The giant tube worm is one of the most conspicuous members of a diverse community that forms around hydrothermal vents. Scientists once thought that no living thing could survive the harsh combination of toxic chemicals, high temperatures, high pressures, and total darkness at these vents. But in 1977, researchers diving in Alvin discovered tube worms and other bizarre organisms thriving at a vent off the Galapagos Islands. Similar communities have since been found at several hundred hot spots around the world. These creatures are like nothing else on Earth.
Vents form where the planet's crustal plates are slowly spreading apart and magma is welling up from below to form mountain ranges known as mid-ocean ridges. As cracks form at these spreading centers, seawater seeps a mile or two down into the hot rock. Enriched with minerals leached from the rock, the water heats and rises to the ocean floor to form a vent.
Vents are usually clustered in fields, underwater versions of Yellowstone's geyser basins. Individual vent openings typically range from less than a half inch to more than six feet in diameter. Such fields are normally found at a depth of more than a mile. Most have been discovered along the crest of the Mid-Oceanic Ridge, a 46,000- mile-long chain of mountains that wraps around Earth like the seams on a baseball. A few vents have also been found at seamounts, underwater volcanoes that are not located at the intersection of crustal plates.
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, flaky bacteria 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 benthic worms that wriggle through the mud; and finger-length, dark red palm worms that stand upright, topped with wiglike 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 one species has an extremely sensitive receptor on its head that may be used to detect heat or even dim light 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 miniature lobsters called galatheids, and amphipods resembling sand fleas. They have also seen snail-like limpets the 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 flurry and 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 withstand higher 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 intense heat is limited to a small area. Within less than an inch of the vent opening, the water temperature drops to 2 C, the ambient temperature of deep seawater. Most of the creatures that congregate around vents live at temperatures just above freezing. Thus chemicals are the key to vent life, not heat.
The most prevalent chemical 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 digestive system - no mouth or gut. "The worm depends virtually solely on 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 hemoglobin that binds hydrogen sulfide and transports it to the bacteria housed inside the worm. In return, the bacteria oxidize the hydrogen sulfide and convert carbon dioxide into carbon compounds that nourish the worm.
Tube worms reproduce by spawning: They release sperm and 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 islands," 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.
Studying the life cycle of vent organisms is difficult. Researchers have visited only a fraction of the ocean's hot spots. They have been able to observe vent life only by shining bright lights on creatures accustomed to inky darkness, and many specimens die quickly when removed from their unique environment. Underwater cameras are helping scientists make less intrusive observations, but diving expeditions are still the most useful way to gather information. The 1993 Alvin expedition to the East Pacific Rise was one in a series of dives to the area. The site was first visited in 1989, and scientists observed vent organisms thriving there. But when Alvin returned in April 1991, its flabbergasted occupants witnessed the birth of a hydrothermal vent. A recent volcanic eruption had spread glassy lava across the ocean floor, and the researchers measured temperatures up to 403 C - the hottest ever recorded at a hydrothermal vent. The scientists dubbed the site Tube Worm Barbecue, because the worms they brought back to their ship had charred flesh.
"The most spectacular sight down there was this massive blinding snowstorm of bacteria," says Rich Lutz, a marine ecologist at Rutgers University, who led the expedition. On the ocean floor, the bacteria formed mats several inches thick, but the scientists saw no other living things.
Since the eruption, scientists have been able to watch several stages of colonization at the site. When they returned in March 1992, only a few bacterial mats remained. In their place were colonies of Jericho worms and a variety of small crustaceans. The scientists named the area Phoenix, because new life had arisen from the ashes of the eruption.
The scientists first observed the giant tube worms at Phoenix in December 1993. They also noticed a number of mineral deposits, some towering to heights of more than 30 feet. These structures form where hot vent water meets cold seawater, causing metal sulfides to precipitate out. The precipitating sulfides, which look like smoke, amass to form chimneys called black smokers. Like the vent fields, some smokers have names. Smoke and Mirrors, for example, has shelflike overhangs that trap hot water rising from below, creating upside-down shimmering pools. The largest known black smoker is Godzilla, a 160-foot-tall structure off the coast of Oregon.
During a December 1993 dive to the Phoenix vent field, Alvin accidentally toppled a 33-foot-tall smoker. When the sub returned for a brief visit three months later, the chimney had already grown back 20 feet. Scientists were surprised by the speedy recovery, which seems to parallel the rapid growth of tube worms and other organisms at the vents. The visits to the Phoenix site "give us a sense of how quickly these vents are colonized," says Van Dover.
Another expedition is planned for November. By then, the community of organisms now prospering at the vents may already be a ghost town. When the flow of hot, sulfide-rich water slows to a trickle, death also comes quickly

yanganjie   

12月20日阅读v63（海底火山ocean vent）内容及答案
本人参加了12月20日的考试。考前感到阅读机经内容很不完整，深感遗憾。所以考试时特别注意了阅读，下面是我的回忆(答案顺序不一定对)。

全文共10个题目，第1题、第6题是选择题，第2~5是填空题，第7~10也是填空题。

一．第一段讲：海底火山早已经被发现（据有关资料是1977年，不过文中没提到），1993年一艘叫alvin的潜艇潜入海底进行研究。后来全世界不断发现海底火山，many of them 在2000多米深的海底。
1. 选择题，哪一说法正确（答案次序可能不对）：
（a）潜艇alvin 第一次发现海底火山（错）
（b）所有海底火山的深度都在2000米以上（错）
（c）全世界各地都有海底火山（对）
（d）忘了（错）
二．后面三、四段讲海底火山喷发的过程、海底火山对海洋的影响以及一座黑色火山的形状等。
这部分出的是5个summary填空题，有点象流程图：
冷的海水通过地壳crust的裂缝（2.cracks）与高温岩石接触，变得over-heated，岩石也被熔化，形成岩浆，会吸收（3.minerals）, 然后通过（4.vent）喷出海底（或者应该填cracks？此处拿不准），遇冷会（5.deposited） (沉淀)，形成各种形状的海底火山。
第六题是选择题，四选一。讲曾经发现一个很大的黑色火山，有三层高、10米宽，象一个分层的蘑菇（three stories high, 10meters across , like a layer mushroom）。a选项是一个倒放的蘑菇，b是一个三层形状的蘑菇，c是波浪形曲线，d是螺旋形。正确答案是（6.b）（说来好笑，我怎么看b也不象一个蘑菇，只觉得象棵树，尽管文章看得很清楚，最后还是莫名其妙地选了个a，唉!）

           (注：这里本来有4个插图，请大家看看b象不象棵树？！可是复制不过来）

三．最后几段讲到火山对海洋的影响，即改变温度和化学成分。还到讲火山口有生命，其中之一是一种tube worms，没嘴、没胃、没有任何消化系统，靠体内的细菌bacteria产生的碳水化合物生存。
此处出的是选择填空题，大致内容及答案如下：
ocean vents have deposited （7.minerals）. ocean vents change the （8.temperature） and （9.chemistry）（原文中是chemistry components) of the oceans. giant tube worms live on (10.bacteria).

     呼吁准备考两次以上的烤鸭们：
（1）第一次考试时只做1篇阅读，考完马上写下来，到这里发表。
（2）大家分工合作，考号末位数字1~3的，只做第1篇；考号末位数字4~6的，只做第2篇；考号末位数字7~0的，只做第3篇。