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In the northwest coast of Spain, a delicious clam called the golden carpet shell is suffering from an extraordinary type of cancer—a contagious leukemia.  

Almost every other case of cancer in animals—including humans—begins when a single cell in an individual starts growing and dividing uncontrollably, producing a tumor. If the tumor kills its host, it dies too. But the clam’s leukemia is caused by cancer cells that have become independent parasites; they can travel between individuals, creating fresh tumors in each new host.

And if that wasn’t astonishing enough, this transmissible tumor didn’t even originate in a golden carpet shell. Instead, its genes reveal that it first arose in a related species—the pullet shell. It’s the first known cancer that not only jumps into new hosts but has, at least once, leapt over the species barrier.

Some cancers are caused by contagious things like HPV, the virus that causes cervical cancer, or Helicobacter pylori, a bacterium that causes stomach cancer. But in these cases, the tumor cells themselves stay put. It’s so exceptional for cancers to become infectious in their own right that until a year ago, scientists knew of only two types that did so. One is a facial tumor that infects Tasmanian devils. It evolved recently, spreads through bites, and threatens the future of its hosts. The second is a far older veneral tumor that affects dogs. It arose 11,000 years ago and has spread to six continents.

A third example emerged last year. Along North America’s east coast, soft-shell clams were dying from a strange type of leukemia. Michael Metzger and Stephen Goff,  scientists from Columbia University, studied these cancers and found that they were all genetically identical to each other, but genetically distinct from their hosts. That’s the same pattern seen in the Tasmanian devil and dog tumors—a  clear sign that these cancers arrive in their hosts, rather than originating from them. They drift through the sea, these selfish shellfish cells, traveling from one cancer-ridden clam to another.

Intrigued, Metzger and Goff polled their marine biologist colleagues and learned that many other shellfish species are afflicted by similar rapidly spreading leukemias. They collected cockles and golden carpet shell clams from the coast of Spain, and mussels from the coast of Vancouver. In all three cases, they found the same signature pattern: a genetic match between all the tumors, and a mismatch between each tumor and its respective host.

“Prior to this, we believed that transmissible cancers were bizarre flukes of nature that happened due to a set of unfortunate coincidences in very unlucky species,” says Elizabeth Murchison, a University of Cambridge cancer researcher who studies the Tasmanian devil tumor. Instead, they are “probably relatively common, at least some bivalves, and the processes whereby cancers become transmissible are not as rare as we previously thought.”

Indeed, Metzger and Goff found that cockles have given risen to two strains of contagious cancer. Their tumors belonged to two distinctive lineages, each of which seems to have independently arisen from a different healthy cell. That explains why the same disease presents in two distinct ways, characterized by cells that look different under the microscope. “People noted that, said, ‘Isn’t that odd?’ and moved on,” says Goff. “This explains the mystery.”  

There’s precedent for a dual origin. Earlier this year, Murchison showed that the Tasmanian devil’s contagious tumor also arose twice. “We absolutely couldn’t believe it,” she told me at the time. “It’s the last thing I could have possibly imagined.”

It might now be the second-to-last thing. A bigger surprise came when Metzger and Goff studied the golden carpet shells. Their tumors were not just genetically distinct from their hosts, but wildly so, with matches as low as 78 percent for certain critical genes. “They weren’t even close,” says Goff. “We then realized they were a near perfect match to the cells of another species, the pullet shell.” The cells must have originated there before jumping into the golden carpets.

Oddly, the pullet shells themselves show no signs of the cancer. They may have given rise to it, but they no longer suffer from it. Why? “One could imagine that the species-of-origin is now resistant to the tumor,” says Goff, “but we don’t know that.”

“It would be good to know how the tumors are transmitted in nature,” says Clare Rebbeck, from the University of Cambridge.  On land, devils and dogs can only spread their tumors by biting and mating. In the water, Goff thinks that transmission might be far easier. The infected mollusks release cancer cells in their feces, so every time they poop, they seed the water with transmissible tumors. And since they are filter-feeders that sieve through huge volumes of seawater, they are also well-suited to picking up the cells from their neighbors.

This might also explain why shellfish seem to be so uniquely susceptible to contagious cancers. Goff guesses that their immune systems are involved, too. Human immune systems would usually stop incompatible foreign cells from setting up shop in our bodies; that’s why people have to take immunosuppressive drugs before receiving organ transplants. “The most fascinating aspect of transmissible cancers is their ability to avoid those immune responses,” says Hannah Siddle from the University of Southampton. “And the transmission of cells across a species barrier is even more striking.”

Fortunately—for us, if not the clams and mussels—there’s no evidence that these cells could affect humans, or that we are plagued by any contagious cancer at all. “I would only worry deeply if I was a mollusk,” Goff says. “Could it happen in rare circumstances? We’d be eager to look for that. It would presumably have to happen between genetically closely matched peers, or people who are profoundly immune-compromised.”

Still, his discovery adds to the growing realization that contagious cancers are more common than anyone assumed. There are now eight of them, and they are organisms unlike anything else. Since time immemorial, people have dreamed of immortality. We’ve filled our fiction with vampires, elves, fountains of youth, horcruxes, and the Singularity. Well, in the real world, this is what immortality looks like. Each of these eight contagious cancers represents a single animal—a dog, two Tasmanian devils, and a handful shellfish—whose original body is long dead, but that lives on as dynasties of cells that propagate in new hosts.

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