Freshwater pearl mussels are "highly endangered"
Fears have been raised that rare mussels discovered in a tributary of the River Wye may not be breeding.
Over 50 freshwater pearl mussels have been discovered in the water - but none appear to be under 60 years old.
Environment Agency Wales said they were concerned it meant the pearl mussels were in decline in the river, which runs through Powys to Monmouthshire.
It now hopes to uncover the reasons behind the apparent lack of juvenile mussels in the Wye.
Mike Jenkins, biodiversity officer with Environment Agency Wales, said that it is not clear why the mussels are not breeding.
"Our initial findings show good numbers of the species are present, but it is also cause for concern," he said.
"What we can't be clear about at this stage is why the mussels appear not to be breeding; either the young may not have survived, or the adults may have given up trying."
The agency said that possible factors which could have affected their ability to breed include poor land management, a decline in fish populations and changes to their habitat.
It also intends to carry out a survey to try to identify if any other mussels are present in the tributary.
Freshwater pearl mussels, which are "highly endangered" also need clean and healthy rivers to survive, and Mr Jenkins admits that this could be a crucial factor.
They have a 100 year lifecycle but only very few young become breeding adults.
"Pearl mussels are more sensitive to pollution than many other river creatures and are a crucial indicator of long term river quality," he said.
"Their gradual loss reflects the historic decline in the quality of our rivers."
The long term quality of rivers across Europe is being addressed by the agency's Water Framework Directive, which sets new and more challenging standards for Welsh rivers to benefit people and wildlife.
Mr Jenkins hopes that as a result, "pearl mussels can once again thrive in the rivers that they originally came from".
In the meantime, however, local agency officers have moved the ageing mussels to one of its fish hatcheries, where the water conditions are more suited for breeding.
This year is the International Year of Biodiversity, as the United Nations are attempting to halt biodiversity loss around the world.