Anatomy and Ultrastructure

Riftia pachyptila occurs in dense clumps attached to the seafloor substrate (e.g., basalt) at low flow vents. A narrow, elongate tube composed of chitin and scleroproteins and up to three meters in length protects the soft body of the worm, which is divided into four major regions. The branchial plume lies at the anterior of the worm in direct contact with the surrounding seawater. Infused with blood vessels, this gill-like organ allows an efficient exchange of metabolites (e.g., sulfide, oxygen, carbon dioxide, inorganic nitrogen) and waste products (e.g., ammonia, protons) between the worm and the surrounding seawater. Below the plume is the vestimentum, a circular muscle that houses the heart and brain of the worm as well as glands involved in tube secretion. The vestimentum also mediates the worm's position in its tube, enabling the animal to withdraw from predation or to extend its plume to access both sulfide-rich vent fluids and oxic bottom water.

The tubeworm trunk lies between the vestimentum and the segmented opisthosome that anchors the posterior of the worm to the tube. Encapsulated within the trunk wall in R. pachyptila, as in other vestimentiferan and pogonophoran tubeworm species, is a unique morphological adaptation designed specifically to house bacterial symbionts: the trophosome (Cavanaugh et al. 1981; Felbeck 1981; Jones 1981). The trophosome, which in R. pachyptila appears to develop from mesodermal tissue (Bright and Sorgo 2003) and replaces the transient gut present in larval and young juvenile tubeworms (Jones and Gardiner 1988), is a lobular organ consisting primarily of blood vessels, coelomic fluid, and specialized host cells called bacteriocytes. Bacteriocytes are packed with chemoautotrophic sulfide-oxidizing endosymbionts that are further encapsulated within a host-derived membrane bound vacuole (Fig. 4; Cavanaugh 1983, 1994; Fisher 1990). Bacterial abundance within this tissue is high, with cell density averaging 109 per gram of fresh trophosome (Hand 1987) and bacterial volume estimated to occupy between 15 and 35% of total trophosome volume (Powell and Somero 1986; Bright and Sorgo 2003).

Fig. 4. Riftia pachyptila Jones (A-C: Galapagos Rift; D, E: 21°N, East Pacific Rise). A Photograph showing elemental sulfur crystals (arrows) scattered throughout tropho-some; courtesy of M. L. Jones. B Scanning electron micrograph, showing lobules of trophosome; arrow indicates area of C (below) where surface epithelium was removed to reveal symbionts within trophosome. C Same, higher magnification, showing sym-bionts within trophosome; note spherical cells as well as rod-shaped cells (small arrows); large arrows indicate likely host cell membranes. D Cross section of portion of trophosome lobule, showing variable fine structure of symbionts, including membrane-bound vesicles in many cells; all symbionts contained within membrane-bound vacuoles, either singly or in groups or two or more; arrow, dividing bacterium; b bacteria; m mitochondria; tc trunk coelomic cavity. E Same, higher magnification, showing cell envelope of symbiont (resembling that of Gram-negative bacteria), intracytoplasmic vesicles, and peribacterial membrane; v vesicle; cm symbiont cytoplasmic membrane; om symbiont outer membrane; pm peribacterial membrane. Scale bars: A, 1 mm; B, 250 |m; C, 10 |m; D, 3 |m; E, 0.2 |m. Reprinted with permission from Biol Soc Wash Bull (Cavanaugh 1985)

In R. pachyptila, symbiont morphotype varies depending on location within the trophosome lobule (Fig. 5; Bosch and Grasse 1984a,b; Gardiner and Jones 1993; Bright et al. 2000). Bacteriocytes in the innermost (central) zone of the lobule primarily contain small, rod-shaped symbionts, while bacteriocytes nearer the periphery of the trophosome generally contain small and large cocci (1.6 to 10.7 |m diameter; Bright et al. 2000). Such pleomorphism may be caused either by intra-lobule biochemical gradients that impact symbiont metabolism, morphology, and growth or by differences in life cycle stage among symbiont cells (see below, Bosch and Grasse 1984a,b; Hand 1987; Bright et al. 2000).

Fig. 5. Juvenile Riftia pachyptila transmission electron micrograph. Each trophosome lobule consists of non-symbiotic host tissue (h) surrounding the bacteriocytes of the peripheral zone ( p) with large coccoid symbionts, the median zone (m) with small cocci, the central zone (c) with rods, and the non-symbiotic central axial blood vessel (ax). a Low magnification of lobule in cross section; note intracellular blood sinuses between bacteriocytes, one marked with an arrowhead. b Higher magnification of lobule center. c Higher magnification of periphery with degrading bacteria (arrow) and degrading bacteriocytes (arrowhead) in peripheral zone. Reprinted with permission from Mar Biol (Bright et al. 2000)

Fig. 5. Juvenile Riftia pachyptila transmission electron micrograph. Each trophosome lobule consists of non-symbiotic host tissue (h) surrounding the bacteriocytes of the peripheral zone ( p) with large coccoid symbionts, the median zone (m) with small cocci, the central zone (c) with rods, and the non-symbiotic central axial blood vessel (ax). a Low magnification of lobule in cross section; note intracellular blood sinuses between bacteriocytes, one marked with an arrowhead. b Higher magnification of lobule center. c Higher magnification of periphery with degrading bacteria (arrow) and degrading bacteriocytes (arrowhead) in peripheral zone. Reprinted with permission from Mar Biol (Bright et al. 2000)

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