But a rapid decline in the world’s manta populations means that the gills may soon be going off the market. A save-the-mantas campaign appears to be nudging China toward a ban on the gill trade. It would be a rare success in an otherwise grim litany of ecological ruination tied to the country’s demand for threatened plants and animals.

In part by cultivating support from the government, which is typically loath to take the lead in conservation efforts, the campaign has built up grass-roots and official backing for ending gill sales with a sobering message: The business is not just endangering a vulnerable species; it is also harming consumers.

Conservationists say the Chinese trade in manta ray gills has soared over the past decade, fueled not by a renaissance of tradition but by an unscrupulous network of traffickers looking for ever new ways to profit from the Chinese appetite for wildlife. The world’s manta populations have dwindled as their gills have piled up in this balmy southern city, home to 99 percent of the trade, according to WildAid, a conservation group based in San Francisco that rolled out the current campaign in 2014.

The group, which uses the tagline “When the Buying Stops, the Killing Can, Too,” led two similar efforts that succeeded in reducing demand for both shark fin soup and elephant ivory. The campaigns employ tactics like having celebrities explain the direct link between consumption of a coveted natural ingredient and the calamitous impact on endangered wildlife, a concept for the most part unknown before in China.

Toxicology tests by WildAid have also revealed that many of the gills sold at the Qingping market contain dangerous levels of heavy metals and carcinogens, including arsenic, cadmium, mercury and lead, toxic substances that accumulate as the mantas filter plankton through their gills. The sale of such a hazardous and scientifically unproven cure-all online and in stores, often marketed to nursing mothers, highlights an alarming flaw in the government’s tattered food safety system, which largely ignores traditional medicine.