Biological characteristics of the chemosymbiotic lucinid clam Pillucina pisidium inhabiting the lagoonal seagrass bed of the subtropical island

Abstract

Chemosymbiotic microbes have been identified in the intertidal zones and the deep-sea hy drothermal vents where the environments are enriched with reduced sulfur. Recently, several studies have reported presence of clams in the sediment of shallow subtidal seagrass, harboring chemosynthetic microbes as symbionts. In this study, we investigated an association of chem osynthetic bacteria with a lucinid bivalve Pillucina pisidium occurring in subtidal seagrass bed in a lagoon on the east coast. A total of 360 clams were collected from Zostera marina bed for the analysis. P. pisidium was a small clam with a mean shell length of 7.0 mm and tissue wet weight of 26.2 mg. Histology revealed that P. pisidium has unusually thick gills which at tributed to 16.5% of the total tissue weight. Histology also revealed that the digestive gland and stomach were reduced compared to other bivalves, suggesting that P. pisidium may have another source of food except the filtered food particles from the ambient environment. Scan ning electron microscopy showed symbiotic bacteria in the gills. The results of stable isotope ratio analysis suggest that significantly depleted carbon and nitrogen indicate that bacterial fixation of carbon and nitrogen occurs in the gills. Next Generation Sequencing revealed that thiotrophic and diazotrophic bacteria were dominant gill filaments