energy from oxidation of metallic ions, particularly Fe2+ → Fe3+ and Mn2+ to higher
states (Jannasch 1999). It is not established that any of those are chemoautotrophs; the
energy return from the iron oxidation is much lower, only −26 kJ mol−1. However, the
chemistry of vents exploitable by prokaryotes is remarkably complex and far from
fully studied. For example, the chimneys have bacterial coatings that apparently
utilize the sulfide in solid pyrite (FeS 2 ) and other metallosulfide deposits to drive
chemoautotrophy (Wirsen et al. 1993).
(^) Cary and Giovannoni (1993) have examined the mechanisms by which the young of
vent animals obtain their symbionts. They used PCR-amplified DNA encoding SSU
rRNA from the symbionts to make probes for characteristic sections of their DNA,
then sought that DNA in eggs. The question was, do the mothers install a symbiont
inoculum in the egg? The answer for Riftia is no. Apparently, young worms must
capture their own symbionts from the flow field around a vent. However, for
Bathymodiolus the answer is yes; the egg is equipped with a complement of viable
symbionts.
Biogeography of Vent Faunas
(^) Because vent fields develop, become populated, then cool, lose flow, and die, their
dependent faunas dying with them, their fauna must have reasonably long-range larval
transport to populate new sites. Otherwise, species extinction would accompany the
death of local vent communities. Given long-range larval transport, one might expect
vent faunas to be cosmopolitan, to be about the same everywhere along the entire
ridge system. That is not the case. Tunnicliffe et al. (1998) reviewed the species then
known to occupy vents and concluded that ranges of most species, even major groups,
extend only along restricted ridge segments, surely because there are long stretches
without vents. “The great majority of species are found at only one site” (Tunnicliffe
et al. 1998). Ramirez Llodra et al. (2007) provide an informal global geography with
good photographs of assemblages. Bachraty et al. (2009) provide a statistical analysis
of regional faunal patterns based on a catalogue they have assembled of 592 species
(∼80% regionally endemic) in 332 genera from 63 vent fields. They found that vent
fields examined to date can be divided, with only small overlaps in faunal
composition, into six provinces (Fig. 15.2). That list will likely grow as more sections
of the spreading ridge system are examined. For example, Schander et al. (2010) have
collected fauna from the Mohn Ridge at the north end of the Mid-Atlantic Ridge
(71°N, 500–700 m) by ROV and identified 180 species. In contrast to many low-
latitude sites, no obvious vent specialists were found, but there is enhanced abundance
of regional fauna: sponges, hydroids, anemones, diverse annelids, a gastropod (Rissoa
sp.), and more. Shander et al. found abundant “smoker” bacteria, but no obvious or