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The word biodiversity is a descriptive term for the overall variety of living things in the world or a particular region. Well-known areas of high biodiversity include tropical rainforests and coral reefs, where the varieties of plants and animals can be astounding. An area with a large number of a particular type of plant or animal is often called the center of diversity for that group. For example, the lakes of eastern Africa are known for their vast array of cichlid fishes, the popular aquarium pets. Over 300 species of cichlids live in Lake Victoria alone. Thus, eastern Africa is known as the center of diversity for that group of fish. Few people know that Alabama is the center of diversity for several types of aquatic animals, including freshwater mussels.
Alabama’s diversity of freshwater mussels is greater than anywhere else in the world, including tropical areas. There are 307 species of freshwater mussels found in North America, as recognized by the American Fisheries Society. A total of 179 species have been reported from Alabama, representing 58% of the total. This is remarkable when you take into account the fact that Alabama makes up only a small percentage of the North American land mass.
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. The central basin in Alabama is the Mobile River system, which is comprised of the Alabama, Tombigbee, Tallapoosa, Coosa, Black Warrior and Cahaba rivers, which form the Mobile River that empties into Mobile Bay. Another major river, the Tennessee, flows through the northern reaches of the state. South central and southeastern Alabama are drained by smaller, coastal systems, such as 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. Each river system has a unique assemblage of mussels, including many endemic species, which are those that are found in a small area and nowhere else.
The other factor that played a role in the mussel diversity of Alabama is that the river systems in the state are very old. Rivers in more northerly climes fell under the continental ice caps during the various ice ages in our not too distant (geologically speaking) past. Thus, as those rivers were destroyed and reformed, mussel assemblages were eliminated and the rivers had to be repeatedly colonized by mussels. Alabama lies well to the south of the southern extent of the ice caps, thus our rivers escaped the fate of more northerly rivers. So, though they have shifted position and even changed channels over the years, they have remained basically intact. The fact that the mussels in the Alabama region are separated in the different drainages, and have been isolated for a very long time, have allowed them to evolve into the multitude of species that we see today.
Among this diversity is an abundance of shell morphologies, with many unusual shapes ranging from very long and thin, almost square, triangular or round. Sizes range from the very large washboard, a common commercially valuable species that reaches a length of 10 inches, to the littlewing pearlymussel, which only grows to about 1-½ inches in length. Various species are adorned with shell sculpture, such as ridges, corrugations, pustules, knobs, tubercles or furrows. The thin, outer layer of the shell is called the periostracum. Among species, periostracum color ranges from yellow, through olive green to brown and black. Some of the lighter colored species are two-toned, with green rays or chevrons. The shell nacre, also known as mother of pearl, which makes up the thick, inner layer of shell, also varies among species. Most mussels have white nacre, but in mussels the nacre may be purple, pink, reddish, salmon or pale orange.
Many of the common names applied to mussels date back to the days when they were harvested for the pearl button industry, which thrived during the first half of the twentieth century. Most of those names simply reflect things they resembled to the mussel fishermen, and were often colorful and unusual. Some examples include: the pigtoes, which have a wide sulcus, giving them the appearance of a cloven hoof; the washboard, which has a heavily ridged and corrugated shell, resembling the scrub boards that were used for laundry in the days before electric washing machines; and butterfly, which has a shell in the shape of a butterfly’s wing. Some mussel common names simply reflect the ornamentation or color of their shells. Examples of these are fiveridge, threehorn wartyback, pimpleback, rainbow and wavyrayed lampmussel. During recent years, scientists that work with freshwater mussels have tried to standardize their common names, assigning common names to those ignored by commercial mussel fishermen. Though generally less colorful than those applied by the mussel fishermen of old, they are usually somewhat descriptive in nature.
In addition to being highly diverse in form, the mussels of Alabama are also diverse in function, demonstrating a variety of life history strategies. The general life history strategy of mussels is very interesting and unique in the animal kingdom. Females brood larvae, called glochidia, until they are mature. Glochidia are parasitic, generally using fish as hosts, so must come into contact with and infest a host upon their release from the female. Glochidia attach to the gills or fins of the fish, which forms a cyst of scar tissue around them. There, the glochidia remain for a period of one to several weeks, depending on the species of mussel. While attached to the fish, the mussels develop all of the organs necessary for a free-living existence. Not only does the cyst offer a safe, secure place for their development, it often allows the larvae to be dispersed over a relatively wide area. Adult mussels are sedentary in nature, seldom moving more than a few feet throughout their lives. The timing of glochidia discharged has been linked with spawning runs of their host fish. Spawning runs of fish allow the glochidia to be dispersed over a large area. Infestations of glochidia have been found to be harmless to the fish and hamper their lives in no way.
The diversity of mussel life history strategies is demonstrated in timing and length of reproductive events, such as spawning and discharge of glochidia. However, the multiplicity of strategy is most notable in the method of making their glochidia available for host infestation. Many species simply discharge their glochidia into the water column and trust their fate to chance. Some discharge glochidia in sticky webs of mucus, through which potential hosts may swim and become infested. Other mussel species have evolved very elaborate methods of attracting hosts. Some discharge glochidia in small packets, called conglutinates, which resemble food items sought by fish.
Conglutinates that resemble fish embryos, insect larvae and worms have been observed. When the host bites into the conglutinate, the packet breaks apart and exposes the host to the glochidia. Some species of mussel have developed intricate lures as modifications to their anatomy. These are in the form of folds of tissue called mantle flaps, and may resemble small fish, crayfish or insects, depending on the species. When the potential host tries to bite the lure, glochidia are discharged by the mussel. Surely the most elaborate host attractor is called a superconglutinate, which is used by a few species in southern Alabama. This is simply a combination of conglutinates, or small packets of glochidia, that are bound together into a single mass. A superconglutinate is formed in such a manner that it closely resembles a minnow, see video. A superconglutinate is discharged into a hollow tube of mucus, which trails in the water current behind the mussel and may be over a yard long. When moving erratically in the water current behind the mussel, superconglutinates look amazingly life-like and have been observed to elicit strikes by predatory fish.
The great diversity of species in Alabama is something of which we can all be proud. However, though diversity of freshwater mussels in the state remains high, many species have been lost from destruction of their habitat by human alterations to river systems. Most species require flowing water; over clean, stable sand and gravel. Water pollution and construction of dams on our major rivers, such as the Tennessee, Coosa, Black Warrior and Alabama, have eliminated many species from our state and even driven quite a few to extinction. Also detrimental to mussel habitat is dredging and channelization of our rivers. These activities cause destabilization of the river bottom and mussels cannot survive in loose, sifting sediments. Particularly hard hit was the Tombigbee River, which was subjected to a combination of impoundment and channelization as part of construction of the Tenn-Tom Waterway. Another effect of destroying habitat on our major rivers is that populations in tributaries become isolated. So, what was once a single large population of a particular species becomes fragmented into a number of smaller populations, separated from each other by expanses of poor habitat, following impoundment of a river. These smaller populations are more susceptible to extirpation than larger populations and often lack the genetic diversity to help them overcome adverse conditions.
But, even though many species have been lost, there are some bright spots. Several areas of good habitat, with diverse mussel assemblages, remain. A few smaller rivers and streams, such as Sipsey River and many of the streams in Bankhead and Talladega national forests, have mussel faunas that are basically intact. The Federal Clean Water Act, which was implemented in 1971, has improved water quality in many rivers and mussel reintroduction efforts are under way. There are plans to reintroduce some species that have not been seen in Alabama for almost one hundred years. So, with our help and continued diligence, the status of our wonderful mussel fauna can be maintained and even improved.