Freshwater mussel species are critical to river ecosystems. But, many species are in trouble. Nearly 70% of freshwater mussel species are considered threatened in some way, according to zoologist Caryn Vaughn at the University of Oklahoma. With support from the National Science Foundation, Vaughn studies the role mussels play in freshwater ecosystems. They work hard filtering impurities from the water. The major problem for freshwater mussels now is habitat fragmentation and channelization by dams.