HOBART, TAS - AUGUST 31: A Tasmanian Devil bears it's teeth at a quarantine facility August 31, 2005 in Hobart, Australia. The Devil, a native marsupial unique to Tasmania, is under threat from Devil Facial Tumor Disease (DFTD) which is decimating numbers throughout Tasmania. Several devils are being monitored under quarantine situations on the mainland, while another group have been moved to Maria Island, to form an "insurance population" should the disease spread. (Photo by Ian Waldie/Getty Images)
In 2012 an angler pulled a brown bullhead out of a lake on the Vermont-Quebec border with what looked like tar smeared across its skin. This month biologists at the University of Vermont reported in Nature that the black melanoma is contagious, the tumor cells themselves passing from one fish to the next. It is the fourth kind of transmissible cancer known to science, and the first in a fish.
Cancer is supposed to be a dead end. When the host dies, its cancer dies too. A few times in the history of life a cancer cell has slipped that fate, breaking free of the body that made it and spreading to new hosts as a living, contagious lineage of its own. The catfish melanoma is the newest of these. Here are the other three.
