|
|
Hot
Topics: Biogeography
of deep-sea hydrothermal vent faunas
By Cindy Lee Van Dover
Driving from New York to Miami across 15 degrees of latitude, hardwood forests
give way to palmetto scrub. From Washington State to Southern California, again
across 15 degrees of latitude, evergreen forests disappear and desert cacti
dominate. Any traveler who has noticed these changing botanical provinces has
practiced the science of biogeography, the study of the patterns of distributions
of organisms and the processes that determine these patterns.
Cindy
Lee Van Dover, Chief Scientist of this expedition, is a professor
of biology at the College of William and Mary in Williamsburg, Virginia.
She is an expert on hydrothermal vent ecology. On land, latitudinal changes in climate - humidity, rainfall, temperature
- help determine the geographic distributions of organisms. But
hydrothermal vents, two to three miles beneath the surface of the
sea, are not greatly affected by surface climate. Instead, distributions
of hydrothermal vent organisms on mid-ocean ridges appear to be influenced
by features such as deep-ocean circulation patterns, by major topographic
characteristics such as deep, cross-cutting fracture zones or changes
in depth of the ridge system, and by the position and movement of
Earth’s
tectonic plates over time.
Based on recent explorations, we know now that across more than 30 degrees
of latitude along the East Pacific Rise, there is a single hydrothermal biogeographic
province! Giant tubeworms, clams, and mussels -- and many smaller species of polychaete
worms, shrimp-like crustaceans, and snails -- have immense ranges, despite physiological
and ecological requirements that restrict the adults to isolated vent habitats
separated by tens to hundreds of kilometers.
Giant
tube worms, called Riftia, grow up to 6 feet long and are commonly
found at vent sites on the East Pacific Rise.
A
different species of tube worm, called Ridgea, live at vent
sites on the Juan de Fuca Ridge in the Northeast Pacific The
composition of the animal communities (fauna) at hydrothermal vents
is far from the same all over the world’s oceans, however.
For example, there is a difference in the vent fauna of the East Pacific
Rise off the western coast of Mexico and the vent fauna of the Juan
de Fuca Ridge off Vancouver Canada. Why should these Pacific vent
faunas be different? The East Pacific Rise goes terrestrial
at the mouth of the Colorado River in the Gulf of California, becoming
the San Andreas Fault. The fault moves back off-shore at Mendocino,
California, and gives way again to a triplet of mid-ocean ridge spreading
centers of the northeast Pacific that includes the Juan de Fuca
Ridge.
Dr. Verena Tunnicliffe, a biologist at the University of Victoria, suggests that
at one time, before the North American Plate overrode the mid-ocean ridge, there
was a single biogeographic province in the eastern Pacific. With placement of
a continental barrier to dispersal, the hydrothermal faunas began to diverge,
eventually forming the sister species we observe today. An example of this is
the closely related tubeworms species found at the East Pacific Rise and the Juan
de Fuca Ridge.
If Juan de Fuca Ridge and East Pacific Rise vent faunas are sisters,
then the vent animals of the Mid-Atlantic Ridge are cousins several times
removed. There are many shared families, some shared genera, but few shared species
between Atlantic and Pacific vents. For example, shrimp in the family Alvinocaridae
are found at both East Pacific Rise and Mid-Atlantic Ridge vents,
but Alvinocaris lusca is the Pacific shrimp species, while Rimicaris
exoculata dominates at some Atlantic vents.
Why should Atlantic and Pacific vent faunas differ? There is no single satisfying
explanation. Because the basic ingredients of hydrothermal systems -- basalt
and seawater -- are relatively uniform, and the major element chemistry of vent
fluids in the Atlantic and Pacific is similar, it seems unlikely that differences
in the chemical setting of the hydrothermal sites plays a dominant role. Even
so, there can be differences in the chemical setting of individual vents that
contribute to the differences in the animals observed.
Perhaps
more importantly, geographic isolation of the Atlantic and Pacific
Ridge systems can account for some of the faunal differences. Trace
the global mid-ocean ridge system starting from the East Pacific
Rise in the Gulf of California. There is currently no direct connection
to the Atlantic without following the Pacific-Antarctic Ridge south
of Australia and into the Indian Ocean along the Southeast and then
Southwest Indian Ridges. The Southwest Indian Ridge, after it passes
the tip of the African continent, loops northward to become the Mid-Atlantic
Ridge. If the East Pacific Rise vent fauna can migrate only by
step-by-step migration along mid-ocean ridges, then isolation by distance
alone might account for much of the difference between Atlantic and
Pacific vent faunas.
Mid-ocean ridges of the Indian Ocean may prove to be the corridor for exchange
of faunas between Atlantic and Pacific vents. The Japanese discovery of the Kairei
Field on the Central Indian Ridge and our detailed sampling on this expedition
are already helping to unravel some of the questions we posed nearly five years
ago when we first proposed this research. Of the species we collected from the
Kairei Field, some appear to be linked to Atlantic vent faunas, while many others
are familiar from Pacific hydrothermal vents.
As is usual for any scientific endeavor, our observations lead us to further
questions. To resolve faunal affiliations and to understand what underlying processes
control bioegrographic patterns at deep-sea hydrothermal vents, many more vents
on the global mid-ocean ridge system need to be explored.
Map
showing the global distribution of major hydrothermal vent sites.
Colored circles show vents with similar animal communities.
Acknowledgements:
Funding to support this research comes from the National Science
Foundation (Biological Oceanography) and The College of William & Mary.
Back to Main Hot Topics
Page
|
Email
to friend