Long ago – we don't know exactly how long – the Pacific shellfish developed leukemia-type cancer somewhere in the northern hemisphere. It all started with a mutation in a single cell, replicated over and over, spreading through hemolymph, blood-like fluid in the body of an animal.
But then the crab did something it didn't need to do: it somehow spread water through other shells. He re-cloned and infected more and more copies in his new hosts.
But even more strange was that they did not stick to the Pacific mussels. Meanwhile, it also appears in two more shells in completely different places: in the common mussels of France and their sister South American species in Chile and Argentina.
The findings were published in a study in the journal "eLife". This is just the latest in a series of studies showing that transmissible cancers are more widespread than thought, especially in the sea. This new area of research could help to better understand the development of cancer in humans and other animals. But it also brings more light into the largely unexplored life of marine animals.
"The fact that it has jumped in two new ways is quite intriguing and concerning," says Elizabeth Murchison, a Cambridge University researcher on infectious cancers. Not only are shells important for their ecosystem, they are also a popular food in many cultures. There is currently no evidence that consuming shellfish affected by cancer has any impact on human health.
Infectious cancers in the sea and on land
Portable cancers, which do not naturally occur in humans, have been detected in two terrestrial animals for the first time in decades. In 2006, researchers discovered that the cancer of DFTD, which is added to the endangered Australian devil bagfish, can be transmitted through the bite to the conspirator. Because such bites are normal animal behavior, more than 80 percent of individuals are infected with this and other similar cancers in the meantime. The species is threatened with extinction.
Also in 2006, scientists discovered that domestic dogs can spread genital tumors. As with all infectious cancers, the morbid cells in this case are copies, originating from a single cell of a dog that lived about 11,000 years ago.
These findings have fundamentally altered our image of cancer because it was previously assumed that cell mutations were restricted to the individual in whom they occurred. It is already known that various types of viruses can increase the likelihood of cancer, such as human papillomavirus (HPV) and feline leukemia. Still, the realization that individual cancer cells could spread within a population was a shock to science.
In the last decade, researchers have discovered half a dozen other cancers that can infect shellfish. Michael Metzger, lead author of the new study and researcher at the Pacific Northwest Research Institute in Seattle, has identified several of them. One of them occurs in the shellfish population (Mytilus trossulus) in British Columbia.
A few years ago, he began collaborating with French and Argentine laboratories to discover new cancer in the local shell population. Cancer cells fell under the microscope through its unique, rounded shape. What Metzger initially thought were two different types of cancer turned out to be one kind: the disease of the French (Mytilus edulis) and Chilean shells (Mytilus chilensis) was identical. And apparently she went for the Pacific mussels (Mytilus trossulus) because cancer cells still contain a genetic signature of this species.
However, the Pacific shells live only in the northern hemisphere on the coasts of Europe and North America. In areas around the equator, they do not occur, so the disease must spread through the shell and ballast water of ships, explains Metzger.
"It's pretty amazing that this single cancer (…) has spread to the ocean," says Murchison, who was not included in the study. "We should probably think more about how such cancers can be spread through human activity."
The mystery of the spread
Similar transmissible cancers, all of which affect hemolymph and resemble leukemia, have been found in sand shells such as Mya arenaria and in European popcorn such as the common rooster. In addition, the butchers and his colleagues discovered that the cancer, the Golden Carpet (Polititapes aureus), first in a flaky carpet (Venerupis corrugata) happened.
These were the first indications that such cancers could move from one species to another. More recent discoveries are even more unusual because they show that cancer is even rising two new species were transferred.
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Although the mussels in question are closely related and therefore have similar vulnerabilities and vulnerabilities, "we don't know where the border is," Metzger says. Cancer cells probably expand when released into the water. Other mussels absorb them when they filter detritus from water. In addition, nothing is known about their distribution.
For now, the cancer does not appear to be fatal to the animals, although it often infects infected individuals. According to Butchers, the newly discovered shellfish and its Chilean sister species affect about 10 percent of the local population.
"At the moment, we can't say how big the (cancer) threat could be," he says. "At least it doesn't seem to be wiping out any supplies."
However, these are widespread shellfish species. Many of them are of commercial importance and are consumed by humans and numerous animal species. Although safe for humans, scientists fear that the cancer may affect other species. As the study of these diseases is just beginning, they are likely to occur much more often than they currently seem.
"I'm very concerned about ecology," says Jose Tubio of the Spanish Center for Molecular Medicine and Chronic Disease Research. He is researching portable cancers in the marine ecosystem. The Tubios Group is funded by the European Research Council for the identification of new cancers. So far, five new cancers have been discovered in cocktails – but no research has been published yet.
"Probably many shellfish have their own portable cancers," Tubio says. "But we don't yet understand the consequences."
Beata Ujvari is a researcher at the Australian Deakin University in Victoria. According to her, these crustaceans could be another of many threats to marine life. Perhaps the decline in oxygen levels in the ocean and the warming of water due to climate change are exacerbating the situation, as cancer cells prefer this environment.
Deliberate or unintended transport of shells between different marine regions can also introduce new cancers into ecosystems where they can cause devastating damage, Tubio said.
Cancer usually results from a mutation in a single cell of the body. If the mutation is not recognized and destroyed by the immune system, a tumor will emerge from it. But most of the time, no tumor is lethal. Only when the cancer has metastasized and spread through the body does it become a deadly danger.
But in the case of mussels, "it's almost as if the metastases spread beyond one host," Murchison says.
Understanding how cancer cells survive transport (…) could potentially decipher the secret of metastatic cancer cells, "says Ujvari.
"Exploring the underlying mechanisms could contribute to our overall understanding of how cancer manages to escape from the immune system," she says. And the results could potentially create new treatment options for all types of cancer patients – including humans.
The article was originally published in English on NationalGeographic.com.