Insights into the divergent evolution of the oceanic squid Sthenoteuthis oualaniensis (Cephalopoda: Ommastrephidae) from the Indian Ocean

Abstract

Sthenoteuthis oualaniensis is known for its complex population structure with three major transoceanic forms (viz. middle‐sized, dwarf, and giant forms) whose taxonomic status has been disputed for decades. This integrated taxonomic study examines these prevenient morphotypes gathered on cruises in the Indian Ocean to ascertain their status in the evolutionary history of the species. Molecular analyses employing mitochondrial (COI, ND2) and nuclear (H3) markers revealed four genetically distinct and novel lineages of the species in the Indian Ocean, representing three morphotypes from the Arabian Sea and one from the Southern Indian Ocean. The mitochondrial‐based phylograms revealed two distinct clades in the species: “dwarf forms + giant form” and “middle‐sized forms,” which further branch into geographically structured evolutionary units. Species delimitation analyses recovered five distinct clades, namely, the Arabian Sea giant and dwarf forms, Equatorial, Eastern Typical, and Other Middle‐sized forms, representing the consensus molecular operational taxonomic units. H3 being heterozygous could not resolve the phylogeny. Haplotype network and AMOVA analysis of mtDNA genes indicated explicit phylogeographic structuring of haplotypes, whereas these outputs and PCA results were incongruent with the morphological grouping. Phenetic features distinguishing the morphotypes were sometimes plastic and mismatched with the genotypes. The giant form was genetically close to the dwarf forms, contradicting the earlier notion that it descended from the middle‐sized form. It may be assumed that the dwarf form evolved following sympatric speciation and adaptation to warm equatorial waters, while the focal features of the Western Arabian Sea guide toward allopatric speciation of the giant form.

Habitat Suitability of the Squid Sthenoteuthis oualaniensis in Northern Indian Ocean Based on Different Weights

“…The results in February and December were poor, and the fishing efforts in suitable habitats accounted for more than 60% of the total, which was lower than that in other months. At the same time, according to the superimposed map of habitat suitability index and CPUE value in 2019, the distribution range of suitable habitat for northern Indian Ocean S. oualaniensis was roughly 12~18 • N and 55~65 • E. This is also the main distribution area indicated by scholars studying the central fishing ground for S. oualaniensis in the northern Indian Ocean [34,35]. The range of suitable habitat in February was concentrated in the north.…”

Size and weight relationships of the purpleback flying squid, Sthenoteuthis oualaniensis (Cephalopoda: Ommastrephidae), from Arabian Sea

“…Similarly, three populations with distinguishable size ranges were identified (Yatsu, 1997). Recently, Jeena et al (2023) reported four genetically distinct and novel lineages of S. oualaniensis in the Indian Ocean, representing three morphotypes from the Arabian Sea (dwarf forms, giant form, and middle-sized forms). The maximum mantle length (ML) of S. oualaniensis is reported as 690 mm dorsal mantle length (DML), and the life span is about 0.5 to 1 year (Trotsenko and Pincukov, 1994).…”