These soft-shell clams (Mya arenaria) from a seafood market in New York are sitting on the lab bench while their hemolymph is checked for the presence of neoplastic cells / Michael J. Metzger

Populations of soft-shelled clams along the east coast of North America are being devastated by outbreaks of a fatal leukemia-like disease. It was first detected in the 1970s, but no one knew what was causing it. According to a new study published in Cell this week, the cancerous tumor cells are being passed from one clam to another. That is, the cancer is contagious, and it all started with just one ill-fated filter feeder.

Until now, we only know of two examples of transmissible cancer in the wild: a venereal tumor transmitted by sexual contact between dogs and the Tasmanian devil facial tumor disease that’s transmitted through biting. Most cancers arise as a result of mutations that accumulate in cells over a lifetime, and while cancers can be caused by contagious things (like viruses), tumors are generally not contagious.

The stricken clams suffer from an overgrowth of abnormally shaped immune cells circulating in the hemolymph (their version of blood). This even turns the clear circulatory fluid into a milky-white, The Scientist explains. They die within weeks or months, though they’re apparently not a health risk for us. Previously, a team led by Stephen Goff of Columbia University discovered a sequence of DNA that’s found at crazy high levels in cancerous clam cells—they named it Steamer. While normal clam cells contain two to five copies of Steamer, leukemic clam shells can carry 150 copies.

In this new study, Goff, Columbia’s Michael Metzger, and colleagues collected soft-shell clams (Mya arenaria) in New York, Maine, and Prince Edward Island in Canada. (Sometimes the clams came from seafood markets, pictured.) Then they analyzed the genomes of cancerous cells isolated from clams living at these different locations and found that they were almost identical to each another at the genetic level: They were clones.

"The evidence indicates that the tumor cells themselves are contagious—that the cells can spread from one animal to another in the ocean," Goff explains in a news release. "We know this must be true because the genotypes of the tumor cells do not match those of the host animals that acquire the disease, but instead all derive from a single lineage of tumor cells."

That means the cancer that’s killing all these clams can be traced to just one incidence of disease that originated in a single clam somewhere and has persisted ever since. “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 spreading over huge geographical distances," Goff adds.

He calls it “a super metastasis, spreading to whole new animals,” according to Science.

The cancerous cells multiply, break free, and survive in seawater long enough to find their way to new clam hosts. For now, the team aren’t sure when the cancer first arose or what role (if any) Steamer plays in that origin. The findings do indicate, however, that the “horizontal transmission” of cancer is more common than we thought.

Images: Michael J. Metzger