Creatures of the Thermal （热量的) VentsThe three-person submersible Alvin sank through the

Creatures of the Thermal (热量的) Vents

The three-person submersible Alvin sank through the cold, dark waters of the Pacific Ocean for more than an hour, finally touching down on the sea floor more than 8,000 feet below the surface. It was December 1993, and the scientists inside the sub had come to thisstretch of the East Pacific Rise, an underwater mountain range about 500 miles southwest of Acapulco, Mexico, to inspect a recently formed hydrothermal vent--a fissure(裂缝) in the ocean bottom that leaks boiling, acidic water.

Peering out through the sub's tiny windows, the visitors were astonished to see thickets of giant tube worms, some four feet tall. The tail ends of the worms were firmly planted on the ocean floor, while red plumes on the other ends swayed like a field of poppies. Alvin had brought researches to the same spot less than two years earlier, when they had seen none of these strange creatures. Previous measurements showed that individual tube worms could increase in length at a rate of 33 inches per year, making them the fastest-growing marine invertebrates. That means tube worms can grow more rapidly than scientists once thought.

The giant tube worm is one of the most eye-catching 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 strange organisms thriving at a Vent off the Galapagos Islands. Similar communities have since been found at several hundred hot sots 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.

Hydrothermal vents are underwater oases (避风港), providing habitat for many creatures that are not found anywhere else in the ocean. Water pouring out of vents can reach temperatures up to about 400℃; 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℃, the ambient temperature of deep seawater. Most of the creatures that assemble 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 (硫化氢), which smells like rotten eggs. This chemical is produced when seawater reacts with sulfate (硫酸盐) in the rocks below the ocean floor. Vent bacteria use hydrogen sulfide as their energy source instead of sunlight. The bacteria in turn sustain large organisms in the vent community.

The clams, mussels, tube worms, and other creatures at the vent have a symbiotic relationship (共生关系) 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 and both partners benefit. The brown, spongy tissue filling the inside of a tube

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