Experts have been looking into the reason for the dead cockles
Scientists are monitoring estuaries and beaches in Cornwall following a spate of unexplained cockle deaths.
Hundreds of dead cockles have been discovered during routine ecological monitoring at a creek of the Ruan River, near the River Fal.
There have been similar reports from the Helford River at Calamansack and also much further west in Penwith.
The Environment Agency said it was urging members of the public to report any unusual findings.
Healthy cockles are usually buried under the surface of muddy sand at low tide.
But the dead shellfish have been found on the surface of the mud with their shells open.
They were also fully grown and still contained flesh, suggesting they had not been targeted by wading birds, the agency said.
However the agency added that: "Strangely, there have been no reports of cockle casualties on the Camel estuary, where cockles are harvested commercially."
In recent years, there have been similar incidents in the Bury Inlet in South Wales on a much larger scale and threatening the survival of a commercial shell fishery.
The Environment Agency is analysing the recent finds and wants to hear from anyone who may have spotted similar incidents on any other rivers in the county.
Samples of the cockles found in the Ruan River have been sent to the Centre for Ecology, Fisheries Aquaculture and Science for analysis.
Paul Elsmere of the agency said: "The cause of these mortalities is still a mystery. First, we need to establish the extent of the problem in Cornwall.
"Hopefully, the work being done by the agency in Wales will eventually shed some light on the cockle deaths in Cornwall.
"The losses could be caused by anything, from a virus that targets cockles just as they are about to spawn, or an algae that produces a toxin fatal to cockles."