The cancer that has killed so many clams traces back to one single incidence of disease.
Stephen Goff, of the Howard Hughes Medical Institute and Columbia University, said: "The evidence indicates that the tumour cells themselves are contagious - that the cells can spread from one animal to another in the ocean.
"We know this must be true because the genotypes of the tumour cells do not match those of the host animals that acquire the disease, but instead all derive from a single lineage of tumour cells."
The cancer originated in an unfortunate clam somewhere and has persisted ever since as those cancerous cells divide, break free, and make their way to other clams.
In early studies of the cancer in clams, Mr Goff and his colleagues found that a particular sequence of DNA was found at incredibly high levels in leukemic versus normal clam cells.
The researchers at first thought that this difference was the result of a genetic amplification process occurring within each individual clam.
However, they later discovered that the genomes of cancer cells collected from clams living at different locations were nearly identical to one another at the genetic level. They were clones.
"We were astonished to realise that the tumours did not arise from the cells of their diseased host animals, but rather from a rogue clonal cell line spreading over huge geographical distances," Mr Goff said.
The results show that the cells can survive in seawater long enough to reach and sicken a new host.
It is not yet known whether the soft-shell tumour can spread to other molluscs, or whether there are mechanisms that recognize the malignant cells as foreign invaders and attack them.
Scientists still do not know when it first arose and how it spreads from one clam to another.
The findings do suggest that transmissible cancers are more common than anyone suspected, as only two other examples of transmissible cancer are known in the wild.
These cancers include the canine transmissible venereal tumour, transmitted by sexual contact, and the Tasmanian devil facial tumour disease, transmitted through biting.
