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egarding the relationship between ridge crest processes and gene flow within the global
vent fauna. First is a finding that species distribution along present day ridges is related
to tectonic plate history. One model suggests that the global distribution of vent species
and groups of species can be explained on the basis of a radiation away from an ancestral
source on the mid-Tertiary ridges in the eastern Pacific. The entire global ridge fauna has
ancestral ties to the eastern Pacific vent fauna, through shared species, genera and
families, and can be seen as a subset of it since the greatest number of vent species is
found on the northern EPR. When comparing the vent faunas of different regions, one
study pointed out that taxonomic similarities reflect distance along the ridge system
rather than shortest oceanic distance, implying a primarily along-ridge flow of genetic
information 10. These authors also showed that some present day relationships between
vent faunas separated by major discontinuities in the global ridge system can be
explained on the basis of past connections between ridges such as the northern EPR and
the northeast Pacific ridges, and between the northeast Pacific ridges and the back-arc
basins of the western Pacific 10.
At the scale of individual ridge systems, studies of the influence of distance and
discontinuities on gene flow are indicating that high levels of long distance gene flow
may be a pre-requisite for success of vent species. However, molecular work is also
showing that the ability of species to move along and between segments can vary
considerably. Eastern Pacific tube worms are very good at dispersing their genes along
ridge axes although neighbouring populations are more similar than more distant ones,
producing a quantifiable effect of along-axis geographic distance on gene flow 20,21.
Discontinuities between ridge axes can also have a measurable effect on gene flow, as has
been shown by comparison of populations of the same species on either side of transform
faults of different length. For the northeast Pacific tube worm, Ridgeia piscesae, no
detectable genetic differentiation was found across the 160km offset between the Juan de
Fuca and Explorer Ridges, while populations on either side of the 360km offset between
the Juan de Fuca and Gorda Ridges had significant genetic differences 21. Depth
discontinuities may also act as a barrier to gene dispersal and confound interpretation of
genetic differences between sites. Vertical mixing is limited in the deep sea so that water
mass and larval transport tend to be horizontal. Mussel populations at the Snake Pit and
Lucky Strike sites on the MAR show distinct genetic differences that may reflect their
separation by transform faults but may also be influenced by depth differences between
the two sites (3489m vs. 1650m) 22. A clearer example of a likely depth effect is that of the
amphipod crustacean Ventiella sulfuris in the eastern Pacific. The species shows low
divergence along the EPR, even across the 240km Rivera Fracture Zone, while the 5000m
deep, 50km wide Hess Deep between the Galapagos spreading centre and the EPR
separates populations with major genetic differences 23.
108 International Seabed Authority