Like a deceptively dreamy blanket of snow, a voracious invader is spreading through Hawaii's deep reefs. The snowflake coral grows so explosively that its white, gelatinous polyps are forming billowy underwater prairies, crowding out native sea life.

Formally known as Carijoa riisei, the alien species has stirred alarm among scientists. Most threatened is Hawaii's state gemstone, the rare and precious black coral that accounts for an estimated $25 million worth of jewelry sales from the islands each year. The full extent of carijoa's reach is not yet known, but researchers say it is choking out black coral colonies at an astonishing rate in depths below 245 feet, leaving only dead brittle skeletons.

"The problem with carijoa is it grows really fast and is really dominating where other creatures may not naturally assume such high densities," said Samuel Kahng, a University of Hawaii Sea Grant researcher. Carijoa grows thicketlike at nearly an inch a week and has tough, fibrous branches beneath its soft exterior tissue, while black coral grows about 21/3 inches a year to its mature height of about 15 feet.

The alien, which shies away from sunlight, does not turn up in the shallow reefs frequented by snorkelers, except on the shady underside of rocks or piers. But in the darker, deep habitat beyond the reach of divers who harvest black coral, carijoa is flourishing. Scientists worry that it is killing off older black coral beds that are believed to supply the larvae that allow black coral to reproduce.

Both corals are filter feeders, living on zooplankton and tiny particles that the currents sweep to their polyp mouths. Both grow in a scraggly, treelike fashion and do not build reefs as many other corals do. Black coral is harvested by divers who chop its branches and retrieve its hard skeleton to be cut and polished into jewelry. Some researchers believe the steady harvesting contributes to the ecological threat black coral faces.

Of the 287 nonnative marine invertebrate species in Hawaii, carijoa is the most invasive, said Richard Grigg, an oceanography professor and reef expert at the University of Hawaii. He first documented the alien growing among black coral during a 2001 submersible dive between the islands of Maui and Lanai.

The finding shocked Grigg. Carijoa, native to the western Atlantic and Caribbean, had been noted three decades earlier in Pearl Harbor, apparently having arrived on a ship's hull or in ballast, but it was thought to be benign. Grigg saw it overgrowing 90 percent of the black coral below 260 feet in the channel off Maui, the prime harvesting grounds for Hawaiian black coral.

The problem is intensifying as alien species plague coral reefs, which are also threatened worldwide from overfishing and ocean dumping. In Hawaii, where 410,000 acres of living reef surround the main islands, Gov. Linda Lingle launched a campaign last month to promote reef protection.

Carijoa, however, presents a separate issue because it affects the lesser-known world of the deep reef, attaching to hard surfaces such as rocks, old tires and plastic, even covering sunken ships.

"It's like Christmas decorations that stay up the year around. That's how dominating it is," Kahng said. "It doesn't just settle these areas; it settles at the exclusion of everything else." Carijoa attaches to dead branches or shells that live on black coral. From there, it climbs and grows in all directions, essentially smothering the black coral and other species in its path.

Not everyone sees snowflake coral as a problem. Robin Lee is one of a handful of divers who harvests black coral off Maui using sledge hammers and axes. It is a hazardous profession that has left him disabled and mostly retired because of a miscalculation about how long he could safely stay underwater.

Lee said carijoa does not appear to have reduced the corals available for harvest at the depths divers visit. "I've seen black coral growing right alongside it," he said. "If it could have killed the black coral beds, it would have done it already."

But Tony Montgomery, aquatic biologist for the state Aquatic Resources division, agrees with Grigg's forecast. It is only a matter of time before carijoa takes its toll on the harvest, he said, because in the deeper reef where divers do not go, it is wiping out mature black corals that are key to its ability to reproduce.

"Carijoa is shifting the habitat, the ecology of the system in that environment, to something that is totally different," he said. "It's basically eliminating a unique habitat. . . . It's turning some high biodiversity areas into lower biodiversity areas, because it's blanketing out everything else."

"Changes that may look minor on the surface may have more far reaching effects on the ecology," he said.

Carijoa has not yet affected the availability of black coral for the marketplace, said Robert Taylor, president of Maui Divers, the islands' major purveyor of black coral jewelry. He said the company's use of black coral has decreased by about 30 percent in recent years as demand for other jewelry categories has grown.

Because of the habitat depth, it is unclear how officials could attack carijoa. Limited invasions have cropped up off the island of Kauai, but officials believe they can manually chop it out there. But off Maui, it appears too entrenched for that remedy.

Grigg is pushing to raise the size minimums on black coral harvesting to protect the more mature, reproductive corals. Two years ago, the Western Pacific Regional Fishery Management Council prohibited divers in federal waters from harvesting anything shorter than four feet or with a stem less than one inch in diameter.

Most of Hawaii's black coral colonies are in state waters, however, and state officials have a more lenient standard. They plan to review the status of black coral in the coming months, Montgomery said. Divers say they do not harvest smaller corals, and Taylor said his company does not accept any black coral smaller than the one-inch federal standard. Grigg contends that the steady overall harvesting levels combined with the carijoa threat spell a dangerous future for the Hawaiian black coral's sustainability.

"My concern is the unknown. I can't predict if this is going to become widespread throughout Hawaii. At the maximum, it could be an ecological disaster. At the minimum, it could be just a temporary outbreak. It's probably somewhere in between," he said.

Snowflake coral is threatening the survival of other sea life, researchers say. They contend it is choking out black coral and, consequently, altering the ecology of that environment.