The absence of the atp8 gene in the mitochondrial genomes of Platyhelminthes and certain other mollusks has garnered significant attention in recent years. Growing evidence suggests that this gene loss may be associated with the high divergence and length variability of atp8, making it particularly challenging for automated annotation. Analysis of putative atp8 genes in the families Dugesiidae and Dendrocoelidae (superfamily Geoplanoidea) revealed substantial differences in gene structure and hydrophobic patterns of amino acid sequences. These variations may result from divergent selective pressures on atp8, driven by distinct ecological niches and survival strategies. This superfamily warrants further in-depth investigation. Phylogenetic tree reconstruction of sampled specimens indicated that Dugesia japonica forms a sister clade to the studied population, albeit with distant genetic relatedness, suggesting the latter may represent a cryptic species within Dendrocoelidae. These findings contribute to expanding and refining the genetic database of Dendrocoelidae.
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