Reassessment of Pectinida (Mollusca: Bivalvia) phylogenetic relationships and description of a new Parvamussium species

Phylogenetic relationships within Pectinida (Bivalvia), particularly the superfamily Pectinoidea, have long been contentious. This study reconstructs a robust evolutionary framework for Pectinida, resolving key uncertainties. Multi-gene analyses confirm Dimyidae, Plicatulidae, and Anomioidea as the earliest diverging clade, being a sister group to all other pectinids and embedded limids. Propeamussiidae is firmly established as monophyletic and a sister-clade to Pectinidae, refuting prior hypotheses of its polyphyly or affinity with Spondylidae. Within Propeamussiidae, Parvamussium and Propeamussium are polyphyletic, while Catillopecten and Similipecten form monophyletic lineages. Two distinct evolutionary lineages are identified within Propeamussiidae, with a core Parvamussium-Propeamussium clade and a pan-propeamussiid lineage comprising four genera. Additionally, we describe Parvamussium liui sp. nov. from New Zealand, characterized by a smooth left valve and parallel ridges on the right anterior auricle. Kimura 2-parameter (K2P) genetic distances based on cytochrome c oxidase subunit I (COI) (4.14%-10.93%) underscore its divergence from congeners. Moreover, we assemble the mitogenomes of the new species and Propeamussium dalli, which align with other propeamussiids in size and base composition but diverge from Pectinidae in gene order, although rearrangements occur in Pr. dalli. Collectively, these findings clarify Pectinida's evolutionary history and redefine Propeamussiidae's phylogenetic boundaries, providing a robust foundation for revising taxonomic frameworks across Pectinida.