FROM 6:00pm onwards, an aquatic product market near Tongchuan Lu comes to life. Piles of crabs, lobsters and different kinds of fish in a variety of containers are stacked rows.

The drizzle outside has made the crowded street muddy and decomposing crabs and fish scattered on the ground are being ground into an unpleasant mass of soggy silt.

When asked if he could supply blood clams, Zheng Chiguang, owner of the Shanghai Jinhui Shui Aquatic Product Shop, was suddenly very alert, although his answer was: "No".

Blood clams, a species of blood-red shellfish banned for sale in Shanghai, were responsible for spreading hepatitis in the city in 1988. But they are also delicious dish. Some people are willing to ignore the ban and risk their health to enjoy the clam's excellent taste.

Despite the owner's denials, the shop actually provides over 50kg of blood clams to big hotels and restaurants in Shanghai every day. In a shabby room near the shop, sacks of blood clams were piled up.

"We have regular customers for the blood clams," said Cai.

Cai said most of the blood clams were from Zhejiang, rather than the countryside around Shanghai. The price of the clams has increased from 10 to 21 yuan (US$1.20-US$2.41) a kilo. Cai said this was because supply of the clams was now so limited. Shops were scared to stock them due to the serious fines that could result.

Cai said three shops in the market had been fined between 20,000-40,000 yuan each (US$2,400-4,800) for ignoring the ban on blood clams introduced by the local health bureau in May to eliminate the dangerous food from the local market.

The crackdown included five big markets for aquatic product markets, 217 vegetables and poultry product markets and 224 restaurants serving seafood in Shanghai.

The examination found nine restaurants and 18 markets serving the blood clams and stocks amounting to 148kg of the forbidden delicacy. From early March to April, the health bureau of the Baoshan District found 49 tons of clams waiting to be exported to Shanghai.

The crackdown did not prevent underground dealing in the clams.

Cai said most aquatic products shops were still engaged in the business and customers were willing to risk their health by consuming the shellfish.

Another shop owner near Cai's agreed it was risky to sell the clams, but said he would smuggle some in if demand was high enough.

"I could suffer a lot if demand was insufficient, because the money earned would not cover the fine," he said.

According to these vendors, the clams come from the countryside around Shanghai and from Zhejiang Province, including Wenzhou and Ningbo. Zhejiang Province produced 65,000 tons of blood clams last year.

Shanghai is a big market for the clams due to the large number of eager customers, especially migrants from Ningbo and Wenzhou.

Chen Gonglin, owner of several large blood clam farms, dreams of selling his produce in Shanghai. He owned a coastal strip with an area of over 14,000 square metres in Leqing City, Zhejiang Province.

But his concern was that it was now difficult to sell the clams in Shanghai, a city normally accounting for half of the entire national market.

Total production can reach 16,000 tons.

Hazardous shellfish

Wenzhou producers have taken measures to improve their production and processing technology in recent years in order to better tap the potential market in Shanghai.

Since 1988, when Shanghai banned the sale of blood clams, producers have been largely locked out of the giant market.

But will Shanghai maintain the ban?

Professor Hu Shanlian, an expert on epidemic disease from Fudan University said the disaster 16 years ago had cast a horrible shadow over the heart of Shanghai. A kind of hepatitis plagued the city in 1988, with over 310,000 suffering the disease. The direct economic cost was put at 1 billion yuan (US$120 million) by government investigators, who identified blood clams as the source of the outbreak.

Health care experts said Shanghai people liked to eat the clams immediately after dipping them in boiling water, a method of preparation that failed to kill the harmful bacteria in the clams.

Professor Ma Zhiying, general engineer with the Shanghai Food Research Institute, said the increasingly polluted state of beaches and coastal areas due to waste water and refuse had affected the environment in which the shellfish lived.

He also noted that the decay of the clams during long-distance transportation generated bacteria which then spread to other clams.

According to Zhang, a blood clam filters 40 litres of sea water every day, absorbing the bacteria and viruses that can cause hepatitis, typhoid or dysentery and storing them in their flesh.

Research has found that the hepatitis virus can survive for as long as three months in the clams.

Shanghai Disease Control and Prevention Centre investigates the clams every year and has found the proportion carrying hepatitis remains between 5-10 per cent.

Gu Zhenhua, an official from the Shanghai Institute of Public Health Supervision, said it was not reasonable to take the risk of removing the ban on blood clams under these conditions. But the system of fines does not seem to be eliminating the hazardous shellfish from the local market.

Gu said although technology could be used to remove the hepatitis virus from the clams before sale, the investigation conducted every year still found virus-carrying specimens.

"So it is still a life-threatening risk to eat these clams," Gu said.