Article 14

Creatures of the Thermal Vents

By Dawn Stover

The three-person submersible Alvinsank through the cold, dark waters of thePacific Ocean for more than an hour, fi-nally touching down on the sea floormore than 8,000 feet below the surface.It was December 1993, and the scientistsinside the sub had come to this stretch ofthe East Pacific Rise, an underwatermountain range about 500 miles south-west of Acapulco, Mexico, to inspect arecently formed hydrothermal vent—afissure in the ocean bottom that leaksscalding, acidic water.

Peering out through the sub’s tinywindows, the visitors were astonished tosee thickets of giant tube worms, somefour feet tall. The tail ends of the wormswere firmly planted on the ocean floor,while red plumes on the other endsswayed like a field of poppies. Alvin hadbrought researchers to the same spot lessthan two years earlier, when they hadseen none of these strange creatures.

Measurements at the site have sinceshown that individual tube worms canincrease in length at a rate of more than33 inches per year, making them the fast-est-growing marine invertebrates. Thatmeans tube worms can colonize a ventmore rapidly than scientists oncethought.

The giant tube worm is one of themost conspicuous members of a diversecommunity that forms around hydrother-

mal vents. Scientists once thought thatno living thing could survive the harshcombination of toxic chemicals, hightemperatures, high pressures, and totaldarkness at these vents. But in 1977, re-searchers diving in Alvin discovered tubeworms and other bizarre organismsthriving at a vent off the Galapagos Is-lands. Similar communities have sincebeen found at several hundred hot spotsaround the world. These creatures arelike nothing else on Earth.

Vents form where the planet’s crustalplates are slowly spreading apart andmagma is welling up from below to formmountain ranges known as mid-oceanridges. As cracks form at these spreadingcenters, seawater seeps a mile or twodown into the hot rock. Enriched withminerals leached from the rock, the wa-ter heats and rises to the ocean floor toform a vent.

Vents are usually clustered in fields,underwater versions of Yellowstone’sgeyser basins. Individual vent openingstypically range from less than a half inchto more than six feet in diameter. Suchfields are normally found at a depth ofmore than a mile. Most have been dis-covered along the crest of the Mid-Oce-anic Ridge, a 46,000-mile-long chain ofmountains that wraps around Earth likethe seams on a baseball. A few ventshave also been found at seamounts, un-

derwater volcanoes that are not locatedat the intersection of crustal plates.

The largest vent field, called TAG(short for Trans-Atlantic Geotraverse), isabout the size and shape of a football sta-dium. Other fields have more whimsicalnames like Clam Acres, Mussel Bed,Rose Garden, Garden of Eden, BrokenSpur, and Lucky Strike. Snow Blower isnamed for the white, flaky bacteria dis-charged from its vents. Genesis is a ventthat sputtered out but came back to life afew years later.

WOODS HOLE OCEANOGRAPHIC INSTITUTIONAND UNIVERSITY OF WASHINGTON

A remotely operated sub spotted this vent-dwelling crab.

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Article 14. Creatures of the Thermal Vents

PHOTO: EMORY KRISTOFF/NATIONAL GEOGRAPHIC SOCIETY

Top:Where hot vent fluids meet icy seawater, minerals precipitate out to form chimneylike structures called black smokers.

ILLUSTRATION: ROB SCHUSTER

Bottom:Birth of a vent: Upwelling magma heats seawater that seeps down through rocks. The hot, mineral-rich water is ejected throughan opening in the ocean floor.

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ANNUAL EDITIONS

Hydrothermal vents are underwateroases, providing habitat for many crea-tures that are not found anywhere else inthe ocean. More than 300 new specieshave been identified since the first ventwas discovered in 1977.

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

Mussels, shrimp, clams, and crabs areabundant at many vents, but these are notthe same species that you find in seafooddishes. The cocktail-size shrimp thatdominate vents in the mid-Atlantic, forexample, have no eyes. However, at leastone species has an extremely sensitivereceptor on its head that may be used todetect heat or even dim light comingfrom vents. Scientists still aren’t surehow shrimp and other vent creaturescope with chemical-laden seawater thatwould kill ordinary shellfish.

Biologists have observed a variety ofsmaller crustaceans around vents, in-cluding miniature lobsters called galat-heids, and amphipods resembling sandfleas. They have also seen snail-like lim-pets the size of BBs, sea anemones,snakelike fish with bulging eyes, andeven octopuses.

While octopuses are at the upper endof the vent’s food chain, bacteria are atthe bottom. They are the first organismsto colonize newly formed vents, arrivingin a snowlike flurry and then settling toform white mats or tendrils attached tothe ocean floor. Bacteria have beenfound living beneath the ocean’s floor,and it seems likely that they emerge frombelow when the conditions are right.

Vent bacteria can withstand highertemperatures than any other organism.That makes them attractive to research-ers who are developing heat-stable en-zymes for genetic engineering, andculturing bacteria designed to breakdown toxic waste.

Water pouring out of vents can reachtemperatures up to about 400°C; the highpressure keeps the water from boiling.However, the intense heat is limited to asmall area. Within less than an inch ofthe vent opening, the water temperaturedrops to 2°C, the ambient temperature ofdeep seawater. Most of the creatures thatcongregate around vents live at tempera-tures just above freezing. Thus chemi-cals are the key to vent life, not heat.

The most prevalent chemical dis-solved in vent water is hydrogen sulfide,which smells like rotten eggs. Thischemical is produced when seawater re-acts with sulfate in the rocks below theocean floor. Vent bacteria use hydrogensulfide as their energy source instead ofsunlight. The bacteria in turn sustainlarger organisms in the vent community.

The clams, mussels, tube worms, andother creatures at the vent have a symbi-otic relationship with bacteria. The gianttube worms, for example, have no diges-tive system—no mouth or gut. “Theworm depends virtually solely on thebacteria for its nutrition,” says microbialecologist Colleen M. Cavanaugh of Har-vard University. “Both partners benefit.”

The brown, spongy tissue filling theinside of a tube worm is packed withbacteria—about 285 billion bacteria perounce of tissue. “It’s essentially a bacte-rial culture,” says Cavanaugh.

The plumes at the top of the worm’sbody are red because they are filled withblood, which contains hemoglobin thatbinds hydrogen sulfide and transports itto the bacteria housed inside the worm.In return, the bacteria oxidize the hydro-gen sulfide and convert carbon dioxideinto carbon compounds that nourish theworm.

Tube worms reproduce by spawning:They release sperm and eggs, whichcombine in the water to create a newworm. Biologists don’t know how the in-fant worm acquires its own bacteria. Per-haps the egg comes with a starter set.

Scientists also don’t know how tubeworms and other organisms locate newvents for colonization. “The vents aresmall, and they’re separated, like is-lands,” says Cindy Lee Van Dover, a bi-ologist and Alvin pilot who studies ventlife. Most vent organisms have a free-swimming larval stage. But scientists

aren’t sure whether the larvae float aim-lessly or purposely follow clues—suchas chemical traces in the water—to findnew homes.

Studying the life cycle of vent organ-isms is difficult. Researchers have vis-ited only a fraction of the ocean’s hotspots. They have been able to observevent life only by shining bright lights oncreatures accustomed to inky darkness,and many specimens die quickly whenremoved from their unique environment.Underwater cameras are helping scien-tists make less intrusive observations,but diving expeditions are still the mostuseful way to gather information.

The 1993 Alvin expedition to the EastPacific Rise was one in a series of divesto the area. The site was first visited in1989, and scientists observed vent or-ganisms thriving there. But when Alvinreturned in April 1991, its flabbergastedoccupants witnessed the birth of a hydro-thermal vent. A recent volcanic eruptionhad spread glassy lava across the oceanfloor, and the researchers measured tem-peratures up to 403°C—the hottest everrecorded at a hydrothermal vent. The sci-entists dubbed the site Tube Worm Bar-becue, because the worms they broughtback to their ship had charred flesh.

“The most spectacular sight downthere was this massive blinding snow-storm of bacteria,” says Rich Lutz, a ma-rine ecologist at Rutgers University, wholed the expedition. On the ocean floor,the bacteria formed mats several inchesthick, but the scientists saw no other liv-ing things.

Since the eruption, scientists havebeen able to watch several stages of col-onization at the site. When they returnedin March 1992, only a few bacterial matsremained. In their place were colonies ofJericho worms and a variety of smallcrustaceans. The scientists named thearea Phoenix, because new life hadarisen from the ashes of the eruption.

The scientists first observed the gianttube worms at Phoenix in December1993. They also noticed a number ofmineral deposits, some towering toheights of more than 30 feet. Thesestructures form where hot vent watermeets cold seawater, causing metal sul-fides to precipitate out. The precipitatingsulfides, which look like smoke, amass

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Article 14. Creatures of the Thermal Vents

to form chimneys called black smokers.Like the vent fields, some smokers havenames. Smoke and Mirrors, for example,has shelflike overhangs that trap hot wa-ter rising from below, creating upside-down shimmering pools. The largestknown black smoker is Godzilla, a 160-foot-tall structure off the coast of Ore-gon.

During a December 1993 dive to thePhoenix vent field, Alvin accidentallytoppled a 33-foot-tall smoker. When thesub returned for a brief visit three monthslater, the chimney had already grownback 20 feet. Scientists were surprised bythe speedy recovery, which seems to par-allel the rapid growth of tube worms andother organisms at the vents. The visits

to the Phoenix site “give us a sense ofhow quickly these vents are colonized,”says Van Dover.

Another expedition is planned for No-vember. By then, the community of or-ganisms now prospering at the vents mayalready be a ghost town. When the flowof hot, sulfide-rich water slows to atrickle, death also comes quickly.

From Popular Science, May 1995. © 1995 by Popular Science.

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