Molecular phylogeny of selected dorid nudibranchs based on complete mitochondrial genome

Dorid nudibranchs are a large group of mollusks with approximately 2,000 recorded species. Although agreement exists on the monophyletic nature of the dorid nudibranch group, the interfamily relationships of the suborder are subject to debate. Despite efforts to elucidate this issue using short molecular markers, the conclusiveness of the findings has been hindered by branching polytomy. Mitogenomes are known to be effective markers for use in phylogenetic investigations. In this study, eight mitogenomes of dorid nudibranchs were decoded and analyzed. Gene content and structure showed little change among species, reflecting the conserved mitogenomes of dorid nudibranchs. For most genes, the direction was typical for nudibranchs; nevertheless, tRNA^Cys had an inverse direction in Cadlinidae species. Phylogenetic trees based on nucleotide and amino acid datasets revealed a relatively consistent pattern of interfamily relationships with little difference for positions of Phyllidiidae and Cadlinidae. Species of Cadlinidae were clustered together and did not form a clade with Chromododidae. Additionally, Goniodorididae was sister to Aegiridae, whereas Discodoridae was sister to Dorididae. This finding was supported by tree topology test based on mitogenome data. The results of the present study indicate that complete mitogenomes are promising markers for investigating interfamily relationships among dorid nudibranchs.