One reason for this certainly is their uncanny ability to resist disease.
Sharks do get sick, but their incidence of disease is much lower than among the other fishes.
D.C. The Smithsonian's enormous database, called the Registry of Tumors in Lower Animals, catalogs tissues suspected of being tumorous, including cancers, from all possible sources throughout the world. To do this, we have established the nurse shark and clearnose skate as laboratory animals. These sharks and skates are maintained at MML under controlled conditions and utilized in much the same way that mice, rats, guinea pigs, and rabbits are used in more conventional biomedical research. This was done by placing the chemicals in their food or surrounding tank water, or by direct injection into their muscle. Then, we monitored subsequent pathways of metabolism or detoxification of the carcinogens in the test animals.