The deep seabed was once considered a biological desert. Life, the logic went, was synonymous with light and photosynthesis. The sun powered the planet’s food chains, and only a few scavengers could ply the preternaturally dark abyss.

Then, in 1977, oceanographers working in the deep Pacific stumbled on bizarre ecosystems lush with clams, mussels and big tube worms — a cornucopia of abyssal life built on microbes that thrived in hot, mineral-rich waters welling up from volcanic cracks, feeding on the chemicals that leached into the seawater and serving as the basis for whole chains of life that got along just fine without sunlight.

In 1984, scientists found that the heat was not necessary. In exploring the depths of the Gulf of Mexico, they discovered sunless habitats powered by a new form of nourishment. The microbes that founded the food chain lived not on hot minerals but on cold petrochemicals seeping up from the icy seabed.

Today, scientists have identified roughly one hundred sites in the gulf where cold-seep communities of clams, mussels and tube worms flourish in the sunless depths. And they have accumulated evidence of many more — hundreds by some estimates, thousands by others — most especially in the gulf’s deep, unexplored waters.

“It wouldn’t surprise me if there were 2,000 communities, from suburbs to cities,” said Ian R. MacDonald, an oceanographer at Florida State University who studies the dark ecosystems.

The world’s richest known concentration of these remarkable communities is in the Gulf of Mexico. The life forms include tube worms up to eight feet long. Some of the creatures appear old enough, scientists say, to predate the arrival of Columbus in the New World.

Now, by horrific accident, these cold communities have become the subject of a quiet debate among scientists. The gulf is, of course, the site of the giant oil spill that began April 20 with the explosion of the Deepwater Horizon drill rig. The question is what the oil pouring into the gulf means for these deep, dark habitats.

Seep researchers have voiced strong concern about the threat to the dark ecosystems. The spill is a concentrated surge, they note, in contrast to the slow, diffuse, chronic seepage of petrochemicals across much of the gulf’s northern slope. Many factors, like the density of oil in undersea plumes, the size of resulting oxygen drops and the potential toxicity of oil dispersants — all unknowns — could grow into threats that outweigh any possible benefits and damage or even destroy the dark ecosystems.

Last year, scientists discovered a community roughly five miles from where the BP well, a mile deep, subsequently blew out. Its inhabitants include mussels and tube worms. So it seems that researchers will have some answers sooner rather than later.

“There’s lots of uncertainty,” said Charles R. Fisher, a professor of biology at Pennsylvania State University, who is leading a federal study of the dark habitats and who observed the nearby community. “Our best hope is that the impact is neutral or a minor problem.”

A few scientists say the gushing oil — despite its clear harm to pelicans, turtles and other forms of coastal life — might ultimately represent a subtle boon to the creatures of the cold seeps and even to the wider food chain.

“The gulf is such a great fishery because it’s fed organic matter from oil,” said Roger Sassen, a specialist on the cold seeps who recently retired from Texas A&M University. “It’s preadapted to crude oil. The image of this spill being a complete disaster is not true.” His stance seems to be a minority view.

Over roughly two decades, the federal government has spent at least $30 million uncovering and investigating the creatures of the cold seeps, a fair amount of money for basic ocean research. Washington has provided this money in an effort to ensure that oil development does no harm to the unusual ecosystems. Now, the nation’s worst oil spill at sea — with tens of millions of gallons spewing to date — has thrown that goal into doubt.

The agency behind the exploration and surveying of the cold seeps is none other than the much-criticized Minerals Management Service of the Department of the Interior — not its oil regulators but a separate environmental arm, which long ago began hiring oceanographers, geologists, ecologists and marine biologists to investigate the gulf seabed and eventually pushed through regulations meant to protect the newly discovered ecosystems.

The minerals service is joining with other federal agencies to study whether the BP spill is harming the dark habitats. Scientists say ships may go to sea as soon as July, sending tethered robots down to the icy seabed to examine the seep communities and take samples for analysis. It is a bittersweet moment for scientists like Dr. MacDonald of Florida State University, who has devoted his career to documenting the ecosystem’s richness and complexity. In an interview, he said the sheer difficulty of trying to fathom the ecological impacts of the spill had left some of his colleagues dejected.

“Once, we had this career studying obscure animals down there,” he said. “And now, it’s looking at this — probably for the rest of my career. It becomes this huge unknown.”

Inky darkness, icy temperatures and crushing pressures conspire to make studying the deep oceans arduous and remarkably costly. Humans are estimated to have glimpsed perhaps a millionth of the ocean floor.

By contrast, people looking at the surface of the gulf have known about the seeping oil for centuries. Spanish records dating from the 16th century note floating oil.

In the early 1980s, scientists investigating the oil seeps wondered if nearby creatures on the seabed might suffer chronic harm from pollution and serve as models for petrochemical risk. They lowered nets about a half mile down and pulled up, to their surprise, riots of healthy animals.

“We report the discovery of dense biological communities associated with regions of oil and gas seepage,” six oceanographers at Texas A&M wrote in the journal Nature in September 1985.

The animals included snails, crabs, eels, clams and tube worms more than six feet long. The founding microbes of the food chain turned out to feed on seabed emissions of methane and hydrogen sulfide — a highly toxic chemical for land animals that has the odor of rotten eggs.

Plants derive energy from sunlight and make living tissue in a process known as photosynthesis. The corresponding method among the microbes of the dark abyss is known as chemosynthesis.

The minerals service proceeded to finance wide expeditions. It issued thick reports in 1988, 1992 and 2002. By then, scientists had discovered dozens of seep communities and found some of their inhabitants to be extraordinarily old.

In the journal Nature, Dr. Fisher of Pennsylvania State University and two colleagues reported that gulf tube worms could live more than 250 years — making them among the oldest animals on the planet.

The latest expeditions have looked at seep communities as deep as 1.7 miles — far down the continental slope toward the gulf’s nether regions. In an interview, Dr. Fisher said investigations of the deeper communities suggested that tube worm species there grew slower and lived longer.

How long? “It’s likely they can live a lot longer,” he answered. “I’m uncomfortable with an exact number, but we’re talking centuries — four, five or six centuries.”

Over the years, scientists have found that the deep microbes not only eat exotic chemicals but also make carbonate (a building block of seashells) that forms a hard crust on the normally gooey seabed. The carbonate crusts can grow thick enough, they say, to reduce the flow of gas and oil through the seep communities and form attachment points for a variety of other sea creatures, especially deep corals and other filter feeders like brittle stars.

By probing the gulf’s deep waters with sound and other imaging technologies, scientists have found evidence for the existence on the northern continental slope of roughly 8,000 regions of hard crust — all, they say, potentially home to old or new seep communities.

On its Web site, the minerals service freely admits “a management conflict” between encouraging oil development and protecting the dark ecosystems. It issued regulations in 1989 and has periodically toughened the rules, most recently in January.

Now, in the wake of the oil disaster, many seep researchers have voiced strong concern about the threat to the dark ecosystems. Dr. Fisher said that thick oil could coat the respiratory structures of the animals and cause them to suffocate, and that high concentrations might otherwise prove toxic.

Samantha B. Joye, a cold-seep scientist at the University of Georgia, told a House science subcommittee on June 9 that the BP blowout represented “an unprecedented perturbation to the Gulf of Mexico system.”

She expressed particular concern about the dispersants that BP is injecting a mile down into the spewing oil — in a largely successful effort to reduce the flow reaching the surface.

Dr. Joye said the surge of oil into subsurface waters could feed microbes that consume oxygen. If their numbers explode, she said, the result could be a spike in oxygen consumption so large that its deep levels drop precipitously.

The dark ecosystems, she noted, “can tolerate reduced oxygen concentrations.” But she cautioned that the BP spill will challenge their tolerance “beyond any previous insult.”

Now, oceanographers are preparing to dive deep to see how the dark communities are holding up. The lessons for oil precautions and regulatory care, they say, could have application not only for creatures in the inky depths of the Gulf of Mexico but also around the world.

“Everywhere they looked, they’ve found them,” said Norman L. Guinasso Jr., director of Geochemical and Environmental Research at Texas A&M. He cited discoveries of seep communities off Angola, Indonesia and Trinidad.

In exploring the gulf, Dr. Guinasso said, scientists are struggling to fathom the strengths and vulnerabilities of some of the planet’s oldest and most novel creatures. “People,” he said, “are still learning.”