ZME Science reports on a Nature article (full article is paywalled) (DOI: 10.1038/nature18599) about a disease called disseminated neoplasia. The disease is a group of cancers which are thought to spread via seawater. They affect mussels, cockles, and golden carpet shell clams.
Among mussels and cockles, the cancer cells come from the same species, but the cancer infecting golden carpet shell clams comes from a different species, Venerupis corrugata , the pullet carpet shell.
https://en.wikipedia.org/wiki/Canine_transmissible_venereal_tumor [wikipedia.org] - Supposedly 5000+ years old. It is the HeLa cell like of dog tumors, and apparently still out in the wild (although rare.)