These pearls are grown in Hyriopsis Schlegeli Anadonata/Plicata hybrid mussels. The Kasumigaura pearls are named after the lake Kasumigaura which is located North of Tokyo.

This lake has much cleaner water and has already been protected because of environmentalism and the fact that they need to protect the freshwater eel habitat that is native to this lake and important for the economy.

PS
I forgot to mention in my last post that Lake Biwa actually had a freshwater red tide in May of 1977. Red tide will decimate any oyster population, and it is rare in freshwater lakes.