Cover Image

HARDBACK
$47.00



View/Hide Left Panel

near-freezing temperatures even at the deepest reaches of the sea, fed by organic material drifting down from the surface. Here, though, the secret of abundant life was a cornucopia of locally derived nutrients. The concentration of food at the oases far surpasses the amounts available elsewhere on the sea bottom. One of our colleagues estimated that the waters surrounding the vents contain 300 to 500 times the nutrients found at nearby sites lacking the benefit of the mineral-rich flow from the vents.

The driving force behind the unique vent communities is the rapid growth of the chemoautotrophic bacteria that are able to use the dissolved oxygen and carbon dioxide present in oceanic bottom waters to oxidize the reduced inorganic compounds (i.e., H2S, S, S2O3, NH4, and NO2) dissolved in the hydrothermal fluids coming out of the vent openings. This chemosynthesis process has been known by microbiologists for many years, but it wasn't until the discovery of the Galapagos Rift hydrothermal vents in 1977 that scientists realized it could form the basis of an entire ecosystem.

Although this process takes place in total darkness, it is still tied to the sunlit surface. For the chemosynthetic process to occur, the bacteria require free oxygen to oxidize the reduced inorganic compounds coming out of the vents. This free oxygen has been generated by green plants as a by-product of the photosynthetic process. An interesting question is: What would happen if the sun suddenly turned off? Clearly, the vent communities would continue to thrive until the free oxygen in seawater was exhausted. But even after that point in time, anaerobic chemosynthesis would persist.

There are many forms of bacteria involved in the chemosynthetic process, which occurs in three basic settings: (1) within the subterranean vent system cutting deep into the volcanic terrain, (2) in large microbial mats covering its surface, and (3) within the internal structure of various symbiotic organisms living around the vent openings. The benthic animals that make up the vent communities have a fascinating strategy for survival. We now know that hydrothermal vents are highly ephemeral or short-lived. They turn on and off in a matter of a few years or tens of years. As a result, vent animals have an "r-type" survival strategy. They are able to settle quickly out of the water when a vent turns on, grow fast, reproduce early, and easily disperse their offspring into the water column to find new vent settings.

The vent communities discovered in 1977 by chemists and geologists and revisited in 1979 by biologists are characterized by large organisms situated in diffuse zonations centered around discrete vent openings where the temperature is the hottest. In the case of the Galapagos vents, the maximum exiting temperature measured was 17°C and the dominant macroorganism living near the vent opening is the giant red tube worm Riftia pachyptila. These spectacular organisms form large clusters or hedges standing 2 to 3 m in height. One of the large populations was termed the "Rose Garden." Without eyes, mouth, or digestive tract, the worm's red tip or obturaculum absorbs food and oxygen from the water by means of hundreds of thousands of tiny tentacles arranged on flaps on the exposed portions of its body. These are the critical compounds needed by the bacteria living inside its body for the chemosynthetic process. Since these ingredients come from both the anaerobic vent fluids (i.e., hydrogen sulfide) and the ambient bottom water (i.e., oxygen and carbon dioxide), the worms position their red tips in the area of mixing just above the vent opening, clustering in thickets to direct the vent fluids up past the tip of the tubes. Sexually differentiated, they most probably broadcast eggs and sperm into the water.

Living directly inside the vent opening itself are a variety of limpets (i.e., Archaeogastropoda) that are also observed living on the white base of the tube worms. Living in close proximity to the vent openings in some cases are large beds of mussels (Mytilidae) attached to the volcanic substrate, as well as other organisms like the tube worms, by strong byssal threads.

In our investigation of the Galapagos Rift vents communities in 1977, the organisms that I found as impressive as the red tube worms were the giant white clams (Calyptogena magnifica) that covered the fresh lava flows. We commonly saw these clams wedged down inside a small fissure cutting across the volcanic terrain, parallel to the rift valley axis. Their anterior end pointed down and their hinge point up, an ideal feeding position with the hydrothermal fluids flowing up past them. The clams of the rift were noteworthy not only for their gargantuan size, but also for the intense blood-red color of their flesh—as with the tube worms, this coloration is due to a high amount of hemoglobin, the pigment of human blood. In numerous cases, you could see that as the clams grew, their enlarging shells conformed to the jagged outline of the fissure opening, wedging them in place.

This vent species was also a critical indicator of past vent activity. Unlike most vent organisms that quickly vanished after a vent turned off, the large white clam shells persisted for many years before finally being dissolved by the ambient bottom water, which is undersaturated by calcium carbonate. In fact, an inactive vent characterized by a cluster of dissolving clam shells was first seen in a deep-tow survey along the Galapagos Rift in 1976 but was not recognized for its importance until after an active vent was found by Angus and investigated by Alvin in 1977.

Other important organisms living in and around the Galapagos vents are a variety of anemones (Actinarians), brachyuran crabs (Bythograea thermydron), galatheid crabs (Munidopsis), jellyfish called "Dandelions" (i.e., rhodaliid siphonophores), and an highly unusual worm (Enteropneust ) clustered in what resembled piles of "spaghetti." The blind white crabs that frequent the oases and feed upon dead mussels and clams are apparently members of a heretofore unknown crustacean family.

What about the so-called dandelions? Animals despite their plant-like appearance, these turned out to be a new siphonophore, related to the Portuguese man-of-war but



The National Academies | 500 Fifth St. N.W. | Washington, D.C. 20001
Copyright © National Academy of Sciences. All rights reserved.
Terms of Use and Privacy Statement