Over the last few decades, advances in molecular techniques have led to the detection of strong geographic population structure and cryptic speciation in many benthic marine taxa, even those with long-lived pelagic larval stages. Polychaete annelids, in particular, generally show a high degree of population divergence, especially in mitochondrial genes. Rarely have molecular studies confirmed the presence of ‘cosmopolitan’ species. The amphinomid polychaete Hermodice carunculata was long considered the sole species within its genus, with...