The freshwater mussel, which gets its name from the striped pattern which sometimes appears on its shell, is not native to Sweden.
The shellfish can in large numbers clog waterways, and their sharp edges can feel like a knife to the feet of an unwary swimmer
The invasive mussels, which threaten native species, have quickly established colonies in Lake Glan and Roxen, according to news agency TT.
"The zebra mussel is considered invasive in many countries and it's threatening to wipe out native species," says Jakob Bergengren for the Swedish Agency for Marine and Water Management.
"I have probably never seen so many mussels," says Jakob Bergengren.
The agency says the species grows at a fast rate and in dense clusters with up to 10,000 individuals per square meter.
"There are positive side effects from the zebra mussel’s presence,” says Bergengren in a press statement. “Its ability to effectively filter plankton algae reduces the risk of algal blooms in fresh water, and they even decrease the cloudiness in water. But the negative effects completely outweigh the positive.”