Plectorhinchus makranensissp. nov. is described on the basis of 16 specimens from the Persian Gulf and Gulf of Oman, in the Northwest Indian Ocean. The new species can be distinguished from congeners by a combination of dorsal fin rays XII, 18–20, pectoral-fin rays 16–17, tubed lateral-line scales 55–57, gill rakers count (10–12 on the upper limb and 16–17 on the lower limb), 17–18 scales between the lateral line and the first anal-fin spine, 30–31 circumpeduncular scale rows and color pattern. Plectorhinchus makranensissp. nov. is distinguished from P. schotaf by having the posterior margin of the opercular membrane grey (vs. red in P. schotaf), fewer circumpeduncular scale rows, and a shorter base of the soft portion of the dorsal fin, 27.6–29.4% of standard length (SL) (vs. 31–32.3% of SL in P. schotaf). The new species resembles P. sordidus but is differentiated from it by having more gill rakers, a smaller orbit diameter 27.5–32.1% of head length (HL) (vs. 35.5–37.2% of HL in P. sordidus), a longer caudal peduncle 19.2–21.3% of SL (vs. 17.1–17.9% of SL in P. sordidus), and the first to third pectoral-fin rays light gray (vs. dark gray in P. sordidus). The new species can also be distinguished from the other species, including P. schotaf and P. sordidus, based on COI and Cyt b molecular markers. The phylogenetic position of this new species indicates that it is a sister taxon of P. schotaf.
The subfamily Plectorhinchinae (sweetlips) is composed of poorly-known species with high commercially and ecologically values that exhibit phenotypic plasticity and various morphologies. Few studies have assessed the validity of sweetlips, intergeneric relationships and evolutionary survey in this subfamily, which have not yet been resolved. This study investigated the DNA sequences of (1) the mitochondrial COI gene to delimit species, and (2) two mitochondrial (COI and Cyt b), and one nuclear (RAG1) markers to infer phylogenetic relationships and evolutionary and biogeographic history. The molecular results could differentiate Diagramma punctatum from the other species, but failed to distinguish D. labiosum as a distinct species with considerably lower genetic distances for the COI (0.53%) and Cyt b (0.51%) markers. However, additional taxonomic investigations are required to shed light on this issue. All previously described nominal species of sweetlips in the northwest Indian Ocean were found to be well supported. The monophyly of Plectorhinchus is not supported and Diagramma pictum and D. punctatum should be assigned to the genus Plectorhinchus. The biogeographic history of Plectorhinchinae likely originated in the Indo-Pacific ca. 34 Ma (30–39 Ma; late Eocene/ middle Oligocene) and subsequently colonised the Western Indian Ocean and the Central Indo-Pacific. Maximum diversification within the subfamily occurred from the middle Miocene to Pliocene, coinciding with dispersal and vicariance events. Diversification was probably driven by both biological and geographical factors.
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