The Philippines has confiscated 200 metric tons of giant clam shells estimated to be worth around $25 million. Conservationists warned Monday the trade in the endangered clams - used as a substitute for ivory - might endanger the country's marine ecosystem.

The smuggling of the giant clams - called the "jade of the sea" - is quickly becoming rampant in Southeast Asia and Europe, according to the executive director of the Palawan Council for Sustainable Development  Teodoro Jose Matta.

The translucent white shells of the clams are used as an alternative to ivory from elephant tusks.

Demand for the clams has risen as world authorities crack down and pay more attention to the illegal elephant ivory trade. According to a recent scientific article, the price of giant clams in the black market can range between $3,000 and $12,000 each.

Giant clams - or Tridacna gigas - from the Philippines are popular.  Matta said large syndicates were involved in the farming and smuggling of the clams.

"They are big. Not only in Palawan. The group claims it operates all over the Philippines - especially in Mindanao," Matta said.

Philippine officials seized a shipment of 200 tons of giant clams over the weekend in Green Island, Barangay Tumarbong, Roxas, Palawan.

Police arrested four suspects and charged them with violating the country's Wildlife Resources Conservation and Protection Act. Breaking the law carries a maximum sentence of 12 years in prison and a fine of up to $20,000.

The latest seizure is one of the largest ever.

Giant clams - known locally as "Taklobo" - can grow up to 1 meter across and weigh up to 250 kilograms. Marine experts said the clams were vital to the local marine ecosystem because they provided a source of food for many food fish species.

"Taking the giant clams from their natural habitat is a form of intergenerational crime. It will permanently affect the marine ecosystem and future generations will be deprived of the benefits accruing from it," Matta said.