Characterization of the Complete Mitochondrial Genome of Dwarf Form of Purpleback Flying Squid (Sthenoteuthis oualaniensis) and Phylogenetic Analysis of the Family Ommastrephidae

Background: The Ommastrephidae family of cephalopods is important in marine ecosystems as both predators and prey. Species such as Todarodes pacificus, Illex argentinus, and Dosidicus gigas are economically valuable but are threatened by overfishing and environmental changes. The genus Sthenoteuthis, especially S. oualaniensis, shows significant morphological and genetic variation, including medium-sized and dwarf forms found in the South China Sea. Methods: Specimens of S. oualaniensis were collected from the South China Sea, their genomic DNA sequenced, and phylogenetic relationships analyzed using mitochondrial genomes from various Ommastrephidae species. Results: The study presents the complete mitochondrial genome of the dwarf form of S. oualaniensis (20,320 bp) and compares it with the medium-sized form, revealing a typical vertebrate structure with 13 protein-coding genes, 21 tRNA genes, and 2 rRNA genes, along with a strong AT bias. Nucleotide composition analysis shows a 12% genetic divergence between the two forms, suggesting a recent common ancestor and potential cryptic speciation, with all protein-coding genes exhibiting purifying selection based on Ka/Ks ratios below 1. Conclusions: The mitochondrial genome of the dwarf form of S. oualaniensis shows a close evolutionary relationship with the medium-sized form and a 12% genetic divergence, suggesting potential cryptic speciation. These findings underscore the importance of mitochondrial analysis in understanding speciation and guiding future conservation efforts.