Deep Sea Hydrothermal Vent Faunas

Biologists do not know how many species exist on earth (see Evolutionary Biodiversity), but the age of exploration for some groups is nearing an end. Although new species of birds, for example, continue to be found nearly every year, no one believes that vast numbers of birds new to science will ever be found again.

That is not so, however, for the creatures that inhabit the remoter, less accessible regions of the earth. And perhaps the deep sea is the least accessible of them all. Light cannot generally penetrate more than a few hundred feet of seawater; thus below that depth photosynthesis is impossible, and the kinds of ecosystems present in all other environments (including the upper levels of the sea, where the food chain is based on the photosynthetic activities of marine microplankton) can not exist. For the most part, the deep-sea fishes, the giant squids, and the sperm whales that prey on them—not to mention the brittle stars and other forms of invertebrate life that have been discovered living on the floors of the oceans miles below the surface—subsist on organic particles of dead organisms that rain down from the oceanic surface.

There is, however, one major exception: the so-called hydrothermal vent faunas, generally found in and near deep ocean trenches. Such trenches are the sites of colliding plates of the earth's crust, where subduction—the swallowing of one plate under another—is occurring (see Plate Tectonics). Heat from deep within the earth's mantle escapes, along with methane and sulfides that some forms of chemoau-totrophic bacteria can metabolize, using energy derived from chemical oxidation reactions that enable them to synthesize organic compounds.

Such bacteria live in the tissues of species of large tube worms and clams commonly found around the vents; other species of marine life simply consume the bacteria directly. Thus hydrothermal vent faunas are the exception to the rule that ecosystems on earth are dependent on sunlight for photosynthesis to form the base of the food chain. Scientists speculate, however, that early in the history of life, such nonphotosynthesizing bacterially based ecosystems were relatively more common.

—Niles Eldredge

See also: Ecosystems; Evolutionary Biodiversity; Food Webs and Food Pyramids; Plate Tectonics

Bibliography

Eldredge, Niles. 1998. Life in the Balance: Humanity and the Biodiversity Crisis. Princeton: Princeton University Press; Margulis, Lynn, and Karlene V. Schwartz. 1998. Five Kingdoms: An Illustrated Guide to the Phyla. of Life on Earth, 3d ed. New York: W. H. Freeman.

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