Here, we report the complete mitogenome information of the terebellid polychaete, Thelepus plagiostoma (Schmarda, 1861). Genome sequencing by Illumina HiSeq platform permitted assembly of a circular mitochondrial genome of 15,628 bp from T. plagiostoma consisting of 67% AT nucleotides, 13 protein-coding genes (PCGs), 2 ribosomal RNA (rRNA) genes, 22 transfer RNA (tRNA) genes, and a non-coding region in the typical annelid gene composition. Gene order of the T. plagiostoma mitochondrion is identical to those of the Terebelliformia mitogenomes. Phylogenetic reconstruction places T. plagiostoma within the monophyletic subclass Sedentaria, a sister to Pista cristata in the suborder Terebelliformia.

The common sunstar, Crossaster papposus, belongs to the family Solasteridae whose ordinal classification has been unstable. Here, for the first time, we assembled and annotated the complete mitochondrial genome of the common sunstar, C. papposus Linnaeus, 1767. The circular genome of C. papposus is 16,335 bp in length and contains 13 protein-coding genes (PCGs), 22 transfer RNA (tRNA) genes, a control region, and large and small ribosomal subunits. The overall genomic structure and gene arrangement were identical to the reported mitochondrial genomes of sea star species, and a phylogenetic analysis of 13 PCGs recovers a closest relationship with the derived cluster of the paraphyletic order Valvatida.