Unionidae are dioecious, meaning they have separate sexes.    From Wordnik.com.  [Freshwater mussels in North America - factors affecting their endangerment and extinction] Reference

In North America, freshwater mussels (Unionidae) are declining at a catastrophic rate.    From Wordnik.com.  [Freshwater mussels in North America - factors affecting their endangerment and extinction] Reference

A little later he put Hyatt at work on the Unionidae, studying the anatomy as well as the shells.    From Wordnik.com.  [Louis Agassiz as a Teacher; illustrative extracts on his method of instruction] Reference

The native freshwater bivalves of Illinois represent three families, of which the great majority belong to the Unionidae -- hence the term "unionid," which is often used when speaking about native mussels in general.    From Wordnik.com.  [Zach Klein Universal Feed] Reference