The vast majority of cancers remain in our innards, turning our own bodies against us. We don't think of them spreading to other hosts like a blight or a disease. Yet, biologists have discovered at least two cancers that do exactly that: One threatens Tasmanian devils and spreads through biting, and another is transmitted sexually among dogs. Now, according to an article in the journal Cell, they've found a third, a leukemia-like agent that has been attacking soft-shell clams (Mya arenaria, aka steamers and little necks) from Maine to the Chesapeake for the past 40 years [sources: Dallas; Gorman; Metzger et al.].
Researchers have long known about the clam cancer, but have focused their search for a cause on environmental elements and, more recently, a possible virus. But when they examined the DNA of cancerous cells from clams across the region, they found something quite surprising. Instead of resembling the genes of their host clams as expected, the cancer cells were clones of a single genetic source, presumably some cancerous "clam zero" from which the cancer had initially escaped and spread [sources: Dallas; Gorman; Metzger et al.]. Such a mechanism is risky for the cancer, not only because its cells cannot long survive outside their host, but also because they must face a victim's immune defenses every time they spread [source: Gorman].
The researchers plan to look into similar illnesses that strike cockles, mussels and oysters to see if a similar marine metastasis affects them as well [source: Gorman].