A mysterious cluster of blackened skin lesions affecting catfish in New England turned out to have a stranger cause than anyone would have predicted: a contagious form of cancer. Researchers in Vermont studied melanoma tumors collected from brown bullhead catfish (Ameiurus nebulosus) living in Lake Memphremagog. The tumors on different fish were genetically much more similar to each other than their hosts, they found, strongly pointing to the cancer’s infectious origin. It’s only the fourth ever kind of transmissible cancer discovered in animals, and there remain many questions about this cancer and others like it, the researchers say. “Transmissible cancers are a puzzling biological phenomenon that are not expected to exist,” the researchers wrote in their paper, published Wednesday in Nature, “with the potential to devastate host populations.” Blackened catfish Lake Memphremagog is a freshwater lake that stretches from Quebec, Canada, to Vermont. Since 2012, both local biologists and fishers have been spotting an increasing number of brown bullhead catfish with blackened skin in the lake. Surveys have indicated that by 2014, roughly a third of catfish in the area had these lesions.