As bad as cancer is in humans, at least it’s not contagious. The same can’t be said for clams, mussels, and other marine bivalves. According to a new study, published online today in Nature, these creatures can suffer from a form of cancer similar to leukemia that appears to be transmitted through the water and can pass not only between members of one species, but even between two different ones. Genetic analyses revealed that, even in different mussels (pictured above), cancer cells were much more similar to each other than to healthy host cells, suggesting that the cancer hadn’t originally developed in host mussel tissue, but may have come from a common outside source. The cockles revealed a similar story, but showed evidence of two distinct lineages of cancer cells that evolved separately over time. The golden carpet shell clams told the most interesting tale of all: Again, dissimilarities between the host cells and the cancerous cells gave the appearance that the cancer cells were from an outside source, but this time the differences were so pronounced that the cancer cells looked much more like host cells from an entirely different species of clam—the pullet carpet shell (Venerupis corrugata). The team concludes that the cancer mutations initially arose in V. corrugata, but crossed species to Polititapes aureus at some point. For the bivalves at least, the results suggest that cancer acts like a new type of infectious agent, similar in many ways to traditional pathogens that continually evolve new genetic tweaks that allow them to survive and reproduce. This type of transmissible cancer has also been discovered in several mammals including Tasmanian devils and certain breeds of dogs. The new results in bivalves, the authors suggest, show that some animals are more susceptible to phenomenon, but humans are in the clear—at least, so far.
*Correction, 23 June, 10:06 a.m.: This item originally stated that contagious cancer was seen in "certain species of dogs." We have corrected the item to read "certain breeds of dogs."