Freshwater pearl mussels have a distinctive appearance
In a burn in the Highlands where the water is coloured brown by peat, scientists are keeping watch over a small colony of freshwater pearl mussels.
They've been reintroduced in an effort to reverse a dramatic decline in the species.
In amongst the dark stones on the river bed they are hard to spot.
Despite that, their location is being kept secret.
Freshwater mussels are increasingly targeted by thieves who kill all the mussels they gather in the hope of finding a precious pearl inside.
Last year, shells discarded by poachers pointed to at least 15 kills on Scotland's rivers.
That's dramatically up on the previous year.
Dr Peter Cosgrove, an ecologist who has spent years surveying Scotland's freshwater pearl population, believes even this may be the tip of the iceberg.
Ecologist Peter Cosgrove says the mussel populations are at risk
"We found a kill of 800 mussels, where the average age of each mussel was 80 years old," he said.
"If you do the maths that's 64,000 years of mussel growth, just ripped from a river and destroyed. The rivers just cannot sustain that."
Throughout the world there are now only 150 rivers where there are known populations of breeding freshwater pearl mussels.
Up to half of those rivers are in Scotland.
The mussels play an important role in river ecology, filtering up to 50 litres of water a day.
Those populations remaining have been protected by law since 1998.
According to Doug Darling from Grampian Police, the thieves are "well versed in what they are doing".
"They know where and when to hit, when there is the least amount of people on a river to see them and catch them."
He adds they can decimate a stretch of river in as little as half an hour.
The penalty for those caught killing a single mussel is as high as £10,000.
But little is known about those who risk such hefty fines.
Mr Darling believes that, to make it worth their while, those involved in the illegal trade are probably part of a network of criminals, and are likely to be getting hundreds of pounds per pearl to make it worth their while.
Those involved in illegally killing the creatures are quick at what they do, and they don't appear to care that very few of the mussels they destroy even contain the precious pearls that they covet.
Mussels are being reintroduced at secret locations to halt the decline
It's believed that so far no-one has been prosecuted for the crime.
Police say if they are to catch the thieves they need people to report suspicious activity where populations of mussels remain.
While the authorities continue working to raise awareness of the prohibitive fines associated with handling freshwater mussels, and appealing for help in catching the culprits, scientists will continue their work on species reintroduction.
But freshwater mussel pearls grow very slowly and can live up to a hundred years.
That means it could be at least 20 years until they know whether that the mussels in that small colony reintroduced into that little burn in the Highlands are successfully breeding and have taken to their new home.
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