There are some experts on this forum who know more about the mussel shell trade than I do and I am blurring the comments made by Drucilla & Cathy:
There is a big trade in the harvest of mussel shells. I believe that most of the mussel shells used in making nuclei come from American mussels. Tons of them are harvested from some of the bigger rivers feeding into the Mississippi and sold to be processed into nuclei. The species that contribute to this trade have very thick MOP on one side of the lip. I believe the nuclei can occur in sizes double 7mm, and they are quite expensive. I am not positive of this, but I think they can run into the $$hundreds each for SSP and some Tahitian nuclei.
Those kinds of American mussels are abundant, although about 40% of the 200 species found in America are protected, endangered or extinct, that appears to be in small rivers, creeks and tributaries in many states.