distinction is not very strongly correlated with the degree of genetic variation along
the ranges of these animals (Audzijonyte & Vrijenhoek 2010).
Faunal Arrangement Around Vents
(^) Different megafauna around a hydrothermal vent have different requirements for flow
rates (exchange) and for specific concentrations of sulfide and oxygen. On the East
Pacific Rise, the smaller vestimentiferan Tevnia jerichonana is an early colonizer of
new vents, settling in vicinities up to 30°C and persisting until they cool to about 5°.
Riftia arrive later, displacing Tevnia and forming colonies in sectors of active vents
where temperatures in their clumps are lower. The mussels do not require quite such
high sulfide levels and are usually found farther away from the vents, although they
may be attached about the bases of the tube worms. The clams appear to prosper in
slower flows with lower sulfide concentrations, and they are most often found along
horizontal cracks in the basalt, through which hydrothermal flow is more of a seep.
They thrust their foot, which is modified for gas exchange, down into the crack. The
foot presumably also anchors them. At the periphery of a vent field is an array of
filterers, particularly tubiculous serpulid polychaetes. The sulfide is already too dilute
and reduced by oxidation to support chemoautotrophy, but sufficient bacteria disperse
from the interior walls of the vents to provide a greatly enriched diet compared to the
deep-sea background. The spatial layout of an East Pacific Rise vent field community
has been described by Hessler et al. (1988). An extensive literature now covers faunal
layout and succession in other submarine hydrothermal regions.
Longevity of Vents and Colonization of New
Vents
(^) Magma-heated venting areas range in longevity from a few years to several decades.
Traverses along the axial rift of the Galapagos Rise on the initial 1977 expedition
found sites of former vents and animal colonies indicated by deposits of shells,
particularly those of C. magnifica. Dead chimneys have been found at sites close to
active chimneys. Since vents do not last indefinitely, the animals must have
moderately rapid growth rates, especially Riftia. Calyptogena can be aged fairly
accurately by dating of the layers of the shell: the largest specimens are of the order of
25 years old. A section of the East Pacific Rise at 9°N with very active spreading was
the site of a lava flow in April 1991, just before a visit by scientists in Alvin (Lutz et
al. 1994). Lava had spread over previously active vent sites that had supported
copious fauna. Fauna not actually buried included scorched clusters of tube worms.