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　　The largest vent field, called TAG （short for Trans-Atlantic Geotraverse）, is about the size and shape of a football stadium. Other fields have more whimsical names like Clam Acres, Mussel Bed, Rose Garden, Garden of Eden, Broken Spur, and Lucky Strike. Snow Blower is named for the white, flakybacteria discharged from its vents. Genesis is a vent that sputtered out but came back to life a few years later. Hydrothermal vents are underwater oases, providing habitat for many creatures that are not found anywhere else in the ocean. More than 300 new species have been identified since the first vent was discovered in 1977.

　　Besides the giant tube worms, which have so far been found only in the Pacific, there are pencil-size Jericho worms with accordion-like tubes; orange worms covered with tiny bristles; small benthicworms that wriggle through the mud; and finger-length, dark red palm worms that stand upright, topped with wig-like fronds. A special class of small worms, called Alvinellids （named after the sub）, live on the walls of mineral deposits that form around vents.

　　Mussels, shrimp, clams, and crabs are abundant at many vents, but these are not the same species that you find in seafood dishes. The cocktail-size shrimp that dominate vents in the mid-Atlantic, for example, have no eyes. However, at least on species has an extremely sensitive receptoron its head that may be used to detect heat or even dimlight coming from vents. Scientists still aren't sure how shrimp and other vent creatures cope with chemical-laden seawater that would kill ordinary shellfish.

　　Biologists have observed a variety of smaller crustaceans around vents, including miniaturelobsters called galatheids, and amphipods resembling sand fleas. They have also seen snail-like limpetsthe size of BBs, sea anemones, snakelike fish with bulging eyes, and even octopuses.

　　While octopuses are at the upper end of the vent's food chain, bacteria are at the bottom. They are the first organisms to colonize newly formed vents, arriving in a snowlike flurryand then settling to form white mats or tendrils attached to the ocean floor. Bacteria have been found living beneath the ocean's floor, and it seems likely that they emerge from below when the conditions are right. Vent bacteria can withstandhigher temperatures than any other organism. That makes them attractive to researchers who are developing heat-stable enzymes for genetic engineering, and culturing bacteria designed to break down toxic waste.

　　Water pouring out of vents can reach temperatures up to about 400 C; the high pressure keeps the water from boiling. However, the intenseheat is limited to a small area. Within less than an inch of the vent opening, the water temperature drops to 2 C, the ambienttemperature of deep seawater. Most of the creatures that congregatearound vents live at temperatures just above freezing. Thus chemicals are the key to vent life, not heat.

　　The most prevalentchemical dissolved in vent water is hydrogen sulfide, which smells like rotten eggs. This chemical is produced when seawater reacts with sulfate in the rocks below the ocean floor. Vent bacteria use hydrogen sulfide as their energy source instead of sunlight. The bacteria in turn sustain larger organisms in the vent community.

　　The clams, mussels, tube worms, and other creatures at the vent have a symbiotic relationship with bacteria. The giant tube worms, for example, have no digestivesystem-no mouth or gut. “The worm depends virtually solelyon the bacteria for its nutrition,” says microbial ecologist Colleen M. Cavanaugh of Harvard University. “Both partners benefit.”

　　The brown, spongy tissue filling the inside of a tube worm is packed with bacteria-about 285 billion bacteria per ounce of tissue. “It's essentially a bacterial culture,” says Cavanaugh.

　　The plumes at the top of the worm's body are red because they are filled with blood, which contains hemoglobinthat binds hydrogen sulfide and transports it to the bacteria housed inside the worm. In return the bacteria oxidizethe hydrogen sulfide and convert carbon dioxide into carbon compounds that nourish the worm.

　　Tube worms reproduce by spawning: They release spermand eggs, which combine in the water to create a new worm. Biologists don't know how the infant worm acquires its own bacteria. Perhaps the egg comes with a starter set.

　　Scientists also don't know how tube worms and other organisms locate new vents for colonization. “The vents are small, and they're separated, like island,” says Cindy Lee Van Dover, a biologist and Alvin pilot who studies vent life. Most vent organisms have a free-swimming larval stage. But scientists aren't sure whether the larvae float aimlessly or purposely follow clues-such as chemical traces in the water-to find new homes.

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