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The freshwater snail Gundlachia ticaga (Marcus & Marcus, 1962) is widely distributed in Brazil, but its morphology has been poorly studied. We compared the shell morphology of samples from four sites (Vila do Abraão, Vila de Provetá, Parnaioca and Praia do Sul) in Ilha Grande (Angra dos Reis, state of Rio de Janeiro) in order to evaluate differences within and among four populations. We used nine morphometric characters representing shell size and shape. To analyze shell morphology we considered aperture shape, sculpture of teleoconch, apex carving and position. The resulting dataset was correlated by Pearson´s linear correlation and shell differences among populations were tested using ANOVA and Discriminant Function Analysis. The results showed that there is significant variation among populations concerning shell shape and morphology. Shells from preserved localities (Praia do Sul and Parnaioca) and shells from highly modified environments (Vila do Abraão and Vila de Provetá) were the most similar to each other. Results from the visual assessment and from the discriminant analysis were almost identical. The shell indices are the most important variables in the differentiation of samples. The observed variation corroborates the hypothesis that G. ticaga displays phenotypic plasticity, which may lead to wrong identifications. Narrower shells with an elongate aperture could be misidentified as Ferrissia Walker, 1903 and, broader shells with a roundish aperture could be wrongly identified as Burnupia Walker, 1912. We confirmed that the absence of radial lines is not a good diagnostic character for G. ticaga. The analysis of the apical micro-sculpture and soft parts is essential for a correct identification

Shell morphology of the freshwater snail Gundlachia ticaga (Gastropoda: Ancylidae) from four sites in Ilha Grande, southeastern Brazil