Ever since it was claimed that a medicine made from the Chinese three-striped box turtle could cure cancer, demand for the product has increased, making the turtle the most expensive to be found in China's food markets. Huge sums of money have been made selling the medicine. The turtle -- along with a few other Cuora species -- was once imported into China by the pet trade. Exporters obtained the specimens in the food markets rather than going out to collect them in the wild. Treatment of the turtles prior to reaching the markets resulted in many of them being diseased. Packed tightly into crates, often for journeys of several weeks without food, it is little wonder that many specimens exported to western countries fared poorly when purchased as pets. Throughout the Far East turtles and tortoises have featured in various cultures and certain religions. They have also been used for hundreds of years for food and medicine. Traditional Chinese medicine is probably the greatest threat to the three-striped box and to the thousands of other turtles that are imported yearly into China. Turtles are well known for their longevity, which it is thought will be passed on to those eating them. Turtle blood, bones and shells -- particularly the plastron (ventral part of the shell) -- are thought to have medicinal properties and be able to cure a number of ailments. Turtle eggs are also used for food and medicine. The medicinal value of the turtles is open to question, and it is possible that herbs could produce the same effects. Any beneficial substances from the turtles could also be synthesized chemically, so as to prevent the killing of turtles. The southern part of China has become more prosperous in recent years, and the demand for turtles has increased dramatically. Turtle dishes are served in restaurants and regarded as an expensive delicacy, even a status symbol. As turtle supplies in China dwindled, traders started importing the animals from Vietnam. As a result, many Vietnamese turtles have now disappeared from their haunts in the wild. To satisfy growing demand, importers have now turned to sources farther afield, including Cambodia, Laos, Nepal, India, Bhutan and Indonesia. Since biologists started studying turtles in Chinese food markets, several previously unknown and extremely rare species have been found some are on Appendix I of CITES. At the present rate of collection such specimens, and even species not yet discovered, could soon be extinct. The three-striped box turtle is simply one among dozens of other species flooding into China, but its high value makes it a special target for collectors. According to this passage, turtles are of great importance in China because they are ______.