Minerals Report - International Seabed Authority

egions, 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
tubeworms 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
tubeworm, 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 center and the
EPR separates populations with major genetic differences 23 .
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