**2. Freshwater mussels: North America's most threatened animals**

Of all groups of threatened aquatic animals, freshwater mussels (also known as unionid or pearly mussels) are the most imperiled, with 67% of North American species considered threatened (Williams et al., 1993). 35 North American freshwater mussel species have gone extinct since 1900 (Williams et al., 1993), and some scientists have estimated a 1.2% per decade extinction rate for this group, with others predicting that unless effective conservation action is taken 127 more species will become extinct over the next 100 years (Ricciardi and Rasmussen, 1999).
