A fatal cancer devastating soft-shell clams along the East Coast originated from a single primordial source, according to a study published Thursday. Leukemic cells from one clam metastasized through hundreds of miles of ocean water, Stephen P. Goff of Columbia University, the study's senior author, said in an interview. The disease eventually kills most of the clams it infects. William Gerwick, a UC San Diego scientist not involved in the study said its researchers amassed "strong evidence" that the cancer cells arose from a single individual source. But the study is not definitive, because researchers didn't do experiments to demonstrate that cells from a leukemic clam could infect healthy clams, said Gerwick, a professor of oceanography and pharmaceutical science at Scripps Institution of Oceanography, UC San Diego.