in vestimentiferan blood is of the order of 100-fold habitat levels, while S2– dissolved
in coelomic fluid is about 10-fold less. Thus, lots of S2– moves to the trophosome,
while relatively little is in contact with the animal’s own cells. At the trophosome,
both oxygen and sulfide are taken off the hemoglobin by concentration gradients, and
chemosynthesis proceeds in the bacteria. In many animal–microbe symbiotic
relations, the symbiont pays its bills by dumping excess organic product into its
vicinity, which the host absorbs and utilizes. Probably symbiosis in Riftia functions in
the same way, but the cycle of bacteriocyte production, death, and resorption may also
enable the worm to acquire bacterial chemosynthate. By-products of the
chemosynthesis are sulfate and protons, which are carried away in the blood,
apparently unbound, and removed at the plume by active transport (metabolic
processes reviewed in detail by Childress & Girguis 2011).
(^) An obvious question is how do the young of animals attached to solid substrate, as
Riftia is, transfer from old active sites to new active sites. Transfer is made more
important because vents can be short-lived. For many vent species, it is now known
and usually involves planktonic larvae. Riftia releases eggs or fertilized zygotes, that
eventually hatch into quite ordinary annelid larvae (trochophores). They are wafted
along the seafloor in the general flow. Sexes are separate in Riftia, with gonopores at
the base of the red gill plume, leading in the male to distinctive external channeling
that some sources say facilitates copulation. However, Van Dover (1994) has observed
external discharges of eggs and sperm (apparently distinctive) in rapidly dispersing
plumes, stating that the eggs sink. Zygotes may be fertilized in the distal oviducts by
sperm from male discharges, not fertilized in the water after egg discharge. Both
ovaries and testes run the length of the trunk adjacent to the trophosome, implying
massive reproductive output.
(^) Brooke and Young (2009) collected zygotes from females with pipettes, then reared
them in both pressurized, thermally controlled incubators and in plastic chambers
moored just above the colonies. They determined that the lipid-rich new zygotes, with
sufficient waxy lipids to qualify them as lecithotrophic, rise very slowly, about 2 m d
−1, and develop into ciliated larvae in a little over 20 days. Buoyancy declines as the
wax is metabolized, so presumably the eggs and larvae stay quite close to the seafloor.
Cell division rate and the fraction developing normally fell off rapidly above 4° to
5°C, both in situ and in incubators. Zygotes do not develop at all without substantial
pressure. Brooke and Young’s rearing results were better at 238 atm than at 170 or
102 atm. There was no development at 34 atm. Based on average respiration rates and
larval composition, Marsh et al. (2001) estimated that larval dispersion can last about
38 days (34 to 44 d). They modeled maximum along-ridge transits in that time of
∼100 km. Other models of along-ridge flow suggest distances over twice that far. In
any event, potential transits around extended ventless barriers (as from the East
Pacific Rise at 21°N to the Gorda Ridge area) are not possible.