With 179 species, Alabama has one of the richest and most diverse assemblages of mussels in the world. Approximately two-thirds of the 307 North American mussel species have been reported in Alabama. This is remarkable when you take into account the fact that Alabama makes up only a small percentage of the North American land mass.
Mussel species within the Cahaba River system have declined from 50 to 37 species over the past 70 years. Of these 37 remaining species, five are federally listed, and three are considered of high or highest conservation concern. Of the original 50, 11 have become extinct locally, and two are likely extinct.
Why does the southeastern United States, and Alabama in particular, have such a high diversity of mussel species? Two factors played a role. One of the factors is the wealth of river systems in the state. Each river system has a unique assemblage of mussels, including many endemic species (those that are found in a small area and nowhere else). The Mobile River system of central Alabama contains the Alabama, Tombigbee, Tallapoosa, Coosa, Black Warrior, and Cahaba rivers. On the northern side of the state, the Tennessee River passes through from its source in North Carolina to join the Ohio. The Tennessee River is currently the most important source of commercial mussels in the world. South central and southeastern Alabama are drained by smaller, coastal systems, including the Choctawhatchee, Conecuh, and Yellow rivers. Finally, the extreme southeastern and southwestern portions of the state lie within the Chattahoochee and Escatawpa River drainages, respectively.
The other factor that played a role in the mussel diversity of Alabama is that the river systems in the state are very old. Alabama lies south of the extent of the last ice age, leaving the oldest mussel species’ habitats intact. Mussel assemblages in northern river systems that fell under ice caps were destroyed, and those rivers had to be recolonized by mussels. The fact that the mussels in the Alabama region are separated in the different river drainages and have been isolated for a very long time has allowed for the evolution of a rich biodiversity.