Alentiana , a genus within the subfamily Lepidastheniinae of Polynoidae, is an obligately commensal deep‐sea group exclusively associated with sea anemones. Currently, only two valid species are recognized, with the genus’s species diversity and phylogenetic relationships remaining poorly understood. In this study, we systematically investigated polynoid specimens collected from four seamounts in the tropical western Pacific. Integrating morphological observations (including SEM observations of chaetal ultrastructure) and genome skimming data, we describe two new species: Alentiana barnichae sp. nov. and A. fiegei sp. nov., clarifying their morphological diagnostic characters and body size‐related intraspecific variations. Phylogenetic analyses based on 13 mitochondrial protein‐coding genes and four ribosomal genes revealed that Alentiana forms a well‐supported monophyletic clade, which is sister to the deep‐sea subfamily Macellicephalinae but not closely related to other genera traditionally assigned to Lepidastheniinae. Lepidastheniinae was clearly recovered as polyphyletic, and notably, it shares terminally inserted lateral antennae with Lepidonotinae without exhibiting a close phylogenetic relationship, thus challenging the reliability of traditional taxonomic characters. Ancestral state reconstruction indicated that terminally or subterminally inserted lateral antennae represent a plesiomorphy of Polynoidae, while deeply incised neuropodia are a product of convergent evolution presumably driven by ecological pressures and cannot serve as an exclusive diagnostic character for Lepidastheniinae. This study enriches the species diversity and geographical distribution records of Alentiana , clarifies its phylogenetic position, and provides insights for revising the taxonomic system of Polynoidae and understanding the evolution of key morphological traits.
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