First insight into the chemosynthetic bacterial symbiosis in the endangered bivalve Pegophysema bialata (Lucinidae: Pegophyseminae)

The family Lucinidae is one of the most diverse groups of chemosymbiotic bivalves, inhabiting a variety of sulfide-rich environments. The lucinid species Pegophysema bialata within the subfamily Pegophyseminae is currently listed as an endangered species in Japan due to the limited records of live specimens. In this study, we investigated two recently discovered live specimens from Aomori and Aichi (northern and central Japan, respectively) to determine the genetic features of both the host and its symbionts and to describe host morphological features related to chemosymbiosis. Bacterial 16S rRNA gene (16S) sequences from gill tissues showed a high degree of sequence identity with the symbionts of other pegophysemine bivalves collected from the South Pacific, Indian, and Atlantic Oceans. The 16S amplicon analysis demonstrated a high relative abundance of symbiotic bacteria within the gills. Phylogenetic analysis placed the symbionts of P. bialata and other pegophysemine species in a distinct clade from other lucinid symbionts, indicating that this specific host–symbiont association is maintained in the Northwest Pacific region. Gross anatomical observations revealed the presence of hypertrophied gills, a typical trait of chemosymbiosis in bivalves. Mitochondrial cytochrome b gene (cytb) sequences of P. bialata were identical among the two Japanese specimens and a previously reported specimen from Hong Kong.