Towards a systematic revision of the superfamily Cyrenoidea (Bivalvia: Imparidentia): species delimitation, multi-locus phylogeny and mitochondrial phylogenomics

Cyrenoidea is a superfamily of bivalves (Bivalvia: Imparidentia) currently comprising three families (Cyrenidae, Cyrenoididae and Glauconomidae). The superfamily is widely distributed in marine, brackish and freshwater environments, with an estimated 60 or more living species. Recent phylogenetic results have confirmed the monophyly of Cyrenoidea and placement in Venerida. Nevertheless, a comprehensive phylogenetic analysis of Cyrenoidea remains elusive and the phylogeny is unresolved due to inadequate sampling in previous studies. Moreover, the taxonomy and delimitation of most species, originally based on shell morphology, have not yet been tested with molecular data. Here, we constructed three molecular datasets by sequencing three markers (COI + 16S rRNA + 28S rRNA) and complete mitogenomes for Geloina coaxans (Gmelin, 1791) and Glauconome virens (Linnaeus, 1767). COI barcoding clarifies the validity of Geloina coaxans and Geloina erosa that have been subject to controversy regarding synonymy. Additionally, the barcoding supports the existence of multiple cryptic species within the Geloina expansa complex. A multi-locus dataset (COI + 16S rRNA + 28S rRNA) provides the most comprehensive phylogeny of all eight recognised genera of Cyrenoidea to date. Phylogenetic results indicate that the currently recognised family Cyrenidae is polyphyletic. The type species Geloina coaxans, Cyanocyclas limosa (Maton, 1811) and Polymesoda caroliniana (Bosc, 1801) that have long been classified within the family Cyrenidae based on shell morphology, have a closer relationship with Cyrenoida floridana Dall, 1896 than with other Cyrenidae. Therefore we transfer the genera Geloina, Cyanocyclas and Polymesoda from the family Cyrenidae to the family Cyrenoididae. The mitochondrial phylogenomics further support the family-level relationships in Cyrenoidea obtained from the three-gene analyses, confirming that the newly defined Cyrenoididae is closely related to Glauconomidae as the sister group. We observed a novel gene arrangement in Glauconome virens, the first report on the mitogenome of the family Glauconomidae, by comparing gene arrangements. Three patterns of gene rearrangement identified in Cyrenoidea are shared by the families Glauconomidae, Cyrenoididae and Cyrenidae, suggesting that gene arrangements can be a valuable tool for phylogenetic studies.