A barrier has been installed between two Scottish rivers in an attempt to block the spread of non-native North American signal crayfish "invaders".
The animals are already present in the River Clyde but it is hoped they can be stopped from reaching the River Annan.
First found in Scotland in 1995, the crayfish have been blamed for eating young fish and destroying habitats.
Scottish Natural Heritage (SNH) described it as a "landmark" attempt to control the spread of the crustaceans.
Environmental experts have identified a "pressing need" to stop the creatures moving from the Clyde into the Annan.
A large number of rivers and lochs across Scotland have already been affected by the signal crayfish.
They include Loch Ken in Dumfries and Galloway where a five-month trapping programme two years ago killed more than 700,000 of them but failed to eradicate the problem.
SNH hopes the new barrier will prevent similar problems on the River Annan, a popular location for salmon and trout fishing.
Recent survey work on the River Clyde has shown that signal crayfish have spread into many of its headwaters sparking fears they might move into the neighbouring River Annan catchment, near the Beattock summit in South Lanarkshire.
A partnership involving SNH, the Annan District Salmon Fisheries Board, the Clyde River Foundation and South Lanarkshire Council has put in the barriers.
"Unfortunately there are no techniques available that will allow us to get rid of signal crayfish from rivers and streams," said Dr Colin Bean, a freshwater adviser with SNH.
"So taking the radical step of developing and installing a physical barrier may offer us the best hope of stopping the species from moving into new catchments."
Two dams have now been installed 20 metres apart at a cost of £50,000.
Nick Chisholm, director of the River Annan Trust and Fisheries Board said: "The presence of crayfish on the Clyde has long been of concern to us.
"The Clyde and the Annan are hydraulically linked under certain conditions and there is no reason why, given time, that crayfish could not effectively walk over the hill.
"We know from bitter experience that once these animals are in an open water course, they are impossible to eradicate and can cause massive damage to the native fauna."
Dr Willie Yeomans, catchment manager with the Clyde River Foundation said the crayfish had made a "measurable difference" to the river within a decade.
"Crayfish have changed the ecology of the Clyde forever and this is a stark reminder to us all about the need for biosecurity and the need to avoid the introduction of invasive non-native species," he said.
If successful, the barrier could be used in other parts of the country to stop the crayfish reaching other rivers and lochs.
Mr Bean said: "We can't prevent them from becoming a problem in rivers where they currently exist, but at least this would give us some hope that we can control their spread in the future."