Dr. Metzger and his colleagues suspect that sick shellfish release cancer cells. The cells float along the currents until they are sucked up by healthy animals as they filter seawater for food. In one case, the researchers found, the cancer cells had moved from one species of shellfish into another.

Nicolas Bierne, a geneticist at the University of Montpellier in France who studies blue mussels, realized that this finding perhaps held the solution to a mystery that he had been struggling with.

Analyzing the DNA from blue mussels in France, he and his colleagues had discovered some genetic markers that looked as if they belonged to another species: bay mussels. He might have expected such a result if these species interbred — but bay mussels don’t even live in French waters.

“It was a puzzle for me,” said Dr. Bierne. “Why do I sometimes find what looks like a hybrid, but I never find one of the parent species?”

Cancer might be the answer. Dr. Metzger had found a contagious form of cancer in bay mussels on the Pacific coast of Canada. Perhaps, Dr. Bierne thought, his blue mussels had been attacked by bay mussel cancer cells.

He and his colleagues isolated cancer cells from French blue mussels and analyzed their DNA. It turned out to be profoundly different from the DNA of the healthy cells, more closely related to that of bay mussels.

Meanwhile, a team of South American researchers got in touch with Dr. Metzger with another potential case of contagious cancer. On the coasts of both Argentina and Chile, Nuria Vázquez of the Institute of the Biology of Marine Organisms in Argentina and her colleagues found beds of mussels in which as many as 13 percent of the animals were sick.