Our Li\'!Hi^ Rt'\('itrrc\ — Aquatic Ecosystems 



251 



Table 2. Freshwater gastropod species presumed extmct 

 in the Mobile Bav basin. 



plants, and water withdrawn for domestic water 

 use. Species such as Lepyhum showalteri and 

 LiopUi.x cyclostomafonnis. 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 

 pebbiesnails (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 infor- 

 mation on the distribution of the freshwater 

 limpets (Ancylidae) is not available, but they 

 appear to have suffered similar range restric- 

 tions. 



The uncertainty expressed in the diversity of 

 the historical gastropod fauna presented in 

 Table 1 is indicative of our lack of infonnation 

 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 distribu- 

 tional 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 water- 

 shed disturbances, including 33 major dams for 

 hydroelectric generation, locks and flood con- 

 trol on the major rivers of the Mobile Bay basin, 

 and numerous smaller impoundments on tribu- 

 tary rivers and streams. Most gastropods inhab- 

 iting shoal areas are gill-breathing species typi- 

 cally 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 sig- 

 nificant portion of the invertebrate biomass liv- 

 ing 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 prac- 

 tices. 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. 



Fig. 3. Illustration of a representa- 

 tive species of the e.xtinct slitshell 

 genus Gyrotoma from Butting 

 Ram Shoals. Coosa River. 

 Alabama. 



