Cancer in clams jumps the species barrier
Leaving the world of mammals behind, we are now diving into another instance of these rare contagious cancers. This transfer happens in bivalves like mussels, clams, and cockles. But before we talk about cancers, Rob Ellis, a researcher in Ecophysiology and Sustainable Aquaculture at the University of Exeter, has gone down to his local estuary to “shellebrate” the wonders of these organisms and Alicia Bruzos from the Francis Crick Institute explains to Julia Ravey what is putting this animals at risk…
Rob - I'm out on the estuary in East Devon and in this estuary, bivalves play a critical role for biodiversity. We have a number of different bivalve populations; probably most visibly noticeable are those of the mussels and the oysters. These two species are classed as ecosystem engineers. The two organisms, when they settle, create a natural reef. In doing so, they create a novel habitat that's able to support a range of other species. If we think about a region, for example the Southwest of England, the number of species supported by bivalve reefs can go up by around 750. As well as improving biodiversity, bivalves obviously play a really important role as a food source for things like crabs, starfish and other predators. They are really important in terms of a food source for humans. Around three miles off the coast of east Devon, it's a bit of a cloudy day today so I can't quite see it, but we've got the largest mussel farm in the UK, Offshore Shellfish Ltd. Mussels are readily filter feeding. The population of mussels or a mussel bed can actually improve water clarity, and therefore really benefit adjacent habitats, such as seagrass beds and associated species like that. They can be a really powerful tool that we can employ to therefore improve those coastal environments and reduce human impact. If we were to lose bivalves, this would be a really important problem in terms of impacting coastal biodiversity, impacting coastal function, ecosystem function, and also in terms of impacting our ability to utilize those resources as a source of human food.
Julia - Based on those insights from Rob, losing bivalves would be pretty disastrous, but these animals are also impacted by contagious cancers. These diseases have only recently been found in bivalves, but could have driven past declines in numbers. Alicia Bruzos, cancer researcher at the Francis Crick Institute who studies these rare conditions in bivalves explains...
Alicia - These cancers, which is a leukaemia-like cancer, in bivalves where reported in the late 60's and in the 70's and 80's of last century. But it was not until 2015 when they discovered that they were contagious. Because we needed to analyse the DNA to know that that cancer cell did not originate in that individual and came from a different individual.
Julia - And do we have any idea about how it spreads from one organism to the next?
Alicia - The hypothesis is that somehow the cell is released to the environment. We don't know if it is an active release, or if it is when he's dying passively, the cells are released to the environment. Then the cells float in the sea water and they survive in that sea water, so maybe the temperature and the salinity and all these conditions might have an impact. Finally what these viruses usually do in nature is that they filter water. By filtering water, they take some cancer cells that were floating in the water, and those cancer cells go inside, and then they start to develop.
Julia - Mammals immune systems have evolved in a way to detect things that are foreign and get rid of them. But I'm guessing the immune system of bivalves is quite different to immune systems like ours. Is there a flaw in their immune system that allows this type of contagious cancer?
Alicia - We do know they have defence, but they don't have the same immune system as us. A lot of research has to be done to actually understand why they are not as successful as our immune system to defend themselves from a contagious cancer. There are many parasites that are affecting these kinds of animals; the bivalves. A lot of research is being done to try to understand 'Why is this happening?' 'Why is the immune system of these animals not fighting against all these parasites?' And of course, the contagious cancers.
Julia - You said that these cells appear to float in the water, passing from one organism into the other. How far do we think these cells can travel?
Alicia - Earlier this year, it was published that the same cancer was found in some clams in the Atlantic coast of Spain and also in the Mediterranean coast of Spain. Those two places are located more than 1,000 miles away. How is this possible? Was the cancer cell able to float and travel that far? We are not sure. We also have to consider that maybe human activity played a role in moving that cancer from the Atlantic coast to the Mediterranean coast. But for instance, in mussels, there's another type of contagious cancer that has been found in Chile, South America, North America, Europe, and even in Russia. But in the case of mussels, mussels attach to boats and boats travel long distances. By moving the mussels from one area to another, that is probably how this contagious cancer was arriving to so many places in the world. In the case of the clams, they usually live buried in the sand, so they don't attach to the boats, it's a very interesting question as well.
Julia - And with these cancers, are they restricted to an individual species in the sense of, if a cancer arises in one species, does it stay within that species because they're genetically similar? Or can it jump the species barrier and move into another?
Alicia - For most of them, it is how you describe it. The cancer arrives in an animal and it spreads to other individuals of the same species. But surprisingly, there are several cases in bivalves where the cancer arose in one bivalve species, and nowadays it is found in other species. By reading the DNA, you are able to study the history of that cancer. We found a cancer in the warty venus clam, which is a clam that lives in both the Mediterranean and the Atlantic coast of Spain. By studying the DNA we found out that the cancer cell did not arise in the warty venus clam but arose in a different species, which was called the striped venus clam. Another case of cancer that crossed that species barrier is the mussels. This one is very interesting because it has gone not only from mytilus trossulus, it is also infecting three extra additional species. That is like the master of cancer.
Julia - It seems like we have these cancers that can not only spread from organism to organism, but they can spread potentially from species to maybe different species and then also can potentially travel in some way we don't know and spread from population to population. What would this mean if these types of cancers impacted lots of bivalve populations? What would be the impact of that?
Alicia - We should be very careful with our economic activities when moving animals from one place to another and not only animals. Even with the sea water. We should now think of cancer, like a parasite. In the case of continuous cancer, it does behave as a parasite. And try to track at least these movements, so we don't put the disease into a disease-free area, because imagine that these cancers become very aggressive and they potentially reduce the number of individuals and they drive it to extinction; that will be an ecological problem. But if we keep the disease-free areas disease-free, so no cancer goes into there, at least not by our activities, in a scenario where this happens, that all the population reduces, we can repopulate with the ones that didn't have the disease.