Distribution of freshwater snails in the man-made Oyan Reservoir, Ogun State, Nigeria

Abstract

The factors influencing patterns of snail distribution in Oyan Reservoir, a typically medium sized man-made reservoir in southwest Nigeria, were investigated once a month, for 28 months (August 1990-November 1992). During each monthly visit, seven stations were sampled for relative snail density, vegetation cover and physical and chemical properties of the lake water. Statistical analyses were performed to determine the influence of these factors on snail abundance and distribution. Seven aquatic snail species were found in the reservoir, namely Bulinus globosus, B. forskalii, B. truncatus, Lymnaea natalensis, Indoplanorbis exustus, Potadoma freethi and Potadoma moerchi. The relative proportion of these species was 17.6% (B. globosus), 16.1% (B. forskalii), 0.2% (B. truncatus), 12.9% (L. natalensis), 23.4% (I. exustus) and 29.8% (both Potadoma spp. combined) of 28 183 snails collected over the study period. The seasonal trends of the species were variable but basically bimodal with peaks in August (rainy season) and November/December (early dry season). These trends were similar to the water level and rainfall patterns, suggesting that seasonal changes in snail density were probably influenced by these factors. Spatial distribution of each snail species was similar along each shoreline but significantly different across the E-W transect. Similar trends occurred for conductivity, dissolved oxygen and biochemical oxygen demand suggesting that these factors may have affected snail distribution across shorelines. Ten species of aquatic vegetation were identified, six of which correlated positively and significantly with various snail species. The most important of the associating vegetation was Impatiens irvingii which correlated with all snail species and served as an indicator plant for Indoplanorbis exustus, an introduced snail species in Nigeria.