Some cancers are contagious, previously these cancers were only known in dogs and Tasmanian devils. Dogs transmit cancer through sex while Tasmanian devils transmit cancers through face biting.
Recent research demonstrates that shellfish are also known to carry a type of contagious cancer. It is spread infectiously between mollusks such as oysters and mussels around the world. Cancer cells can escape an organism and spread to other organisms through the water. These cells are clones that are nearly identical to the original cancer cells. Mussels, cockles, and clams from all over the world share tumor cells with the same genetic markets indicating transmissible cancer.
Mussels are filter feeders, and as they filter algae and other particles they may also accidentally filter tumor cells. These cells can then make it into the mussels hemolymph (invertebrate blood) and infect the organism. Tumor cells may be released into the water when an organism dies or perhaps through feces. Researchers are still uncertain the exact mechanism through which shellfish cancer is transmissible.
Transmissible cancers might be more widespread than we thought. Fortunately, humans cannot get cancer from eating shellfish (as far as we know).