by G. Thomas Watters
Freshwater mollusks include members of the Gastropoda and Bivalvia. To our knowledge, no cephalopods, chitons, tusk shells, or other groups ever invaded the freshwater environment. These invasions have happened over and over again, at different times, with different groups. The result is a hodge-podge of very unrelated animals all living in that creek in your back yard.
Freshwater snails, for example, are a mix of neritoids, cerithioids, rissoids, and others. Some are the result of marine invasions, others represent a return to an aquatic habitat by terrestrial forms. Those evolved from marine forms may retain an operculum and breathe with a gill. Those evolved from terrestrial ancestors usually lack operculae and breathe with a 'lung' comprised of mantle tissue. In North America, the more diverse groups include the Pleuroceridae, Lymnaeidae, Physidae, and Vivipariidae. Sizes range from large 'apple' snails the size of baseballs, to microscopic hydrobiids and bithyniids. Most live by grazing algae, often tenaciously clinging to stones in swift currents.
Freshwater bivalves also are a melting pot of independent invasions. Our Recent forms include species derived from veneroids, trigonioids, mytiloids, and even cardioids. They range from the tiny fingernail clams to the massive freshwater mussel Megalonaias nervosa, which reaches a length of 11 inches and weighs several pounds. Many have evolved specialized life cycles including brooding their young and producing larvae parasitic on fishes and amphibians. Some are free-living, burrowing in the substrate with their foot. Others may possess byssal threads for some or all of their life. And a few cement themselves to objects in the manner of marine ostreids.
Freshwater species are perhaps the most imperiled group of all mollusks. Confined to narrow ranges the width of a river or creek, they are easily extirpated by ecological disasters - manmade or otherwise. Over 50 freshwater mussels are considered endangered in the United States. Perhaps another two dozen have become extinct in the last 200 years. Most of the dozens of pleurocerid snail species that inhabited the southern rivers of North America are extinct. Many freshwater mollusk species have been reduced to pitifully small populations, their continued existence at the mercy of the vagaries of nature and the ignorance of man.