Vignette of mitogenome and evolutionary status of the giant form of Sthenoteuthis oualaniensis (Lesson, 1830) endemic to the northwestern Arabian Sea

The Indo-Pacific purpleback flying squid Sthenoteuthis oualaniensis has the most complicated population structure reported to date among the ommastrephid squids. The highest density occurs in the Arabian Sea in the northwestern Indian Ocean, where fishing interests in this species are developing. This study offers the first report on the mitogenome of the enigmatic giant form of this species believed to be endemic to this region for better resolution of its evolutionary history. We used low-coverage whole genome sequencing in Illumina and assembled the complete mitogenome using the NOVOPlasty pipeline. The mitochondrial genome was AT-rich, 20318 bp in length, and contained 18 PCGs, 23 tRNAs, two rRNAs, and two non-coding regions. The plus stand encoded 22 of the 43 genes while the others were encoded by the minus strand. In the mitogenome, six genes and the non-coding region were all duplicated. Codon usage analysis suggested three predominant codon families that are AT-rich. The skew statistics of the PCGs suggested a compositional symmetry between different forms of S. oualaniensis. Ka/Ks analysis in its lineages indicated that PCGs are under purifying selection. Phylogenetic analyses recovered all lineages of the species into two sister clades, one containing the giant form along with the dwarf form and the second containing all middle-sized forms, despite the morphological similarity of the giant form with the middle-sized form. The major lineages appear to have originated through adaptive radiation. The lack of distinct morphological differences between certain forms indicates that they are probably in the grey zone of speciation