elaborate symbioses. This suggests that arctic vents (there are also some on the
Gakkel Ridge farther north, known only from temperature studies) are relatively new,
not old enough for evolution of vent specialists. When the very long ridge system
surrounding Antarctica can be fully explored (which the prevailing weather makes
less attractive than tropical ridges) more vents and possibly species will be found.
Fig. 15.2 Map showing six biogeographical areas statistically identified for
hydrothermal vents by Bachraty et al. (2009). The arrows represent the apparent
directions of faunal exchange with “coefficients of dispersal” (see the paper).
(After Bachraty et al. 2009.)
There are vestimentiferans on long sections of the system in the eastern Pacific and
in the western Pacific, but not in the Atlantic, or Indian Ocean. They differ at the
(admittedly somewhat arbitrary) genus level among the East Pacific Rise (Tevnia and
Riftia), western Pacific (Alaysia and Arcovestia) and the vents of the Explorer–Juan
de Fuca–Gorda ridge system seaward of Canada and the northwest USA (Ridgea).
The eastern Pacific vent systems lack the provannid snails dominant in the western
Pacific and the alvinocaridid shrimp (e.g. Rimicaris) of the Mid-Atlantic and Central
Indian Ridges.
(^) The Atlantic system lacks the vesicomyid Calytogena clams of the east Pacific, but
does have related mussels. Bathymodiolid mussels are the most cosmopolitan of the
larger fauna. Atlantic sulfide chimneys support swarms of alvinocaridid shrimp, but
lack the vestimentiferans and alvinellid worms, a complementary exclusion which is
not explained, except by distance and isolation. It would seem that the roughly 40-
million-year existence of deeply submerged, oceanic ridges in the Atlantic should
have been long enough for some species of vestimentiferan to establish there, but it
has not been. The barriers are too extensive. Also, but not likely given the time for
evolution to operate, differences in the chemistry of the vent plumes, deriving from
differences in rising magmas, may support different strategies for exploitation of