The trophosome of adults of Rifia 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 myoepitheiium 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.

Oxygen concentration at the proximal part of Vorticella sp. did not increase during contraction, whereas during slow extension deoxygenated seawater was transported upwards and rapidly mixed with the surrounding oxygenated seawater when the ciliate started to beat its cilia. In both species rapid stalk contraction and subsequent slow extension enhanced the mixing of oxygenated and deoxygenated, H 2 S-containing seawater; the feeding currents (toroidal vortices) drew the surrounding seawater within reach of the zooid's external surface at high speed. It is suggested that this advective fluid transport supplies the ectobiotic bacteria with O 2 and H 2 S simultaneously. The high fluid velocity may cause a decrease in cell boundary layer thickness, thereby enhancing rates of nutrient uptake by the ectobiotic bacteria.

Glycogen storage in host tissue and symbiotic bacteria in the anterior trophosome of the vestimentiferan tubeworm Riftia pachyptila Jones, 1981, was investigated using transmission electron microscope and stereological methods. The relative glycogen content (RGC) of each partner was calculated from the percentage of host and bacterial cytoplasm area taken up by glycogen and from the percentage area that host and symbionts occupy within a trophosome lobule section. Our results show that host and symbionts contribute equally to the total glycogen content of the trophosome; this ratio remains similar for up to 40 h of hypoxia. Furthermore, there is a glycogen gradient in the lobule. In both symbiotic partners, the glycogen content increased from the lobule center toward the periphery, implying different metabolic activities of host cells and bacteria depending on their location within a trophosome lobule.