| Summary: | The spread of freshwater invasive species through aquarium trade poses a threat to the ecosystem, economy and human health. The availability of the exotic freshwater gastropod mollusc, Planorbarius corneus (Linnaeus, 1758) (Gastropoda: Planorbidae), in pet shops in Kolkata, West Bengal, India, raises concern for its potential establishment as an invasive species. Assuming the role of the native predators as a potential biotic barrier for invasion, observations on the predation pattern of the malacophagous leech, Glossiphonia weberi (Blanchard, 1897) (Clitellata: Glossiphoniidae) against P. corneus was made in single as well as in combination with the native snail Indoplanorbis exustus (Deshayes, 1834) (Gastropoda: Planorbidae). Using varying size classes and densities of P. corneus, the predation potential of G. weberi was estimated after a 24 hour period. In another set of experiments, the predation of G. weberi against the snail P. corneus was observed in the presence of I. exustus under four different habitat conditions. The purpose was to justify - (1) the effect of conspecific and heterospecific conditions of the prey availability and (2) the effect of habitat complexity on the predatory efficacy of G. weberi. The results indicated that G. weberi consumed varied numbers of P. corneus, depending on the size and the densities of the prey and predator. Although the prey heterogeneity and complex habitats caused significant reduction of predation on both of the prey species, G. weberi significantly preferred P. corneus over I. exustus in open (P < 0.0001), macrophyte (P = 0.002) and pebbles and macrophyte containing habitats (P < 0.0001). Apparently, G. weberi preferred the exotic snail P. corneus in presence of alternative prey snail I. exustus, under different habitat conditions. Therefore, G. weberi may act as a significant biotic resistance against the colonization and establishment of P. corneus in the Indian context. However, further studies including the multiple prey and predators are required to ascertain the food web level impact of the exotic snail P. corneus in invaded freshwater ecosystems.
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