Three New Mitochondrial Genomes of Semisulcospiridae J. P. E. Morrison, 1952 (Caenogastropoda: Cerithioidea) from China and Insights into Their Phylogenetic Position

<b>Background:</b> Semisulcospiridae is a family of freshwater gastropods with over 100 species, primarily distributed in East Asia and North America. They play crucial ecological roles and are of medical importance as intermediate hosts for parasites. However, their phylogenetic relationship remains unclear. Most previous studies, which focused on fewer molecular markers (e.g., <i>COI</i>, <i>16S</i>, <i>28S</i>), have shown limitations in resolving relationships with low resolution. Mitochondrial genomes, with their richer phylogenetic information, offer a promising tool to infer the evolutionary relationships within this family. <b>Methods:</b> This study sequenced, assembled, and annotated the complete mitochondrial genomes of three Semisulcospiridae species from China: <i>Koreoleptoxis friniana</i>, <i>Hua textrix</i>, and <i>Hua yangi</i>. Phylogenetic analyses were conducted using Maximum Likelihood (ML) and Bayesian Inference (BI) methods on five distinct datasets derived from the mitochondrial genomes, including nucleotide sequences of protein-coding genes (with and without third codon positions), amino acid sequences, and combinations with two ribosomal RNA genes. <b>Results:</b> The complete (or near-complete) mitochondrial genomes of <i>K. friniana</i>, <i>H. textrix</i>, and <i>H. yangi</i> were 15,474 bp, 15,660 bp, and 15,744 bp in length, respectively, showing typical gene content and an A+T bias. The gene order was highly conserved. Phylogenetic analyses consistently recovered the family Semisulcospiridae as monophyletic and revealed three well-supported, distinct clades corresponding to the genera <i>Semisulcospira</i>, <i>Koreoleptoxis</i>, and <i>Hua</i>. While the overall tree topologies were robust for Semisulcospiridae, some incongruences were observed in the placements of other cerithioidean families depending on the dataset used. Evolutionary rate analysis (Ka/Ks) indicated strong purifying selection across all protein-coding genes, with <i>COX1</i> being the most conserved. <b>Conclusions:</b> This study provided three new mitochondrial genomes for Semisulcospiridae: <i>K. friniana</i>, <i>H. textrix</i>, and <i>H. yangi.</i> Phylogenetic analysis based on mitochondrial genome datasets offers new evidence that supports the monophyly of the three Asian genera of Semisulcospiridae. Future research should include broader taxonomic sampling, particularly of the North American genus <i>Juga</i> and the atypical Japanese <i>Semisulcospira</i> lineages, to achieve a comprehensive phylogenetic framework.