Shedding light on the Holocephali taxonomy, the mitogenome of Chimaera opalescens

Cartilaginous fish (Chondrichthyes), i.e. sharks, rays, and chimaeras, are extremely interesting from a biological perspective as they represent one of the oldest and most ecologically diverse groups of jawed vertebrates. Their K-selective reproductive traits, make them vulnerable to overfishing. Chimaerid are also a frequent by-catch of deep-water fisheries. Holocephalans comprise a single surviving order, the Chimaeriformes, and are allocated into three different families: Callorhinchidae, Rhinochimaeridae and Chimaeridae (Weigmann 2016). Furthermore, the family Chimaeridae only includes two genera: Chimera and Hydrolagus. Recently, several new species have been described (Iglesias et al., 2022), including Chimera opalescens from deep-sea assemblages (Luchetti et al., 2011). However, previous records of C. opalescens were erroneously classified as Chimera monstrosa (Luchetti et al., 2011; Catarino et al., 2020), due to the similar morphology (Luchetti et al., 2011), which highlights the critical importance of molecular approaches to support species identification. Mitogenomes have been a powerful tool used to elucidate phylogenetic relationships, both at deep and at shallow evolutionary nodes. The development of long-read sequencing technologies a precise and reliable assembly of complete mtDNA genomes. The sequencing and characterization of the complete mitogenome of the opal chimera Chimera opalescens (Luchetti, Iglesias et al., 2011) was carried out, using the long-read technique PacBio HiFi. The entire mitogenome was 23,411 bp long and shows the same overall content, i.e. 13 protein-coding genes, 22 transfer RNA and 2 ribosomal RNA genes, as all other examined chondrichthyan mitogenomes. Phylogenetic reconstructions using all available Chondrichthyan mitogenomes, including 11 Holocephali (chimeras and ratfishes), places C. opalescens within the Chimaeridae family. Furthermore, the results reinforce previous findings, showing the genus Chimera as paraphyletic and thus highlighting the need to expand molecular approaches in this group of cartilaginous fishes.