In this study, the complete mitochondrial genome of the marbled eelpout, Lycodes raridens Taranetz & Andriashev, 1937 was sequenced using the primer walking method. The mitogenome was 16,569 bp in length and encoded with 13 protein-coding genes, 22 transfer RNA genes, two ribosomal RNA genes, and one Non-Coding Region. The overall nucleotide composition of L. raridens is 25.5%, 25.3%, 18.7%, and 30.5% for A, T, G, and C, respectively. Phylogenetic analysis using the ML method showed that L. raridens was clustered into one branch with L. ygreknotatus and L. toyamensis.

The full-length mitochondrial genome of Pseudohelice subquadrata (Dana, 1851) was analyzed by the primer walking method. Its mitogenome is 16,884 bp in length, comprising 13 protein-coding genes, 2 ribosomal RNA genes, and 22 transfer RNA genes. The phylogenetic tree confirmed that P. subquadrata belonged to the subsection Thoractotremata within the Crustacea. This is the first record of the complete mitogenome for the genus Pseudohelice.

The complete mitochondrial genome of a herbivorous amphipod, Ampithoe lacertosa, has been determined for the first time in the present study. The total mitogenome length of A. lacertosa was 14,607 bp with 13 protein-coding genes, two ribosomal RNA genes, and 22 transfer RNA genes. Phylogenetic analysis showed that A. lacertosa belonged to the families Caprellidae, Hyalellidae, Hyalidae, and Talitridae in the same clade, and to the suborder Senticaudata within Amphipoda. This is the first record of the complete mitochondrial genome sequence of the family Ampithoidae.

The present study reports, for the first time, the complete mitochondrial genome (mitogenome) of Undinula vulgaris. The total mitogenome length of U. vulgaris was 15,303 bp with 13 protein-coding genes (PCGs), 2 ribosomal RNAs (rRNAs), 22 transfer RNAs (tRNAs), and 1 non-coding region. Phylogenetic analysis showed that U. vulgaris belonged to the same family. This is the second report of the complete mitogenome sequence of the family Calanidae.

The complete mitochondrial genome of the majoid crab, Oregonia gracilis, was determined from a specimen collected in Korea. The mitochondrial genome is 15,737 bp long and contains 13 protein-coding genes (PCGs), 22 transfer RNA (tRNA) genes, and two ribosomal RNA (rRNA) genes. A maximum-likelihood phylogenetic tree based on the 13 PCGs of the mitochondria showed that O. gracilis is closely related to the genus Chinoecetes. The complete mitochondrial genome of O. gracilis provides valuable information on the mitochondrial evolution of majoid crabs.