Two new species of Idas (Bivalvia, Mytilidae) from sunken wood in the East China Sea: description, phylogenetic position and symbionts

Mussels of the genus Idas (Bathymodiolinae) are small bivalves inhabiting organic falls and cold seep ecosystems. Despite their ecological significance, the taxonomy and phylogeny of Idas remain understudied. In this study, 30 Idas specimens were collected from a piece of sunken wood in the East China Sea at depths of 460–550 m. DNA barcoding and phylogenetic analysis revealed three distinct clades: 23 specimens matched Idas sp. ESU D, previously reported from Japan and the Philippines, five samples clustered with Idas iwaotakii (Habe, 1958), and two samples formed a new clade related to I. iwaotakii. These findings resulted in the description of two new species, Idas sinensissp. nov. and Idas pacificussp. nov. We present the complete mitochondrial genomes for both species and investigate the diversity of their symbiotic microbes through full-length 16S rRNA amplicon sequencing. Phylogenetic analyses of mitochondrial genomes further support the taxonomic placement of the new species. The gill-associated microbial communities of these mussels include sulfur-oxidizing SUP05 bacteria, suggesting chemosynthetic symbiosis.