114 Lynn B. Starnes and Arthur E. Bogan 



to these deeper, swift areas, anchored between rocks or in sand-filled 

 cracks in bedrock. 



Populations of Corbicula were widespread and four to six times as 

 dense as unionid populations, with 10.75, 43.73, and 46.59 individ- 

 uals/ m 2 at the three quantitatively sampled stations (Table 4). Relict 

 Corbicula shells were found in great numbers along banks and in pools. 

 While muskrats harvest both unionids and Corbicula, the ratio of Cor- 

 bicula to unionid shells in 1981 seemed disproportionately high, indicat- 

 ing possible changes in Corbicula populations. 



Gastropods identified from Little South Fork included: Goniobasis 

 semicarinata (Say, 1829), G. ebenum (Lea, 1941), Pleurocera acuta 

 (Rafinesque, 1831), Physella sp., and Campeloma crassulum (Rafin- 

 esque, 1819). Campeloma rubrum is widely distributed, yet relatively 

 uncommon in the river outside of its preferred habitat. Live specimens 

 were restricted to mud (loamy) banks. Consequently, Campeloma was 

 not collected in quantitative surveys. The upstream limit for gastropods 

 was not determined; however, at the uppermost collecting locality their 

 numbers were significantly lower, ranging from to 8 individuals/ m 2 . 

 Densities of gastropods ranged from 1 to 25 individuals/ m 2 at Station 4, 

 from 5 to 38 individuals/ m 2 at Station 8, and from to 12 indivi- 

 duals/ m 2 at Station 16. 



DISCUSSION AND CONCLUSIONS 



The Little South Fork of the Cumberland River contains perhaps 

 one of the last extant representative populations of the Cumberlandian 

 mollusk fauna in Kentucky. A total of 24 unionid, one corbiculid, and 5 

 gastropod species are reported in these surveys. After clarification of 

 unionid taxonomy, recent Little South Fork samples were compared 

 with historical Big South Fork records, revealing that approximately 

 one-third of Big South Fork species are absent from Little South Fork. 

 We can only speculate on the factor(s) responsible for the absence of 

 these 15 species. Along these lines, Stansbery and Clench (1975, 1978) 

 listed factors possibly limiting molluscan populations, including avail- 

 able nutrients, stable substrate or dissolved calcium. Unionid distribu- 

 tions in Little South Fork are most likely limited by a combination of 

 factors, including the possible absence of the proper fish host. 



Unionid densities were greatest in current-swept substrates. Opti- 

 mal habitat was riffles with a relatively coarse substrate, in water 10 to 

 25 cm deep. Downstream density decreases were related to increases in 

 percentage of bedrock in pools and riffles, which reduced available 

 habitat. 



Distribution of the naiad fauna is curtailed where the river gradient 

 increases from 1.2 m/km to 1.8 m/km before reaching the 3.8 m/km 



