The complete mitochondrial genome of Conger erebennus (Jordan & Snyder, 1901) (Anguilliformes: Congridae) from Liaoning Province, China

Abstract This study aims to explore the complete mitochondrial genome and phylogenetic relationship of Conger erebennus (Jordan & Snyder, 1901) (Anguilliformes: Congridae). The information about the mitogenome was obtained using Illumina’s next-generation sequencing technologies. The results revealed that the length of the circular mitogenome was 18,777 bp and it contained 37 genes and three unidentical control regions (CRs). The mitogenome contained 62.78% AT bases with an anti-G bias similar to that in most marine fishes. The maximum-likelihood phylogenetic tree of Anguiliformes based on encoding genes revealed that C. erebennus was in the same clade as other species of Congridae. The results of this study will progress the future of evolutionary research and species identification.

Conger erebennus (Jordan & Snyder, 1901) (Anguilliformes: Congridae) is mainly found in the Japanese waters of the south of Hokkaido and the Korean Peninsula (Odani et al. 2005), and was first reported from the coastal waters of China in the present study. C. erebennus is a rare nocturnal rare deep-sea fish occasionally caught by fishing nets. During the day, it hides in caves within sandy-mud substrates. Partial molecular sequence analyses of 16S rRNA (GenBank accession number: AB922176.1) and 12S rRNA (GenBank accession number: LC421664.1) have been performed on C. erebennus; however, the complete mitogenome remains undeciphered. In this study, we have revealed the circular mitogenome and phylogenic relationship of a C. erebennus specimen.
The specimen of C. erebennus (no. LHYT2020-021) was collected from Zhangzi Island in Changhai County, north of the Yellow Sea (122 01 0 E, 39 10 0 N), on 20 September 2021, and was kept in the museum of Liaoning Ocean and Fisheries Science Research Institute (Dong Guo, guohongtiansky@163. com). The total genomic DNA of the muscle tissue was extracted by the modified cetyltrimethylammonium bromide (CTAB) method (Sambrook and Russell 2001). NEBNext Ultra DNA Library Prep Kit was used to construct the 500-bp paired-end DNA library for Illumina sequencing. The complete mitogenome was deciphered by Illumina NovaSeq 6000 sequencing platform. The mitogenome of C. erebennus was assembled and annotated using the online MITOS tool, with the mitogenome of a closely related species, Conger japonicus (GenBank accession number: NC_027186.1), as reference.
The circular mitogenome of C. erebennus is 18,777 bp long (GenBank accession number: OM691699), making it the longest mitogenome sequence published from the order Anguilliformes (Zhang et al. 2021). The gene component of its mitogenome is highly similar to that of most vertebrates (Boore 1999). It contains 12S rRNA, 16S rRNA, 13 protein-coding genes, 22 tRNA genes, and three control regions (CRs). ND6 and eight tRNA genes (Ala, Asn, Cys, Gln, Glu, Ser-UCN, Tyr, and Pro) are encoded on the L-strand, while other mitochondrial genes are encoded on the H-strand. The mitochondrial nucleotide composition of C. erebennus is 33.62% A, 29.16% T, 22.11% C, and 15.11% G. The mitochondrial base composition of C. erebennus has an anti-G bias like most marine fishes (Miya et al. 2003), and a slight excess of AT (62.78%). The initiation codon for 11 protein-coding genes (ATP8, ATP6, COII, COIII, ND1, ND2, ND3, ND4, ND4L, ND5, and ND6) is ATG, while it is ATA and ATT for Cytb and COI, respectively. Thirteen protein-coding genes had different stop codons. The stop codon for 11 genes (ND1, ND4L, ND5, ND6, ND2, ND3, ATP8, Cytb, COI, COIII, and ATP6) is TAR (TAA and TAG), whereas for ND4 it is AGA. The stop codon of COII is a single base T, which may be finished by post-transcriptional polyadenylation which adds poly A tails to the 3 0 end of mRNA (Ojala et al. 1981). The longest and shortest proteincoding genes in the mitogenome are ND5 (1842 bp) and ATP8 (168 bp), respectively. The 12S rRNA and 16S rRNAs are separated by the tRNAVal gene and are situated between the tRNAPhe and tRNALeu (UUR) genes.
We performed the phylogenetic relationship analyses to choose one species in the order Saccopharyngiformes as an outgroup. The maximum-likelihood (ML) phylogenetic tree for encoding genes (Figure 1) was constructed using the software PhyML 3.0 and the MtGTR þ IþG nucleotide substitution mode. The ML phylogenetic tree of 18 Anguilliformes species indicates that C. erebennus is classified in the family Congridae, and the Congridae is closely related to the Nettastomatidae. Currently, Nettastomatidae is a polyphyletic group.

Ethics statement
This study was approved by the Academic Committee of Liaoning Ocean and Fisheries Science Research Institute. All of the experimental animals used in the study were used in accordance with the regulations of the National Administration of Laboratory Animals.

Author contributions
Dong Guo and Weikuang Wang conceived and designed the experiments. Long Yan and Jie Liu carried out the whole experiment, analyzed and interpreted the data. Long Yan drafted the paper herself. Dong Guo and Weikuang Wang revised it critically for intellectual content, and approved the version to be published. All authors agree to be accountable for all aspects of the work.