Only two known forms of cancer are contagious, affecting dogs and Tasmanian devils. Now a third has been discovered — infecting clams by spreading through the water in the Northeast Atlantic.
It's fascinating stuff, and freaky. For years, the only contagious cancers were a facial tumor that spreads between Tasmanian devils when they bite each other, and a venereal tumor that affects dogs, spread via sexual contact.
This new tumor spreads even more promiscuously — the cells appear to float around the ocean, going from clam to clam. There's a paywalled article in Cell by the scientists who've been studying this, and Ed Yong did a great writeup over at National Geographic's Phenomena blog.
This clam cancer raises an interesting question: Are there other cancers that spread so widely — and if so, what sort of organisms can they infect? As Yong writes:
How is the cancer spreading? Unlike dogs and devils, clams aren't mounting or biting each other. But they are filter-feeders: they sieve bits of food from the water, so they could easily draw in floating cancer cells too. Certainly, the disease can spread between animals that share the same aquarium tank, even if they aren't touching. "It's not efficient or quick but in the course of months, it happens," says Goff. It's a terrifying thought: transmissible cancer in the water. Thankfully, it's just the clams that are affected.
Or is it?
"We are actively looking in other species too," says Goff. "There are leukaemias like this in other molluscs in Europe." In his paper, he already hints that he has discovered a contagious cancer that affects cockles. He also wants to know if these cancers can spread from one species of shellfish to another.
(That lovely picture of Shijimi clams is a CC-licensed pic from coniferconfifer's Flickr stream. To be precise, I don't know if Shijimi clams are affected by this contagious cancer; but the picture looked so cool I wanted to use it.)
