Biologist Ed Gomez was in his office at the University of the Philippines Marine Science Institute about 25 years ago when the phone rang. The woman on the line told him she worked as a broker and had a client who wanted to buy some of the young giant clams his team was culturing.

“She said, ‘I have a buyer who will purchase your clams,’” Gomez recalls. “I asked her, ‘Are these for local sale or intended for export?’” Both, she said.

“That’s when I told her, ‘I’m sorry to tell you the Bureau of Fisheries will not allow the export of giant clams.’” Months before, Gomez and his team had applied for—and been denied—export permits for giant clams. His original plan had been to figure out how to breed the clams in captivity, so Filipino fishermen could supplement their income by growing them themselves and selling them abroad.

“‘Oh don’t worry about it,’” Gomez remembers her saying. “‘Just release the clams, and [I’ll] take care of it.’”

The University of the Philippines has been cultivating giant clams in captivity for several decades in an effort to restock the species in the region. Here a volunteer helps rearrange young clams at the Bolinao marine laboratory.

Gomez again declined and hung up the phone. He thought nothing more of it.

A few weeks after that call, the institute’s clams, which lived in sea pens offshore, started disappearing by the hundreds. It went on for weeks. “They were stolen,” Gomez says.

The team notified the police, but the culprits were never found.

At the time, the colorful species of giant clams were in demand for the aquarium trade. Gomez’s lab was the only one of two institutions raising the slow-growing clams—whose soft tissues glow in hues from seafoam green to deep blue and bright purple—and he suspects that his babies were ending up in people’s aquariums.

He hatched a plan. The lab would switch from raising the colorful species to growing the biggest of the 12 species of giant clams, Tridacna gigas. They are plain-looking as juveniles and not so appealing to collectors. Breeding them would focus the team’s purpose: to help restock this nearly extinct species in the Philippines and South China Sea.

SOURCES: COUNCIL ON FOREIGN RELATIONS; U.S. ENERGY INFORMATION ADMINISTRATION; OCEANASIA 2015, REPORTED AND ESTIMATED UNREPORTED CATCHES; RANDALL AND LIM, 2000; CENTER FOR STRATEGIC AND INTERNATIONAL STUDIES

Gigas clams can grow up to nearly four feet (1.2 meters) long and weigh 500 pounds (227 kilograms), and they play a significant part in keeping the South China Sea’s shallow-water reef habitats alive and well. They provide a home for seaweeds, sea sponges, snails, and slugs, and protection for young fish. They also fill a valuable role as filter feeders, cleaning the water of pollutants as they ingest algae or plankton.

Giant clams have taken a hit in recent decades, and they’re considered vulnerable to extinction. During the 1970s much of the Asia-Pacific region was picked clean of giant clams for their meat, which was prized in China as a supposed aphrodisiac. Then came the aquarium trade in the 1980s. Now there’s a new problem: Giant clams are in vogue in China for carvings, jewelry, and other ornaments—as status symbols for the wealthy and as protective charms in Chinese Buddhism.

Meanwhile reefs in the South China Sea—some of the most biodiverse on Earth—are under assault: More than 60 square miles (160 square kilometers) of reefs, nearly 10 percent of the total, have fallen victim to the ongoing dispute in the South China Sea. Gomez estimates that this reef destruction means that the South China Sea region faces $5.7 billion a year in potential economic loss.

Fishing supports millions of people in the countries ringing the South China Sea, and they’re responsible for more than 10 percent of the world’s fish catch. As reefs disappear, so do fishermen’s livelihoods .

In an effort to extend its reach, China has built more than 3,200 acres (1,300 hectares) of new land in the Spratly Islands by smothering shallow reefs with sand to create new land and dredging to create deeper ports. In July, a tribunal at the Permanent Court of Arbitration in The Hague, Netherlands, ruled that China’s island-building activities violated the country’s international obligations to protect the marine environment.

But giant clam poaching is destroying reefs more extensively and indiscriminately than island building. Poachers face few consequences, and the degradation has garnered little attention on the international stage.

Scars on the Reef

In 2012 Philippine authorities found a Chinese fishing boat loaded with corals, live sharks, and giant clam shells at Scarborough Shoal, some 138 miles (220 kilometers) from the Philippine coast.

The government contacted Gomez with a question. Why in the world would the Chinese have so many giant clam shells? Gomez didn’t know, but he soon found out. A friend gave him a tip, advising him to visit the port town of Tanmen, on the island of Hainan.

When he got there, he was flabbergasted: “Rows and rows of shops selling nothing but giant clam carvings, giant clam shells, and corals,” he says. “There must have been, I would guess, a mile and a half of stores.”

It turned out that the giant clam handicraft market in China had exploded, and the South China Sea was its epicenter.

By the late 1990s Hainan fishermen had overfished their coastal waters. Their catches were getting smaller, and they were looking for ways to supplement their income, says Zhang Hongzhou, a research fellow at Nanyang Technological University in Singapore.

The government helped them out with a special fuel subsidy to travel more than 500 miles (800 kilometers) south to the Spratly Islands, as well as subsidies for bigger and better boats.

Within a decade or two, the demand for giant clam handicrafts also began growing, possibly, according to Zhang, because the government became stricter on the trade of elephant ivory. The newly rich were on the lookout for a status symbol that was less stigmatized and less likely to get them in trouble with the law. Plus, devout Buddhists wanted giant clam carvings for luck, and speculators wanted them for investment. In short, Chinese fishermen had the motivation to poach, the means to do so, and the market demand to absorb their supply.

Eugenio Bito-onon, Jr., then the mayor of the Kalayaan municipality, an island group in the Spratlys claimed by the Philippines, had his first encounter with giant clam poachers in 2009. By 2012, he was seeing them regularly around Thitu, known locally as Pag-asa, the biggest island occupied by the Philippines in the Spratlys.

View Images The captive-bred giant clams pictured here are about five or six years old. More of the South China Sea's reefs have been destroyed by giant clam poaching than by China's island-building. Photograph by Adam Dean, National Geographic

“As soon as we reached Pag-asa, the whole reef was in smoke,” he says about a 2012 return trip to the island. Chinese skiffs were spread across a nearby reef, anchored in place. Their outboard motors were below the water, engines revving and clouds of diesel exhaust billowing into the air. The propellers were being used to grind up the reef and loosen giant clam shells encased in the coral.

In December 2015, a BBC team caught the poaching on camera and broadcast the reef destruction to the world.

John McManus, a marine ecologist at the Rosenstiel School of University of Miami who studies the South China Sea’s reefs, went diving off Thitu earlier this year. “There was absolutely nothing alive on the bottom except for two pieces of seaweed,” he says. “We have a very serious problem. [Giant clam poaching is] going to have an effect on the whole system, from coral reefs in general to pelagic fish that come swimming through and feed on reef fish.”

Putting a Price on Reefs

In economic terms, coral reefs are the single most valuable ecosystem on Earth, according to a paper published in 2012 by ecologist Rudolf de Groot at Wageningen University and Research Centre, in the Netherlands. A hectare (about 2.5 acres) of reef has a potential value of approximately $350,000 a year.

View Images The port town of Tanmen, China, has numerous shops that sell giant clam carvings and jewelry. Most customers do not realize that harvesting the clams often entails extensive reef destruction. Photograph by Farah Master, Reuters

De Groot’s team assigned a dollar value to various natural regions by accounting for everything from the food, water, and raw materials each one can provide at a sustainable level, to their roles in regulating water flow, protecting genetic diversity, and providing a place for recreation.

Few coral reefs are as productive and diverse as those in the South China Sea, which is often considered to be part of the “Coral Triangle,” a region of tropical waters that hosts more marine biodiversity than anywhere else in the world.

Using de Groot’s data, Gomez came up with the estimate: $5.7 billion a year in potential economic loss because of reef destruction in the South China Sea.

The sea’s reefs protect and replenish species elsewhere in the region. Fish larvae spawned on one reef in the Spratly Islands, for example, are carried by currents to other reefs, resupplying stocks there. Reef flats grow sea grasses, which sustain sea turtles. And they provide homes for reef fish like groupers and support lower-level fish like sardines and herring, important food sources for tuna and other bigger pelagic fish, not to mention humans.

View Images There are now about 30,000 captive-bred giant clams in the Bolinao lab’s sea pens. Gomez hopes to restock the Spratly Islands, but the last time his team tried, the newly introduced giant clams were almost immediately poached. Photograph by Adam Dean, National Geographic

Gilbert Elefane, a Filipino fisherman in the municipality of Quezon, has seen the effects for himself. On a sweltering day, we’re sitting in the shade as fishermen swim with their catches from boat to shore to be weighed and transported. Stacks of Styrofoam boxes in wooden crates are yet to be filled with the fresh fish.

Elefane, who the day before had returned from a trip to the South China Sea, tells me he’s seen poachers dive down to the reef and surface with a rare giant clam. He says he’s also seen fishermen using the boat’s propeller shaft as a pulley to lift clams out of the sea. “When you lose the giant clams, and you lose the turtles, the ocean system suffers,” he says.

Taking Responsibility

“Most Chinese citizens have no idea what kind of environmental devastation China has done in the South China Sea,” says Gregory Poling, the director of the Asian Maritime Transparency Initiative at the Center for Strategic and International Studies, a Washington, D.C.-based think tank.

That’s why efforts to reduce demand for giant clam handicrafts are badly needed, McManus says.

“A person owning products from tigers or elephants is likely to be socially ostracized,” McManus says. “However, owning a product from a threatened species of clam generally carries no such stigma.”

View Images Many Chinese Buddhists believe giant clams have protective powers. Others buy giant clam handicrafts as a financial investment or status symbol. Photograph by Adam Dean, National Geographic

People aren’t aware that the giant clam is a threatened species, McManus says, nor are they likely to know that huge swaths of reef likely had to be destroyed to harvest it.

In early 2015 Beijing began to take a tougher stance against illegally harvested clams, says Zhang, the research fellow in Singapore. “On one hand, they tried to prevent fishermen from going out to harvest clams in the South China Sea. On the other hand, they’re trying to take legal measures against people who purchase giant clams.”

China’s State Ocean Administration and Ministry of Agriculture, which Zhang says were involved in the efforts, did not respond to requests for details.

For now, marine biologists and political scientists alike are watching to see how China will react to its loss at the international tribunal, which found that Chinese officials were aware that their fishermen are poaching sea turtles, coral, and giant clams “on a substantial scale” and that China’s island building has violated international environmental agreements.

China has often defended its island building by saying it limits construction to reefs that are already dead. But according to McManus, satellite imagery shows that at least some of the reefs were dead only because giant clam fishermen had ground them up before the construction crews moved in.

Perhaps the tribunal decision will eventually encourage China to cooperate.

“That’s the hope, that this case over the long term will prove a useful bit of leverage to getting Beijing to the table,” Poling says. “In the short to medium term, I think we’re in for more of the same, if not worse.”

View Images Giant clams were poached in the 1970s because their meat was believed to be an aphrodisiac in China. Then they were popular in the 1980s for aquariums. Now they're in demand for carvings and beads. Photograph by Adam Dean, National Geographic

Aurora Almendral contributed to this report.

Read the other story in this series: One of the World's Biggest Fisheries Is on the Verge of Collapse.