Fig. 1

Distribution of bacterial symbionts in the gill filaments of Conchocele bisecta collected in a deep-sea hydrothermal vent field. A Transmission electron micrography (TEM) of bacteriocytes in gill filaments. Extracellularly symbionts were densely aggregated in the large apical vesicles (black arrow; v: vesicle) covered by microvilli (mv) and smaller vesicles (white arrow). Phagolysosome-like organelles (pl) and nucleus (n) located in the basolateral membranes of the bacteriocytes (scale bar: 5 μm). B Details of phagolysosome-like organelles with whorls of lysed bacterial products (scale bar 2 μm). C Living bacterial symbiont (b) with a clear border located in the apical vesicles, and the symbionts were small cocci with a diameter around 500 nm (scale bar 250 nm). D Lysed bacterial symbionts with blurred cell walls in the phagolysosome-like organelles (scale bar: 250 nm). E Fluorescence in situ hybridization (FISH) of 16S rRNA of the dominate symbionts in the gill filaments. The image shows the overlay of DAPI-stained DNAs and 16S rRNA tagged with Cy3 dye (scale bar: 50 μm). F Schematic representation of typical bacteriocytes, including small, and large apical vesicles (v), phagolysosome-like organelles (pl), and lysed symbionts (b)