Decline in the Freshwater Gastropod Fauna in the Mobile Bay Basin
Arthur E. Bogan
Freshwater Molluscan Research
J. Malcolm Pierson
U.S. Fish and Wildlife Service
|The historical freshwater gastropod fauna of the Mobile Bay basin in Alabama, Georgia, Mississippi, and Tennessee was the most diverse in the world, comparable only to the diversity reported for the Mekong River in Southeast Asia. This fauna was represented by 9 families and about 118 species. Several families have genera endemic to the Mobile Bay basin: Viviparidae: Tulotoma; Hydrobiidae: Clappia, Lepyrium; Pleuroceridae: Gyrotoma; and Planorbidae: Amphigyra and Neoplanorbis. The greatest described species diversity was in the Pleuroceridae (76 species). The pleurocerid genera Pleurocera, Leptoxis, and Elimia had their greatest radiation in the Coosa River drainage.|
|Although this extremely diverse aquatic gastropod fauna received little attention in the past 50 years, it was actively studied during the second quarter of this century (Goodrich 1922, 1924, 1936, 1944a, 1944b). During the last 60 years, this unique gastropod fauna has declined precipitously (Table 1; Athearn 1970; Heard 1970; Stansbery 1971). More recent documentation of the decimation of this fauna was presented by Stein (1976) and Palmer (1986). The endemic genus Tulotoma (Figs. 1 and 2), formerly widespread in the main channel of the Alabama and Coosa rivers, was presumed extinct until recently rediscovered (Hershler et al. 1990). The pleurocerid genus Gyrotoma, restricted primarily to the shoals of the Coosa River, contained six recognized species, all of which are presumed extinct (Table 2; Fig. 3).||Table 1. Summary of the aquatic gastropod fauna of the river systems in the Mobile Bay basin.|
Status and Trends
|Fig. 1. Live specimens of the endangered tulotoma, Tulotoma magnifica, from Kelly Creek, Elmore County, Alabama, 1993. Courtesy J.M. Pierson|
|Literature records were compiled to document the gastropod species present historically. Recent surveys of the aquatic gastropod fauna of the Coosa and Cahaba river drainages in Alabama have been conducted by using standard field techniques (Bogan and Pierson 1993 a, b). Additional unpublished data (Bogan and Hartfield) are included.|
|Recent surveys of the aquatic gastropod fauna at about 800 sites (Table 1) have documented population declines, decreases in species' ranges, and the loss of a major portion of the gastropod diversity, especially in the Coosa River. The Coosa River drainage had at least 82 species historically (Table 1); today 26 species are presumed extinct in six genera, and four genera (Clappia [2 species], Gyrotoma [6 species], Amphigyra [1 species], and Neo-planorbis [4 species]) are presumed extinct (Tables 1 and 2). The genus Leptoxis has been reduced to a single species restricted to three creek tributary systems in the Coosa River.||
Table 2. Freshwater gastropod species presumed extinct in the Mobile Bay basin.|
|The fauna of the Cahaba River drainage has fared much better (Table 1). Although the Cahaba River drainage does not suffer from the numerous dams and the siltation problems of the Coosa River drainage, it is heavily affected by nonpoint-source runoff, siltation, acid mine drainage, pollution from wastewater treatment plants, and water withdrawn for domestic water use. Species such as Lepyrium showalteri and Lioplax cyclostomaformis, formerly much more widespread in the basin, are now apparently restricted to one or two shoal areas in the Cahaba River main channel. The status of the pebblesnails (Hydrobiidae) is uncertain. The former diversity of the genus Somatogyrus in the Coosa River has probably suffered the same fate as most of the main channel shoal-dwelling pleurocerid species--extinction. Detailed information on the distribution of the freshwater limpets (Ancylidae) is not available, but they appear to have suffered similar range restrictions.|
|Fig. 2. Historical and current distribution of Tulotoma magnifica. Filled circles represent a single or two closely located collection sites (after Hershler et al. 1990). Map modified from the U.S. Geological Survey 1:500,000 scale--State of Alabama sheet (1970 ed.).|
|Fig. 3. Illustration of a representative species of the extinct slitshell genus Gyrotoma from Butting Ram Shoals, Coosa River, Alabama. Courtesy A.E. Bogan|
|The uncertainty expressed in the diversity of the historical gastropod fauna presented in Table 1 is indicative of our lack of information regarding all aspects of the historical gastropod fauna of the Mobile Bay basin. There are a lack of detailed data on the ecology and life history of all of the species, and a paucity of distributional information for most of the families other than the Pleuroceridae, making estimation of gastropod diversity by drainage difficult.|
|Declining species diversity can be directly linked to the inundation of the shoal areas of the rivers of the Mobile Bay basin by impoundment and siltation resulting from a variety of watershed disturbances, including 33 major dams for hydroelectric generation, locks and flood control on the major rivers of the Mobile Bay basin, and numerous smaller impoundments on tributary rivers and streams. Most gastropods inhabiting shoal areas are gill-breathing species typically grazing on the plant life growing on the rock substrate in shallow riffle and shoal areas. They formerly lived on rocks in the shallow shoal areas with highly oxygenated water. The pleurocerid gastropod fauna represented a significant portion of the invertebrate biomass living on these shoal areas.|
|When this habitat was impounded, the snails were not able to survive the deep, cold, and often oxygen-depleted water. Many areas not impounded have suffered because of the heavy siltation of shoal areas, smothering the plant life that formed the diet of these gastropods. Major sources of siltation include poor agricultural and silvicultural practices, lack of riparian buffer zones, and generally poor land-use practices. The drastic decline in gastropod diversity is especially evident in the Coosa River main channel where numerous species formerly found on shoals have disappeared after the damming of the river (Bogan and Pierson 1993a). Other species have had their ranges fragmented by the damming of the rivers and have become restricted to the unimpounded areas below the dams with clean current-swept gravel and bedrock outcrops.|
|Tulotoma magnifica (Figs. 1 and 2) is the only aquatic gastropod now federally listed as endangered; none is listed as threatened, although 104 species of aquatic gastropods from Alabama are on the federal candidate list. Most are from the Coosa and the Cahaba rivers (Table 1). Conservation and recovery of the remaining diversity will require immediate action to prevent further declines and extinctions. This will necessitate action to improve water quality across the basin and to decrease the amount of silt entering the streams and rivers. In addition, the survey of the aquatic gastropod fauna of the Mobile Bay basin is not complete, and additional fieldwork in the main channels of the larger rivers is needed, especially on the vertical limestone wall habitats.|
Freshwater Molluscan Research
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