Diversity and distribution of freshwater molluscan fauna in reservoirs and headwater streams of the Kala Oya river basin in Sri Lanka

In this study, the variation of aquatic molluscan diversity concerning seasonal water level fluctuations (WLF) and sediment texture was investigated in some perennial irrigation reservoirs and the associated head headwater streams in the Kala Oya river basin in Sri Lanka. Altogether 11,752 molluscan specimens belonging to 10 gastropod taxa (96.6%) and 2 bivalve taxa (3.4%) were identified. Of these, 11,282 specimens (96.0%) were from reservoir sampling sites and the remainder 470 (4.0%) was from the associated headwater streams. Of the gastropods, the individuals of the genus Melanoides showed the highest distribution index (DI) of 0.75 followed by Bellamya (DI = 0.63) and Thiara (DI = 0.60). The DI of the gastropod molluscs showed a significant positive relationship with their shell size (p < 0.05). Although molluscan diversity and taxa richness were comparatively lower in headwater streams than in reservoir sampling sites, Viviparidae (Bellamya; 0.06±0.03), Bithyniidae (Bithynia; 0.21±0.07) and Thiaridae (Thiara; 4.8±1.9 and Melanoides; 1.1±0.8) showed higher relative abundance (expressed as individuals per dm³) in the headwater streams. The reservoir water level fluctuation showed a significant negative relationship with molluscan taxa richness (p < 0.05), but not with the Shannon-Weaver diversity index (p = 0.073). The distribution of molluscs was also found to be related to the sediment texture where the most abundant gastropod genera Bellamia, Bithynia, and Thiara were widely distributed in sandy substrates in reservoirs and associated streams in the Kala Oya river basin.