The project has drawn national attention, coming after alarming reports in the last decade that up to 90 percent of the coral that surrounds many of Okinawa’s islands has died off. This raised a rare preservationist outcry in a heavily industrialized nation whose coastal vistas tend toward concrete sea walls and oil refineries.

The result has been what marine biologists call one of the largest coral restoration projects in the world, begun four years ago. The goal, say biologists, is to perfect methods that could be used around the world to rescue reefs endangered by overfishing, pollution and global warming.

They say they are using the Sekisei Lagoon Reef, which is named after the broad, shallow lagoon that it created, as a test bed for new techniques that they hope will one day make transplanting coral in the sea as routine as raising tree saplings on land.

“We have been replanting forests for 4,000 years, but we are only just now learning how to revive a coral reef,” said Mineo Okamoto, a marine biologist at Tokyo University of Marine Science and Technology, who has led development of the palm-size ceramic discs. “We finally have the technology.”

Critics, however, say the project might be wasted effort. They say transplanting is futile without addressing the problems that caused the reefs to deteriorate in the first place, like coastal redevelopment and chemical runoff from terrestrial agriculture. There is also the bigger problem of rising ocean water temperatures, for which there may be no easy fix.