This is a pretty amazing paper out in Cell this week showing that clams are getting leukemias that are the result of tumor cells spreading from clam to clam. Non-viral infectious cancers are a pretty rare phenomenon, with one of the more well-known examples being devil facial tumour disease.
As explained in Science, the researchers were originally testing whether the leukemias present in the clams were the result of a virus. Instead, what they found were numerous copies of a retrotransposon (which they named Steamer, hehe) that came from the cancer DNA itself. Also, the pattern of how Steamer inserted itself into the cancer DNA was the same across different clams, which suggested that the cancer cells were related to each other. When they did closer analyses of the cancer DNA, they found that they all matched other, and not the clams that they came from.
So one clam, probably decades ago, developed a leukemia that managed to spread itself across clams found in the US Northeast and Canada. That is insanely fascinating/horrifying, and so so glad humans aren't breeding spreadable supertumors.