Characterization and phylogenetic analysis of the complete mitochondrial genome of Glabracollonia laevigata (Gastropoda: Trochida: Colloniidae) from East China Sea

Organic falls play an important role in the evolutionary transition of chemosymbiotic mollusks from shallow to deep seas. Exploring their genetic characteristics is essential to elucidating adaptive mechanisms. Here, we report the first complete mitochondrial genome of Glabracollonia laevigata (Family Colloniidae), collected from sunken wood at 200 m depth in the East China Sea. The circular genome is 18,056 bp in length with an average sequencing depth of 6847×, and contains 13 protein-coding genes (PCGs), two rRNA genes, and 23 tRNA genes, including a duplicated trnY. The nucleotide composition is biased toward A + T (69.0%). Most PCGs use ATG as the start codon, except nad6 (GTG) and nad4L (ATA), and all terminate with TAA or TAG. Codon usage analysis revealed a preference for A and U at the third codon position. Phylogenetic analyses based on concatenated PCGs, using Bayesian inference and maximum likelihood approaches, strongly support a close relationship between Colloniidae and Phasianellidae, consistent with recent studies. These findings offer new insights into the phylogenetic placement of Colloniidae within Trochoidea, while contributing to a broader understanding of biodiversity and evolutionary adaptation in organic-fall ecosystems.