Mitochondrial Genomes of Three Species of the Family Camaenidae (Gastropoda: Stylommatophora): Structural Features, Codon Usage Patterns, and Phylogenetic Implications

This study sequenced the complete mitochondrial genomes of three Camaenidae snail species (Acusta ravida, Bradybaena similaris, and Trichobradybaena submissa). Each mitogenome contained the typical 37 genes (13 PCGs, 22 tRNAs, and 2 rRNAs), with most tRNAs forming clover structures (except trnK, trnH, trnN, trnS1, trnS2, and trnR). Comparative analysis with nine existing Camaenidae mitogenomes revealed: (1) The nucleotide composition of mitochondrial genes was significantly skewed toward A and T; (2) Most protein genes used ATN start codons and TAA/T- stop codons; (3) Synonymous codons ending with T/A were dominant. Analyses (PR2-plot, neutrality plot) indicated codon usage was influenced by both mutation pressure and natural selection. Phylogenetically, the three newly sequenced species clustered within Bradybaeninae of Camaenidae, consistent with previous classifications. The results support Hygromiidae and Geomitridae as sister groups and identify Polygyridae as the sister clade to Camaenidae. Additionally, they also partially support elevating Satsuma and its subgenus to subfamily status. This study enriches mitogenomic resources for Camaenidae and provides foundational data for future research on codon usage, adaptive evolution, and phylogenetics within this family.