It’s Catching, If You’re a Clam: Infectious Cancer Spreading in Soft-Shell Clams, Other Mollusks

Scientists have discovered cancer that’s transmissible from mollusk-to-mollusk, including soft-shell clams. (Photograph: Michael Metzger)

It sounds like the plot of a summer horror flick: Malignant cells floating in the sea, ferrying infectious cancer everywhere they go.

The story is all too true, say scientists who’ve made a discovery they call “beyond surprising.”

Outbreaks of leukemia that have devastated populations of soft-shell clams (Mya arenaria) along the east coast of the U.S. and Canada are the result of cancerous tumor cells making their way from one clam to another.

“The evidence indicates that the tumor cells themselves are contagious – that they can spread from one clam to another in the ocean,” says biochemist and immunologist Stephen Goff of Columbia University, co-author, along with Michael Metzger of Columbia, of a paper reporting the results in the journal Cell.

These mussels are one of four species of mollusks affected.
The mussels at Copper Beach in West Vancouver, Canada, are infected with the disease. (Photograph: Annette Muttray)

This week the team reported new findings in the journal Nature. The transmissible cancer has been discovered in three more bivalve species – mussels (Mytilus trossulus) in West Vancouver, Canada; cockles (Cerastoderma edule) in Spain; and golden carpet shell clams (Polititapes aureus), also in Spain.

Mytilus trossulus is the main native intertidal mussel in the northern Pacific. In North America, it’s found from California to Alaska. Cerastoderma edule is widely distributed from Norway to the coast of West Africa; Polititapes aureus is common in the coastal waters of Spain and nearby nations.

The plot thickens: Soft-shell clams…and their relatives

A total of four mollusk species has been diagnosed with transmissible cancer.
A disease first found in soft-shell clams is now confirmed in mussels, cockles and gold carpet shell clams. (Photograph: Michael Metzger)

The range of the soft-shell (Mya arenaria) extends along the eastern North America coastline from Canada to the U.S. Southeast. The species is also found along the U.K. coast, as well as in the North Sea’s Wadden Sea, where it’s the dominant large clam.

Soft-shell clams – also called steamers, longnecks and Ipswich clams – are popular in seafood markets and on restaurant menus.

For those who favor clams on the half shell, the researchers believe that clam leukemia can’t be contracted by eating potentially infected clams, nor by swimming in the sea.

Mya arenaria’s shell is made of calcium carbonate and is thin and easily broken, hence the name soft-shell. The clam lives buried in tidal mudflats, some six to 10 inches under the surface. It extends its paired siphons up through the mud to filter seawater for food. Water often spurts from the siphons, a tip-off for clam diggers.

Cockles near Galicia, Spain, have the disease.
Cockles like these were collected near Galicia, Spain, and tested for the disease. (Photograph: David Iglesias)

Means and opportunity: The disease

Clam diggers likely won’t wipe out a mudflat’s soft-shells, but clam leukemia may. The cancer, it’s believed, originated in one unfortunate mollusk. It’s astounding, Goff says, that a leukemia that has killed countless clams traces to one incidence of the disease.

As the cancer cells divide, break free, and make their way into other clams, leukemia has infected soft-shells along more than 600 miles of coastline. It’s now found from northern Newfoundland to Chesapeake Bay, nearly the soft-shell’s entire range. “The prospects for disease control therefore aren’t very promising,” says Goff.

Only two other transmissible cancers are known in the wild: Canine venereal disease in dogs and Tasmanian devil facial tumor disease, spread when one Tasmanian devil bites another.

Will soft-shell clams and related mollusks go the way of Tasmania’s devils, now listed as Endangered on the IUCN (International Union for Conservation of Nature) Red List of Threatened Species? No one knows.

Golden carpet shell clams near Galicia, Spain
Along with cockles, golden carpet shell clams near Galicia, Spain, have leukemia. (Photograph: David Iglesias)

On-the-loose: From New York to Maine to Prince Edward Island

In their studies of clam cancer, Goff and colleagues found that a particular sequence of DNA, which they appropriately named Steamer, was found at high levels in leukemia-ridden clam cells. While normal soft-shell cells contain only two to five copies of Steamer, cancer cells may have 150 copies.

The researchers at first thought this difference was the result of a genetic amplification process within each individual clam. But when Metzger analyzed the genomes of cancer cells from soft-shells collected in Port Jefferson, New York; St. George, Maine; Larrabee Cove, Maine; and Dunk Estuary, Prince Edward Island, he was astounded. The cancer cells were identical to one another at the genetic level. “They were clones,” says Metzger.

Adds Goff, “We were astonished to realize that the tumors did not arise from the cells of their diseased host animals, but rather from a rogue clonal cell line that had spread over large geographic distances.”

The cells can survive in seawater long enough to reach and infect a new host, the scientists found. They aren’t sure, however, how many mollusk species ultimately might be able to contract the leukemia. But the new findings suggest that transmissible cancers are more common than researchers suspected.

Mussels in West Vancouver, Canada, tested positive for the mollusk leukemia.
Mussels from Copper Beach in West Vancouver, Canada – potentially diseased – on ice. (Photograph: Michael Metzger)

Where’s the trigger?

Biologist Anne Bottger of West Chester University in Pennsylvania believes environmental contaminants may be the sparks that set off mollusk leukemia. She and colleagues studied soft-shell clams in three coastal New England locales: New Bedford Harbor, Massachusetts; Hampton Harbor, New Hampshire; and Ogunquit, Maine.

“Frequencies of terminal clam neoplasia are correlated with chronic environmental contamination,” Bottger and colleagues reported in a 2013 paper in the journal Northeastern Naturalist. “That’s likely involved in disease transmission by compromising their [the clams’] innate immune systems and making them more susceptible to infectious agents.”

Bottger found the most clam leukemia in New Bedford Harbor. Of the three research sites, New Bedford Harbor had the highest levels of contaminants, including PCBs.

Once leukemia is established in a soft-shell population, Bottger discovered, it kills 40 to 100 percent of the clams.

What will happen in other mollusk species?  Ominously, says Goff, “It’s too soon to know.”

For now, the best he or anyone can offer is: Stay tuned for the sequel…

Infected cockles near Galicia, Spain.
Is disease in these cockles near Galicia, Spain, an indicator of the future for still other mollusk species? It’s too soon to know, scientists say. (Photograph: David Iglesias)
Award-winning science journalist and ecologist Cheryl Lyn Dybas, a Fellow of the International League of Conservation Writers, brings a passion for wildlife and conservation to National Geographic, Natural History, National Wildlife, BioScience, Yankee and many other publications, and is a Field Editor at Ocean Geographic. Eye-to-eye with the wild is her favorite place to be.
  • Mike

    It’s Fukushima, which has been in full-blown meltdown since March 2011.

  • heart of sorrow

    incredible, all this “searching for answers” when the
    Fukushima radiation is pouring into the sea, anemones, other species, bony fish, mammals, are affected. How long are we going to listen to this blather?

  • heart of sorrow

    Fukushima radiation has been pouring into the sea for quite some time. Many species are affected. Anemones, fish, mammals. thiis “searching for causes” is worse than drivel.

  • Grant

    Fukushima,,,,but scientists are baffled,,,,unreal .

  • John Frymire

    Fukushima is bad news but doesn’t explain Atlantic incidences of the disease. Probably toxic overload as the article suggests.

  • Don Uber

    Ah, the era of the new C-Food.
    It is not disputed that Fukushima radiation has circled the world’s oceans and air column. Heck, airborne particles of Fukushima Plutonium were found in Vilnius, Lithuania and Cs has been found in kelp/seaweed across the globe. Google it.
    It’s been known since the 1960’s that even very small doses of ingested radioactive particles can cause cancer in humans. It seems possible that the thousands of tons of radioactive isotopes deposited into the sea from Fukushima could affect the species that feeds by filtering micronutrients from sea water. Additionally, one of the main types of cancer seen from ingestion of radiation is leukemia. Some day 1 + 1 will equal 2 for the science community.

  • Old 333

    The ocean still seems healthy here, but all these comments regarding radiation – well, yes. We should be doing more and better testing, and we should have protected our exposed croplands and water reservoirs during the aerial fallout period. Expensive, but cheaper than…infectious cancer.

  • Kevin

    I am pleased to hear people mentioning Fukushima in these posts. The radioactivity has spread throughout the entire world and will continue to do so for thousands of years. The Pacific Ocean is dying…not just because of Fukushima but for multiple reasons.

    When are we going to have an honest discussion in our media about what is happening? And….the Tokyo Olymipcs in 2020?!

    Are you kidding me???

  • Marc the Arcturian

    I also told them all this too,… years ago – Ocean’s dying – Soylent Green is coming… Ahh, they never listen.
    My film told all, should have been on NG:

  • Marc the Arcturian

    Oh yeah, I’m a rocket scientist but I meant THIS film:

About the Blog

Researchers, conservationists, and others share stories, insights and ideas about Our Changing Planet, Wildlife & Wild Spaces, and The Human Journey. More than 50,000 comments have been added to 10,000 posts. Explore the list alongside to dive deeper into some of the most popular categories of the National Geographic Society’s conversation platform Voices.

Opinions are those of the blogger and/or the blogger’s organization, and not necessarily those of the National Geographic Society. Posters of blogs and comments are required to observe National Geographic’s community rules and other terms of service.

Voices director: David Braun (dbraun@ngs.org)

Social Media