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.

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

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.

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.

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.

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…