Cancer: Uncontrolled Cell Division

Pretty much everyone has heard of a terrible disease called cancer. In humans, specific types of cancer can affect organs like the brain, lungs, or skin. The body’s normal, healthy cells only divide as much as necessary, but cancer results from a massive, out-of-control multiplication of cells. Typically, cancers are caused by mutations in the genes that regulate normal cell division.

Cancer does not only affect humans—it can affect other animals and even plants. All living organisms have cells, so they can all develop cancer, whether they live on land or in water. In fact, many marine (ocean) species develop cancers, such as whales, dolphins, sea lions, walruses, turtles, fish, sponges, and corals. In this article, we focus on a peculiar type of cancer that affects shellfish species.

Contagious Cancers?

Most cancers are not contagious, meaning they cannot spread from one person to another like the flu or COVID-19 can. This means that, when a person with cancer dies, the cancer also dies. However, researchers have recently found some types of cancer that can survive even after the organism dies. A type of cancer discovered in certain shellfish called bivalves can spread between individuals and is thus known as a contagious cancer. In contagious cancers, cancer cells leave the affected individual, survive in seawater, and infect another individual. These cancers have been found in many bivalve species, including cockles, clams, and mussels.

The Peculiar Case of Contagious Cancer in Bivalves

The scientific term for contagious cancers affecting bivalves is disseminated neoplasia. “Neoplasia” means “new growth” and “disseminated” means that the cancer spreads through all organs. In bivalves, the cancer spreads through their haemolymph, which is their form of blood. Scientists can remove a drop of haemolymph using a syringe and observe it under a microscope. Cancer cells are easily identifiable because they are larger than healthy cells. Furthermore, as cancer is a disease in which cells divide uncontrollably, it is very common to find cancer cells in the midst of mitosis (Figure 1). The rapid division of cancer cells causes the bivalve’s organs to deform, which prevents their proper functioning. For example, if the gills no longer have their normal structure, the bivalve will be unable to get oxygen from the water, which can ultimately lead to death.

DNA studies have been important in the discovery that these bivalve cancers are contagious. In an individual with a non-contagious cancer, the DNA of healthy cells is very similar to the DNA of cancer cells. However, in an individual with a contagious cancer, the DNA of cancer cells is quite different from the DNA of healthy cells because those cancer cells originated in a different organism—the one that infected it.

Which Bivalve Species Get Contagious Cancers?

Several contagious cancers have been found in species of clams, cockles, and mussels all over the world (Figure 2). These shellfish live in coastal areas, so they can be easily caught when the tide goes out or by using small boats. The first contagious cancer in bivalves was diagnosed in softshell clams along the Atlantic coast of the United States and Canada [1, 2]. In Europe, contagious cancers have found in clams from the Baltic Sea [3], and in common cockles from the Atlantic coast of Europe [1, 4]. Lastly, two mussel species found worldwide have also been found to be affected by contagious cancers. In fact, contagious cancers in mussels have been detected in locations that are very far from each other, such as the East and West coasts of the United States, Canada, Argentina, Peru, France, Russia, and Japan [5, 6].

Super-Powerful Contagious Cancers

On rare occasions, contagious cancers can spread to a different bivalve species. This is the case with two contagious cancers detected in clams off the Spanish coast. DNA studies of the contagious cancers detected in warty venus and golden venus clams revealed that these cancers originated in a different clam species [1, 7]. For example, a contagious cancer that originated in striped venus clams was found in warty venus clams (Figure 3). Although researchers have seen several cases of contagious cancers spreading to closely related species, contagious cancers do not spread to all the bivalve species that are living close to the infected individuals.

Can Humans Get Cancer From Eating Shellfish?

Contagious cancers in bivalves are highly specific to those organisms and have only been found to infect closely related species. Not all bivalves living near affected individuals become infected [8]. Thus, it is unlikely that humans could ever become infected with cancer from eating shellfish. When we eat bivalves, our bodies break down the food to extract nutrients and energy—and a contagious cancer cell from a bivalve could not survive that process. Even if a contagious cancer cell could enter the human body, humans have an effective immune system that can defend against many diseases. In a nutshell, it is extremely unlikely that humans could become infected with cancer from shellfish.

Why are Marine Contagious Cancers Important to Study?

Studying marine contagious cancers is important for three main reasons. First, bivalve cancers can be fatal for the affected organisms because cancer cells reach all their organs, preventing them from carrying out their normal functions. Second, contagious cancers have revolutionized our understanding of cancer in marine organisms. We now know that each individual with cancer poses a threat to all healthy individuals, as they can release cancer cells that can survive in the marine environment for days, waiting to infect another organism. In other words, these cancers behave like infectious diseases, such as the flu or COVID-19. Third, understanding why contagious cancers affect many bivalve species could help scientists to understand what makes a cell become cancerous in the first place.

Impacts On Biodiversity

The discovery of contagious cancers puts a new twist on understanding a disease that kills many people each year. Contagious cancers have also been found in species that live on land, including dogs and Tasmanian devils. In the case of Tasmanian devils, contagious cancer is driving the species to extinction, decreasing Earth’s biodiversity. However, the contagious cancer found in dogs does not kill the animals quickly, so the cancer can survive for years. Studies have found that this contagious dog cancer could be the oldest known cancer. Since it can remain alive after death of the original host, it has been able to survive for thousands of years [9].

In summary, we now know that, although it is rare, cancer can be contagious in some cases. Will we find more contagious cancers in nature? This is a question that researchers will try hard to answer in the years ahead.

Glossary

Mutation: ↑ Any change in the genetic material of an organism, specifically a modification of its DNA sequence.

Contagious: ↑ Something can be passed from one person to another, like a cold or a flu when you’re close to someone who’s sick.

Bivalves: ↑ Aquatic organisms with bodies that are enclosed by two “valves”, which are commonly known as shells. Examples include clams and mussels.

Haemolymph: ↑ Fluid that circulates within the bodies of some invertebrates such as insects, arachnids, and mollusks. Its function is similar to blood.

Mitosis: ↑ The process by which a single cell divides into two cells, which involves replicating and splitting up the DNA.

DNA: ↑ Deoxyribonucleic acid, the molecule that contains the genetic information of cells.

Biodiversity: ↑ It refers to the rich variety of life on Earth, including different species of plants, animals, and living things in a place.

Conflict of Interest

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Acknowledgments

SD has received funding from the European Union’s Horizon 2021 research and innovation programme (HORIZON-MSCA-2021-PF-01) under the Marie Skłodowska-Curie grant agreement No. 101066116.