First mitochondrial genomes of Capitellidae and Opheliidae (Annelida) and their phylogenetic placement.

The complete mitochondrial genomes of Notomastus sp. (15,776 bp) (Annelida: Capitellidae) and Armandia sp. (18,538 bp) (Annelida: Opheliidae) were assembled for the first time. A group II intron (303 bp) was found in cox1 of Notomastus sp. A phylogenetic analysis revealed that Notomastus sp. and Armandia sp. were monophyletic, and this clade was clustered with echiurans, although the possibility of the effect of long-branch attraction should be considered.