Vertical transmission of chemoautotrophic symbionts in the bivalve Solemya velum (Bivalvia: Protobranchia)

Adults of the bivalve species Solemya velum live in symbiosis with chemoautotrophic bacteria in spe- cialized gill bacteriocytes. The bacteria play an essential nutritional role in the mature association, fixing CO2 via the Calvin cycle with energy obtained through the oxi- dation of reduced sulfur compounds. To understand how the continuity of this partnership is maintained between host generations, we investigated the mode of symbiont transfer in S. velum. A diagnostic assay using the poly- merase chain reaction and primers specific for the S. velum symbiont ribulose- 1 ,5-bisphosphate carboxylase (RubisCO) gene consistently detected bacterial sequence in female gonad tissue, suggesting the presence of sym- biont cells in host ovaries and a vertical mode of symbiont transmission from mother to offspring. Furthermore, in- tracellular bacteria were present in the developing gills of juveniles that had not yet hatched from the gelatinous capsule in which larval development occurs ( 11 days after fertilization). By 64 days postfertilization, the typical adult gill ultrastructure of alternating bacteriocytes and sym- biont-free intercalary cells was apparent. Knowledge about the mode of symbiont transfer in S. velum allows further study into the dynamics of host-symbiont interactions in chemoautotrophic associations.