Abstract
The sabre‐toothed blennies (tribe Nemophini) are renowned for vivid colouration and several evolutionary adaptations, including a venom that resembles opioids in both chemistry and function. The genus Plagiotremus contains 11 species distributed across the Indo‐Pacific and Red Sea, including two nominal species endemic to Hawai'i. Here, we use mitochondrial COI and cytb in addition to select morphological characters to examine the relationships of Hawaiian endemics to their widely distributed sister species, along with their phylogeography and evolutionary relationships above and below the species level. The Hawaiian endemic P. ewaensis was fully diverged from the widespread P. rhinorhynchos at COI (~4.5%), with genetic diversity patterns suggesting the possibility of a Hawaiian origin for the species pair. In contrast, the endemic P. goslinei was polyphyletic with the widespread P. tapeinosoma , sharing several haplotypes in both COI and cytb and indicating recent/ongoing contact and population‐level separation. The Marquesan population of P. tapeinosoma was morphologically and genetically distinct and ~1.7%–2.9% divergent from all other groups, and may warrant elevation to species. The P. goslinei / tapeinosoma group showed significant population differentiation across the Indo‐Pacific (COI Φ st = 0.786, p st = 0.801, p ewaensis / rhinorhynchos group showed low non‐significant differentiation (COI Φ st = 0.073, p = 0.06) with one frequent COI haplotype shared within P. rhinorhynchos across the Indo‐Pacific. Ancestral reconstruction analyses of P. goslinei / tapeinosoma / azaleus invoke the tantalizing possibility of an East Pacific origin for the species group. Our findings provide further support for biodiversity feedback in the Indo‐Pacific and show that endemic Hawaiian species with widely distributed sister taxa may have dramatically different evolutionary histories.