Thrissina mystica sp. nov., a new thryssa (Teleostei: Clupeiformes: Engraulidae: Coiliinae) from the northern Indian Ocean

Abstract Thrissina mystica sp. nov. is described on the basis of 62 specimens collected from the northern Indian Ocean from Kuwait to Sri Lanka. The new species closely resembles Thrissina katana Hata, Lavoué and Motomura 2022 and Thrissina malabarica (Bloch 1795), all having a moderate maxilla (extending posteriorly beyond the posterior margin of the opercle, but not reaching the pectoral-fin insertion), a pair of dark longitudinal lines on the dorsum, a black blotch behind the upper part of the gill opening, the snout tip above the level of the eye center, and small conical teeth on both jaws. However, the new species differs from T. katana and T. malabarica in having fewer longitudinal series scale rows (38–42 vs. 40 or more) and a greater number of branched anal-fin rays (usually 36–40 vs. 38 or less); from T. katana by a lower number of total vertebrae [44–46 (mostly 45) vs. 45–47 (modally 46)], and shorter pelvic fin [7.4–8.7% of standard length (SL) vs. 7.5–10.2%]; and from T. malabarica in having greater numbers of gill rakers on each gill arch, and a narrower body (26.1–30.0% of SL vs. 28.3–32.0%). The morphology of genetically assigned specimens of the new species and T. katana and T. malabarica was investigated using traditional morphology methods applying non-parametric (Kruskal-Wallis) and parametric (ANOVA) tests to identify and evaluate the performance of meristic and morphometric characters, respectively. Combined with Principal Component Analysis (PCA), the results show that features such as scale rows in longitudinal series, branched anal-fin rays, total vertebrae, gill rakers, pelvic scutes, body depth and pelvic-fin length provide significant level discrimination to separate the new species from T. katana and T. malabarica . A molecular analysis of T. mystica sp. nov., T. katana and T. malabarica revealed they form a monophyletic group within the genus Thrissina . Thrissina mystica sp. nov. and the latter two species diverge by 1.2% and 1.8% mean p-distances in the mitochondrial cytochrome oxidase I (COI) gene. Thrissina mystica sp. nov. and T. malabarica represent another example of the west-east differentiation along the northern region of the Indian Ocean.