Ultrastructural reinvestigation of the trophosome in adults of Riftia pachyptila (Annelida, Siboglinidae)

The trophosome of adults of Riftia pachyptila (Vestimentifera) was reinvestigated using 3‐dimensional ultrastructural reconstruction and quantitative morphological analysis. The symbionts make up 24.1%, the symbiont‐containing cells (bacteriocytes) are 70.5% of the trophosome's volume. The trophosome is composed of lobules that have a central axial blood vessel surrounded by a myoepithelium containing bacteriocytes, in turn surrounded by an apolar tissue of bacteriocytes. Part of the splanchnic peritoneum lining the trunk coelom encases the bacteriocytes and forms a ramifying network of peripheral blood vessels. Based on the morphology and ultrastructure of the adult, we hypothesize a mesodermal rather than endodermal origin of trophosome and its constitute bacteriocytes. Some of the central bacteriocytes are part of the myoepithelium surrounding the axial blood vessel and act as stem cells for a proliferating tissue produced in the center and ultimately degraded at the periphery of each lobule. Similarly, the rod‐shaped symbionts in the center act as stem cells and exhibit a simple cell cycle. Differentiation into cocci takes place in the median and peripheral zone. Lysis of cocci occurs in the degenerative zone.