Researchers in Vermont and across the U.S.-Canada border have identified a contagious form of melanoma in brown bullhead catfish, marking the first confirmed transmissible cancer in a freshwater fish. The cancer cells act like parasites, spreading between fish rather than arising independently, and have afflicted roughly a third of Lake Memphremagog’s brown bullheads since 2012. Genome analyses show the tumor cells from different fish are much more closely related to each other than to their hosts, indicating a single transmissible cancer lineage. While the health impact on the fish populations remains uncertain, the discovery raises questions about origins, potential environmental factors such as arsenic, and how cancers can spread across species barriers in nature. The study, published in Nature, adds to the very small number of known transmissible cancers and prompts further research into their mechanisms and ecological consequences.