These invading cells "seem to be the most frequent causes of these leukemia-like cancers in mollusks," said study senior author Stephen Goff, a molecular biologist at Columbia University Medical Center in New York. (Usually in cancer, a trigger causes the organism's own cells to multiply uncontrollably.)
For example, the transmissible cancers seen in golden carpet shell clams did not genetically match any members of this species, but instead matched the pullet shell clam, where they likely originated.
"I think the most surprising aspect is the realization that these infectious clones spreading between individual animals in the oceans are indeed very common," Goff said. "It awakens us to the fact that animals living in the sea are really in a shared environment - that viruses, bacteria and even cells are freely circulating between individuals."
RELATED: Second Form of Contagious Cancer Found in Tasmanian Devils
It remains uncertain how shellfish are spreading cancers to each other. Tasmanian devils spread cancers by biting each other on the face, and dogs transmit the diseases through sexual intercourse, but neither method is a possibility with these immobile shellfish. One possibility is that cancer cells may enter seawater via the excrement of their original hosts and then invade the digestive or respiratory tracts of new victims, Goff said.
The scientists noted that there is no evidence that these transmissible cancers they saw would ever spread beyond mollusks.
"I would want to reassure people that there is no reason for concern that these cancers will be transmitted to humans," Goff said. "In the one case of interspecies transmission, the transmission was to a closely related species."
Future research could investigate whether human cancers might ever spread from human to human.
"We expect such events to be very rare and only occurring under special circumstances," Goff said. "These could likely only occur between genetically closely matched pairs of individuals, or to a profoundly immunocompromised recipient."
Future research can also investigate what mutations allow such cancers to spread from one body to another. "We think these mutations will be informative with respect to the mechanisms that human cancers use during metastasis," Goff said.
The scientists detailed their findings online June 22 in the journal Nature.
Original article on Live Science.
Copyright 2016 LiveScience, a Purch company. All rights reserved. This material may not be published, broadcast, rewritten or redistributed.