Reassessment and systematic position of the sinistral snails of genus Hemiplecta from Thailand (Eupulmonata: Ariophantidae), with description of two new species

Indochina land snails of the family Ariophantidae are in need of thorough systematic revision. Here we comprehensively revise the systematics of the large-shelled, sinistral (counterclockwise) coiling snails from Thailand and Peninsular Malaysia. Molecular phylogeny based on mitochondrial (coi and 16S) and nuclear (28S) gene sequences demonstrates that these sinistral snails are not members of Dyakiidae as previously thought, but instead are more closely related to the genus Hemiplecta in the family Ariophantidae. Comparative morphology also reveals similarity of reproductive organ features (globular gametolytic organ, well-developed dart apparatus, and lack of amatorial organ complex). Based on this evidence, we propose to transfer these sinistral snails to the genus Hemiplecta. Molecular phylogenetic analyses further strongly support the monophyly of this sinistral lineage with respect to other members of Hemiplecta. This monophyletic clade consists of five members including three species that were previously classified as “Dyakia”, H. lahatensis, H. retrorsa and H. salangana, and two new species described herein, H. ligorica n. sp. and H. thailandica n. sp. This study also suggests that the anatomy of the gametolytic organ in the genus Hemiplecta corresponds well with the phylogenetic relationships and appears to be a taxonomically informative character, while the penial verge has little utility for generic recognition.