The pearl mussel was abundant in the Sudety Mountains until the late 19th century. No live
mussels were found within the present borders of Poland since the early 20th century. In 2006–2007 we did
a detailed survey of 50 streams and rivers within the former range of the pearl mussel to verify its current
status and assess the habitat conditions. No live mussels were found. On the whole, the rivers and streams
were found to be degraded. Only four streams met the habitat requirements of the species. Well-preserved
shells with a nacre layer were found at the site of the last known population in the Koci Potok stream. To
determine whether the shells represented an extinct population or rather indicated the presence of the last
survivors, we placed
Unio crassus
shells in the stream within the historical range of occurrence. Those shells
dissolved at the rate of approximately 20% per year, indicating that empty shells would not have remained
intact within the channel. Possibly the earlier-found shells had been preserved in the banks above the water
level. The possibility that they represent the last survivors can not be excluded. Further search for the last
survivors, extended to cover northern Poland, is recommended.
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