Bacterial symbiont transmission in the wood-boring shipworm Bankia setacea(Bivalvia : Teredinidae)

The Teredinidae (shipworms) are a morphologically diverse group of marine w ood-boring bivalves that are responsible each gear for millions of dollars of damage to wooden structures in estuarine and marine habitats worldwide. They exist in a symbiosis with cellulolytic nitrogen-fixing bacteria that p rovide the host with the necessary enzymes for survival on a diet of wood c ellulose. These symbiotic bacteria reside in distinct structures lining the interlamellar junctions of the gill. This study investigated the mode by w hich these nutritionally essential bacterial symbionts are acquired in the teredinid Bankia setacea. Through 16S ribosomal DNA (rDNA) sequencing, the symbiont residing within the B. setacea gill was phylogenetically character ized and shown to be distinct from previously described shipworm symbionts. In situ hybridization using symbiont-specific 16S rRNA-directed probes bou nd to bacterial ribosome targets located within the host gill coincident wi th the known location of the gill symbionts. These specific probes were the n used as primers in a PCR-based assay which consistently detected bacteria l rDNA in host gill (symbiont containing), gonad tissue, and recently spawn ed eggs, demonstrating the presence of symbiont cells in host ovary and off spring. These results suggest that B. setacea ensures successful inoculatio n of offspring through a vertical mode of symbiont transmission and thereby enables a broad distribution of larval settlement.