Twenty Thousand Leagues Under the SeaApril 11, 2001 / Posted by: Shige Abe
Miles below the ocean surface exist some of the most fascinating habitats for life on Earth. Here, where sunlight never reaches, live complex ecosystems that can appear and disappear within a matter of decades. What provides the thermal and chemical energy that fuels these ecosystems are deep-sea hydrothermal vents, one of the unofficial wonders of the natural world.
These vents occur at oceanic “spreading centers,” mountainous ridges where magma from deep within the Earthâ€™s crust forces its way up to the ocean floor, creating new ocean crust and pushing the old crust out of the way. This is the engine that drives apart the Earthâ€™s tectonic plates, moving continents about and causing volcanic eruptions and earthquakes.
From time to time, hydrothermal vents, known as “black smokers,” occur along these ridges. They are underwater geysers. At these vent sites, cold ocean water seeps down through cracks in the seafloor to hot spots underground. The water gets superheated to several hundred degrees Celsius and is spit back up in a mineral-rich broth of scalding fluid. And in this bizarre environment, life flourishes.
Until a little over 20 years ago, no one knew that deep-sea hydrothermal vents existed, much less that they were teeming with life. The first such vent was discovered in 1977 east of the Galapagos Islands. Since then, dozens of vents have been discovered and explored along ridges in the Atlantic and Pacific.
Active vents are inhabited by a complex ecosystem of organisms containing both microbial and more complex animal life. (There is no plant life in the deep ocean, because sunlight cannot reach down that far to drive the process of photosynthesis on which plants depend.) The animal life includes tube worms, shrimp, clams, mussels and crabs.
Last year scientists discovered a vent along a ridge in the Indian Ocean. (Itâ€™s located south of the southern tip of India and east of the African island nation of Madagascar.) An expedition is currently underway to explore this vent. Japanese scientists visited this vent in August, 2000, but spent only four days there. The new expedition plans to spend several weeks at the new vent. Cindy Lee Van Dover, of the College of William and Mary in Williamsburg, VA, is chief scientist on the research cruise.
Van Dover has been exploring ocean vents for many years. “I really never thought that one could be an explorer in this day and age. But in the ocean, itâ€™s absolutely true. Youâ€™re going places that nobodyâ€™s ever been before.”
Van Dover studies the morphology (body shape) of vent animal life. Mussels are her specialty. Bob Vrijenhoek, a senior scientist at the Monterey Bay Aquarium Research Institute in Moss Landing, CA, takes a different approach. He studies vent animals, as well as the bacteria that inhabit the vents, by analyzing their DNA. Members of Vrijenhoekâ€™s research group are participating in the Indian Ocean expedition.
The study of how animal populations evolve and disperse geographically is known as “biogeography.” Hydrothermal vents offer a unique opportunity for biogeographers because the underwater environment is affected by fewer factors than are land environments. “People study biogeography on land and itâ€™s always got superimposed on it the effects of latitude and climate,” says Van Dover. Hydrothermal vents, in contrast, are “largely decoupled from climate. They are isolated from what goes on above.”
Most vent organisms, scientists believe, can exist in their adult form only near an active vent site (although many of these organisms have swimming larval stages, which can travel for great distances). Individual vents remain active for anywhere from a few decades to a few thousand years. When a vent shuts off, the adult animals living there die. Yet as soon as a new vent emerges, it is rapidly colonized. Within a few years, a new vent undergoes a complete transformation from uninhabited to fully populated.
By studying the similarities and differences among the animals that live at different vents, scientists have begun to piece together a picture of how organisms move from one vent to another, what are the natural barriers to such movement, and how the geography of the deep ocean affects the evolution of the species that inhabit it.
Most of the research into the fauna (animal life) that inhabit hydrothermal vent systems has been done in the northern region of the Mid-Atlantic Ridge and along the East Pacific Rise, which runs roughly parallel to the west coast of South America. Although similar types of animals can be found at both Atlantic and Pacific vent sites, there is more similarity among the vent ecosystems along the same ridge than there is between the two ridges. For example, shrimp are found at both Atlantic and Pacific sites, but one particular type of shrimp, known as “swarming shrimp,” is found only in the Atlantic.
The Pacific is a very old ocean, while the Atlantic is relatively young, having fully formed only about 120 million years ago. One question scientists are interested in is how the animals that inhabited the Pacific ridge system made their way to the younger Atlantic ridge.
One theory is that some of the organisms may have arrived by way of the Tethys Sea. Donâ€™t look for it on a map, unless itâ€™s a map of what Earth looked like 100 to 200 million years ago. All thatâ€™s left of it today is the Mediterranean. The Tethys Sea was a much larger body of water, which once connected the Indian Ocean to the Atlantic. Scientists theorize that animals could have migrated along ocean ridges from the Pacific to the Indian Ocean, and from there through the Tethys Sea to the North Atlantic.
Some vent organisms, for example, vent shrimp, havenâ€™t been around all that long. They are thought to have evolved only 20 million years ago. So they couldnâ€™t have arrived by way of the Tethys Sea, because by 20 million years ago it had closed up. Another possible route for organisms to have traveled is through the Indian Ocean around the Cape of Good Hope to the South Atlantic.
In either case, the Indian Ocean vents may provide a “missing link” between Atlantic and the Pacific vent ecosystems. Early photographs from the Indian Ocean site taken by Japanese scientists show shrimp and mussels that appear very similar to those found at Atlantic vents. “If you had shown me one of those pictures and asked me where that picture came from,” says Vrijenhoek, “Iâ€™d have told you it came right from the mid-Atlantic ridge.” But, he cautions, “we could get fooled just by superficial appearance.” He is looking forward to the results of the DNA analysis that his colleagues will perform on these animals.
Recently developed, highly efficient DNA-based tools have dramatically changed the way scientists study evolution. Scientists like Vrijenhoek use these tools to determine the similarities and the slight mutational changes between the genes in organisms found at different vent sites. Using this information leads to a better understanding how the life cycle of an organism interacts with the changing typography of the seafloor to affect both the geographic dispersal and evolution of that organism. “We do the same thing that a forensic scientist would do,” Vrijenhoek explains. “We basically extract DNA from the organism and then we use that DNA to look at the degree of relationships within populations, and then between populations.”
For example, Vrijenhoek and his colleagues have found what he calls “genetic discontinuities” among populations of vent amphipods (small crustaceans) that donâ€™t appear among populations of other vent organisms. This is due, he explains, to the fact that there is no swimming larval stage in the amphipodsâ€™ life cycle. As a result, one population of organisms can easily be cut off from another, causing the two populations to drift apart genetically.
Says Vrijenhoek, “The amphipods probably just ride up and down these ridge axes like a corridor. So if thereâ€™s a disruption in that corridor, through a transform fault or lack of habitat, or something like that, they simply canâ€™t get from point A to point B.” The isolated populations then evolve along separate pathways.
Genetic isolation is less likely to occur among populations of animals that have do have a swimming larval stage, because they can more easily cross such physical barriers. This is just what is seen in mussels, clams and tube worms.
Although there is plenty of work yet to be done in the Indian Ocean, Van Dover and Vrijenhoek are also excited about taking a look at other vent sites that remain completely unexplored. The southern Atlantic is one such region.
But if given a choice (and funding), Van Dover would head for the Arctic Ocean, because of its isolation. “The Arctic Basinâ€™s been separated from the Atlantic and the Pacific since the Arctic Ocean was formed by shallow fill. So the deep fauna of the Atlantic and the Pacific, the ones that occur at the vents, may not have gotten up into the Arctic. If you wanted to pick the place to go find the most unusual vent organisms, Iâ€™d have to choose the Arctic.”
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