A long time ago, though we don’t know how long, a bay mussel somewhere in the Northern Hemisphere developed a leukemia-like cancer. It began as a mutation in a single rogue cell, which copied itself over and over and over again, and spread through the mussel’s hemolymph, the blood-like fluid in its body.
But then the cancer did something it was not supposed to do, or so we thought: It somehow spread to other mussels through the water. Cloning itself again and again in these new hosts, the malignancy continued to proliferate and infect new individuals.
Even stranger still, the disease didn’t remain in bay mussels alone: It’s now been found in two other shellfish species on opposite ends of the world: blue mussels in France, and Chilean mussels in Argentina and Chile.
This finding, described in a paper published Tuesday in the journal eLife, is the latest in a series of studies showing the transmissible cancers are more common than previously believed—especially in the ocean. This new field of research could help us better understand how cancer develops in both animals and humans, as well as illuminate the murky lives of marine creatures.
“The fact that it’s crossing into two new species is quite fascinating,” says Elizabeth Murchison, who studies transmissible cancers at the University of Cambridge, “and concerning.” Besides being environmentally important, mussels are a favorite food of many cultures—although there’s no evidence that eating cancer-infected shellfish has any impact on human health.
Land and sea
Transmissible cancers, which do not naturally occur in people, were first recognized in two land animals in recent decades. In 2006, researchers discovered that a cancer infecting Australia’s endangered Tasmanian devil—called Tasmanian devil facial tumour disease—could be spread when the animals bite each other, which is a normal part of their behavior. More than 80 percent of the animals have since been infected and killed by this contagious disease and a second, very similar transmissible cancer, which threatens the species with extinction.
Also in 2006, scientists discovered that domestic dogs can spread venereal tumors, which cause cancerous masses on their genitalia. Like all transmissible cancers, the cells are identical, and in the case of dogs, derive from a single canine that lived about 11,000 years ago. (Read on: Dogs carry the oldest known living cancer.)
These findings seriously changed our understanding of cancer, which was previously thought to be confined to cell mutations within individuals. Though various types of viruses can cause damage and pave the way for cancer, such as human papillomavirus (HPV) or feline leukemia virus in domestic cats, it was a shock to find that individual cancer cells could spread among a population.
In the last decade, researchers have found another half dozen cancers that infect shellfish. Michael Metzger, lead author of the new paper and a researcher at Pacific Northwest Research Institute in Seattle, has identified several of them, including one in a British Columbia population of bay mussels (Mytilus trossulus).