Cryptic biodiversity in the commercial diamondback squid Thysanoteuthis rhombus Troschel 1857

Cephalopod fisheries are increasing, but little is known about the cryptic diversity of some key commercial species. Recent studies have shown that cryptic speciation is common in cephalopods, including several oceanic squids formerly considered ‘cosmopolitan species.’ Further efforts are needed to investigate the cryptic diversity of commercial species, to inform management and support sustainable fisheries practices. Thysanoteuthis rhombus is an oceanic squid, currently recognized as the single species of the family Thysanoteuthidae. Thysanoteuthis. rhombus has a global distribution in tropical and subtropical waters and is an economically important species, with the highest catches occurring off Okinawa in Japan and of potential fishery resource for other countries due to its high abundance and large size. Here, we used sequences from 12S rRNA, 16S rRNA, and cytochrome c oxidase I to characterize its cryptic diversity using samples collected throughout most of its known geographic range. We identified three different putative species whose distributions are concordant with main ocean basins: Thysanoteuthis major, the most abundant species, is widely distributed in the North Pacific Ocean, North Indian Ocean, and limits of the South Atlantic Ocean; Thysanoteuthis rhombus is distributed in the North and South Atlantic Ocean and Mediterranean Sea; and Thysanoteuthis cf. filiferum, likely the least sampled to date, is found in the southwestern Pacific Ocean. A sister relationship was observed between T. rhombus and T. major, and T. cf. filiferum was found to be the most divergent species. Based on our divergence estimation, we hypothesize that the closure of the Isthmus of Panama during the early Pliocene played a significant role in the split of T. rhombus and T. major, while the split of their ancestor from T. cf. filiferum coincided with an increase in the Pacific Walker Circulation and the longitudinal gradient of surface temperatures in the Pacific Ocean during