About the pearly mussel in question--- Margaritifera margaritifera.

Here's a fairly old, but very good article. According to it, just a little over 10 years ago, in 1996, things were looking optimistic for the mussels of the South River Esk. What a difference today.

http://www.jncc.gov.uk/protectedsite...eIntCode=S1029

Slraep