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
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.