trunk called the trophosome. When the skin is opened, it looks like an elongated
bunch of greenish grapes. Each lobule is a colony of cells, of bacteriocytes, with a
large blood vessel at its core and smaller vessels extending like spokes to the
periphery and then opening into a sinus beneath an outer tissue layer. Blood flow is
from the periphery toward the central vein. The bacteriocytes are generated in a
central epithelium and then repeatedly divide and migrate along the spoke vessels
toward the cell layer at the outer surface. While dividing repeatedly in this transit, the
symbiotic bacteria change shape: rods, then small cocci, then large cocci. At the
periphery, bacteriocytes digest the symbionts, die, and are resorbed (Pflugfelder et al.
2009). This recurrent cycling is likely to be part of the harvesting of nutriment from
the bacteria by the worm.
(^) Gas exchange at the plume is promoted by its extremely fine surface subdivision,
averaging 22 cm^2 g−1 of dry body mass (Anderson et al. 2002), a ratio greater than
that of all measured gills, except that of another vent annelid, Paralvinella (47 cm^2 g
−1). In addition, the diffusion distance through tissue from water to blood is extremely
short at the tips of the gill filaments, only 1 to 2 μm. The blood flow, driven by a heart
in the vestimentum section, must transport S2–, O 2 , CO 2 , and fixed nitrogen (as NO 3 −
in Riftia, unusual for animals but an adaptation because the deep-sea mostly lacks
NH 4 +) from the gill to the trophosome, return the by-products of sulfide oxidation to
the gill for removal and distribute nutrition from the symbiosis throughout the body.
Substantial complexity for these exchanges, particularly of carbon species, is entailed
by the necessity to maintain alkaline internal pH against the inward gradient of H+
from the acidic vent fluid–seawater mixture. The details of this physiology are
reasonably well studied (see Bright & Lallier 2010). A few aspects deserve mention
because they illustrate the high level of specialized adaptation achieved by this worm.
(^) The vascular blood and coelomic fluid are rich in specialized hemoglobins, actually
two molecular versions in blood (HbV1 at 3600 daltons and HbV2 at 400 daltons) and
one in the coelomic fluid (HbC1 at 400 daltons). A complex of linked subunits, HbV1
is large but not close to the size of human, tetraplex hemoglobin (68,000 daltons).
Riftia hemoglobins are not contained within cells. Both sulfide and oxygen bind to
HbV1 in roughly equal amounts from roughly equal external concentrations (Arp et
al. 1987). Oxygen is bound at the heme group, much as for other oxygen-transport
hemoglobins. Sulfide is bound in the protein superstructure of the molecule, possibly
at some sectors rich in cysteine and methionine (S-containing amino acids), possibly
at ligated zinc ions (Flores & Hourdez 2006), but that remains unresolved. Binding of
S2– has been shown for HbC1, likely providing protection for other molecules that
might be poisoned by it, such as the heme-centered mitochondrial cytochromes that
are key to oxidative electron transport. There is some evidence that cytochromes of
Riftia have lower affinity for S2– than those of other animals. Hemoglobin-bound S2–