Poor mussels. Mark A. Wilson on Wikipedia Scientists have discovered the first contagious type of cancer that seems to jump between animal species, and they suggest that cancers that spread like viruses could be more widespread than we thought.

Eight different types of contagious cancer have now been identified. We've known about one type in dogs and two types in Tasmanian devils for several years now, but a new study has identified five more types of contagious cancer in four species of molluscs and their close relatives.

According to the head of the research team, Stephen Goff from Columbia University Medical Centre, it seems that certain kinds of cancer are jumping between these types of sea creatures, collectively known as bivalves.

The leukemia-like cancer was found in mussels collected near Canada and cockles and golden carpet shell clams from the coast of Spain. In all three cases, Goff and his team found evidence of cancer being transmitted between the species living in these underwater colonies, Sarah Kaplan reports for The Washington Post.

It's possible that these sick bivalves release cancerous cells when they die - and these cells survive long enough in the water to be picked up by another host. Mussels, cockles, and clams are all passive filter feeders, all have underdeveloped immune systems, and all lack the necessary resources to fight off an attack.

A genetic analysis of the tumours revealed that they came from outside sources, rather than the existing host.

"The tumour cells didn't have the same DNA as their host. Instead, every mussel was being killed by the same line of cancerous cells, which were jumping from one individual to the next like a virus," Kaplan reports.

The affected species would still have to be closely related for cancerous cells to pass between them, the researchers said, which means us humans are in the clear for now.

"These findings seem to paint a picture of shellfish beds around the world that are awash with microscopic cancer cells metastasising both within and between species," Elizabeth Murchison of the Cambridge Veterinary School, who wasn't involved in the study, wrote in an accompanying analysis for Nature.

"I guess that many, many of the cancers that are known will turn out to be of this [contagious] type," Goff told the Washington Post. "How many other marine species might turn out to suffer from this, we don't really know."

And the effects can be devastating, as was seen in the case of the clams studiedoff the coast of Canada last year.

"We could often find beds or areas where most of the clams were dead from [the cancer], particularly if the summer was hot and the disease gets going," study co-author, Jim Sherry, told CBC News.

The next step is to look more closely at the specific types of mutations that cause cross-species contagion.

The study has been published in Nature.