In shellfish, the Live Science headline says, cancer can be contagious. “Recently scientists discovered that cancer cells can sometimes escape an organism and spread to others. These cells are clones that are nearly identical to the originals, save for mutations that might have popped up since they diverged from the initial cancer cells.”

Genetic analysis of the cancers and their hosts revealed that in nearly all of these cases, the genetic makeup of these cancer cells did not match those of their hosts. Instead, the cells came from other animals. Then comes the kicker. The finding suggests that transmissible cancers might be far more widespread than previously thought.

I knew that there was a contagious facial cancer afflicting Tasmanian devils (and in fact, decimating the population.) They nip each other in the face like you and I shake hands, but while we once spread warts (mom told us) they pass on a horrific cancer. And I didn’t know they’d found contagious cancers in dogs, spread by puppy love. But these involve intra-species transmission, though, one he dog to a she dog, or one snarling Tasmanian devil to another. The thing about this news story, though, is that they’ve found a cancer had spread from a clam to a mussel, that is, inter-species transmission. And while there’s no way you or I will ever catch this cancer–we are way too different from clams and mussels, even the dumbest of us–it does mean that we might be able, some day, to catch a cancer from another primate species, at least. Or maybe even from Fido, wagging his tail every time you look his way? But as no cancer has jumped from human to dog or vice versa in the 20-30K years that dogs (née wolves) and people have been hanging together (I assume we could have detected any such transmission genetically) it seems unlikely. Specific cancers are too tied into specific genetics to be able to just flit from one mammal species to a distantly related other, and people and dogs and all their carnivore and primate predecessors and their pre-carnivore and pre-primate predecessors have each been on their merry but separate evolutionary way for maybe 80 millions years, as dinosaurs still stomped about. And though both are mollusks, the last common ancestor of mussels and clams existed over 480 million years ago. Yet half a billion years later a clam can “catch” cancer from a mussel. A half billion years is an incredibly long time, both in terms of deep time and genetics.

We might catch the same sort of cancer from the same carcinogen as a dog, but it’s highly unlikely a dog’s cancer cell could settle amid our healthy people cells and metastasize. But here a cancer in mollusks that has done just that. No one knows how, though there is one revolting hypothesis that it comes in excrement that floats on in with sea water, a mussel leukemia transmitted in the way we transmit cholera, though all we ever did was drink and bathe in the water, while mussels eat and breathe it. The sea is their air. Imagine breathing in cholera laden air, as if all the old ideas of bad air causing epidemics (“malaria” comes from the Italian for bad air, mal aria) were true. Perhaps that is how this cancer (a mussel leukemia) spreads. Not that the mussels are thinking about it. But I am, and the entire idea is creepy. You have to wonder how often this happens. In a small population a contagious cancer could theoretically push a species to the point of no return. Perhaps even a hominid species. What have the evolutionary implications been over the past half billion years as cancers crossed from one species to another closely related species? Has genetic variation been driven, in some small sense, by mutations that allowed an individual to fend off the wandering cancer cells from a closely related species that had devastated his otherwise identical siblings?

Thinking about cancer is bad enough. But thinking about contagious cancer gives me the willies. Change the subject, please.