Hydrothermal venting was already active through fissures in the new lava, and
particulate matter clouded the water. Bacteria were abundant as mats up to 5 cm thick
on the bottom. On a return visit in March 1992, venting was more localized, and the
bacterial mats much less extensive. Possibly, crabs active in the area had eaten back
the surface coating of bacteria. Venting fissures were surrounded by populations of
Tevnia, a small (to 30 cm) vestimentiferan that appears soon after new vents open and
is apparently suited to bathing in very hot water. No Riftia were present at all. A final
visit in December 1993 showed masses of Riftia with individuals exceeding 1.5 m in
length completely covering sites tagged for relocation that had been bare rock with
dispersed Tevnia 20 months earlier. Small mussels and small Calyptogena were also
found around localized areas of active venting. The active sites were supporting
complex hydrothermal communities. Thus, the build-up time for the communities is
quite short, a few months to a few years. Their demise can be nearly instantaneous
due to lava burial or termination of venting.
(^) In a study similar to that by Lutz et al., Tunnicliffe et al. (1997) followed the
colonization of a completely new vent on the Juan de Fuca Ridge. It was downstream
in the poleward mean flow from a new axial volcano on the ridge. The vent stream
included some flocculent material, so they called the vent “Floc”. The Floc site was
barren when discovered in summer 1993. By summer 1994 there were eight metazoan
species, and by summer 1995 there were 21, which compares well with 24 at an
obviously older site upstream they named “Source”. A final visit only slightly later in
1995 found Floc to be dying, and the species count had already dropped to 12.
Colonization is relatively rapid; so is denudation after flow stops. Most submersible
and ROV visits to ridge areas with vents find both live and dead assemblages
scattered along the axis. Dead vents are indicated mostly by cold chimneys, clam
shells, and calcified worm tubes.
(^) Like Riftia (see above), most vent animals have planktonic dispersal, and settlement
on experimental plates has been observed (Craddock et al. 1997; Vrijenhoek 1997) for
limpets, several worms including alvinellids, and both Bathymodiolus and
Calyptogena. Some larvae reach new vents, certainly vastly more do not. Clearly,
many can settle in the vicinity of their parents, which allows large colonies to
accumulate. Arriving at new, habitable vent sites must be facilitated to some extent by
channeling of near-bottom flow along the axial valleys of spreading centers, where
new vents are most likely to erupt. However, the overall loss rates must be higher than
for almost any other oceanic, benthic animals. The reproduction required to
compensate for these losses must be allowed by the copious food supply, which
supports both large body size and high fecundity.
(^) Several workers, particularly Craddock, Vrijenhoek, and colleagues, have examined
the degree of genetic differentiation between stocks of vent animals at different
distances along and between ridge systems. The tools have been isozyme frequency