, for the first couple of years, the team left the containers well alone, with only minimal cleaning. As it turned out, the mussels were actually quite resilient to disturbance, and the most important factor in breeding was keeping the gravels clean. A three-stage system has now been developed. First, the larvae are put in a tank in which a gentle flow passes through the gravels from top to bottom. Next, they are transferred into a tray with a flow from the side; finally, they move into an outdoor flume, with a flow more resembling a natural river. This process can take up to eight years. In 2017, there was a trial release of 70 nine-year-old juveniles into the River Irt.
Subsequent monitoring has confirmed that these youngsters are still present and growing too. In summer 2019, juveniles reared during the first year of the Ark in 2008 released glochidia themselves, proving that captive- bred individuals are able to contribute to the next generation of mussels within their native catchments.
A larger population reinforcement project began on the Irt in 2019, with the release of 1,300 juveniles. By this point, the mussels were being tagged: on one valve the team superglues a tag with a colour and a number; on the other, a PIT tag (a small radio transponder) encased in dental cement, allowing researchers to electronically scan the riverbed and check on progress. Finally, the team thinks they have now got the captive breeding method about right. “It has been a long, long process,” says Lavictoire.
The River Irt is 22km long. Fed by Wast Water, it runs through Wasdale Valley and enters the sea at Ravenglass. It should – and once did – hold good populations of pearl mussels. Historically, it has suffered from poaching, but the main reason for the species’ absence in its waters is decades of mismanagement and abuse, in common with so many rivers in the UK.
The upper Irt is dominated by sheep farming. Lower down, it becomes more intensive, with beef and sheep farming; lower still, it is dairy farming. There is some limited potato cropping where the soil is sandy and sufficiently fertile.
Over centuries, this river has variously been polluted with agricultural run-off, straightened, over-abstracted and dredged. Its gravels have been dug out (some were used to build the foundations of the local nuclear power station) and what remains is clogged with incoming silt. The land beside it has been overgrazed and denuded of trees, with bankside vegetation repressed.
The job of restoring natural processes to the river – and thus giving it and its inhabitants a fighting chance of developing resilience in
the face of climate change – falls to Chris West, project officer at the West Cumbria Rivers Trust. If the river can be made welcoming for the freshwater mussel, then it will also be good for the other species that call it home, including the trout and salmon that are vital as early hosts.
“The most visual thing we are doing is to try and create a wooded buffer corridor on the Irt by putting up fencing and planting trees,” says West. “That helps keep grazing away from the river by allowing thick resilient vegetation to grow, which also stops soil getting washed into the river. It also creates space for wildlife and provides shade.
“Where we think the river used to run in a different course, we are also opening up old channels to make the river more diverse and more resilient, so it is able to express natural processes. That way you end up with better quality habitats.”
West sees a large part of his job as giving the river a ‘voice’ in the community, and explaining why they are making the changes they are making. During an early public meeting in 2015, when the river restoration project began, a farmer stood up and asked indignantly “What is more important, a mussel or a cow?” “But it’s a fair challenge,” says Shaun Davies.
Hailing from farming stock, West understands both sides, describing himself as not being “a clipboard guy”. “I’m trying to find a solution that works for the environment and farms,” he says. Indeed, West is keen to communicate that restoring the river is about more than the freshwater pearl mussel alone. “Managing a catchment for a single species would be a waste of time,” he says, “because we’ve got to look at the whole approach: from nutrients to fish habitats, to the river bed, to temperature – that is why we are trying to champion natural processes.
“What might be fashionable in terms of river restoration today might be shown in the future to be ‘old hat’. But what we are fairly confident about is that Mother Nature knows best. If you allow space for the river to express natural processes, to be as wide or as narrow as it wants to be, you are probably on the right track.”
West knows that reestablishing the freshwater pearl mussel in his Cumbrian river is a long game. I ask him how he imagines he will feel when he sees the first mussel that has bred naturally in the wild, spent its years in the gravels and found its way back to the beds again. “Elated,” he says. “But it has got to be a lifetime’s work.”
Main image: Freshwater pearl mussel (Margaritifera margaritifera), © Joel Berglund CC BY-SA 3.0 <http://creativecommons.org/licenses/by-sa/3.0/>, via Wikimedia Commons
