Morphological similarity and molecular divergence of Trochulus striolatus and T. montanus,and their relationship to sympatric congeners (Gastropoda: Pulmonata: Hygromiidae)

Shell morphology has played an important role in the differentiation of mollusc species. However, extensive morphological variation and the lack of readily available diagnostic characters often lead to confusion and controversy in taxonomy of closely related species, such as the genus Trochulus. Two nominal species Trochulus striolatus and T. montanus show only subtle differences in their reproductive systems and are indistinguishable on the basis of sole conchological traits. Therefore, we performed molecular and morphological analyses to establish robust species limits among the taxa. The canonical discriminant analysis (CDA) of shell characters confirmed strong similarity between the species. It also revealed high intraspecific variability of the shell morphology, which allowed the distinguishing of some populations as two extremely distinct forms, while simultaneously making it difficult to discriminate between the two species. Analysis of the genital morphology indicated that differences were continuous and practically negligible among the species. In contrast, phylogenetic analyses based on newly obtained 114 Trochulus sequences of the mitochondrial cytochrome c oxidase subunit I (COI) gene and 55 homologues retrieved from the GenBank database showed clear genetic divergence between T. striolatus and T. montanus. Moreover, they revealed a close relationship between some T. montanus, some T. caelatus and T. clandestinus samples, which formed a monophyletic group. Some of their haplotypes were even identical. It suggested that their recent common origin or recurrent gene flow occurred between these populations. However, T. sericeus sequences were split into independent diverged lineages, which were separated by many unidentified Trochulus species. It suggested that this taxon may represent a paraphyletic species complex.