Clams’ Contagious Cancer

Two facts about cancer are constant across nearly all species on earth: the disease results from mutated cells growing out of control, and it’s not contagious. Or so scientists thought. Although scientists have learned in recent decades of two cancers that are contagious, one in dogs and another in Tasmanian devils, they’re now discovering that multiple cancers can spread like epidemics through shellfish populations, as well.

The first hints of this phenomenon came in the 1970s, when researchers discovered soft-shell clams dying in Chesapeake Bay and coastal waters of Maine. It wasn’t until 2015 that biologists found the cause: free-floating cancer cells that infect healthy clams and cause tumors. A year later, scientists identified three more species vulnerable to an infectious cancer: mussels, cockles, and golden carpet shell clams. Today, scientists know of ten susceptible shellfish species. The drifting mutated cancer cells that infect mussels and cockles are specific to those species alone. But the cancer in golden carpet shell clams actually comes from cells of a different species, the pullet carpet shell, living in the same area, even though researchers did not find any pullet carpet shells with the cancer.

The most recent development came last October, when genetic analysis found the ancestry of these infectious cancers dates back hundreds, maybe thousands, of years. The cancer of each species appears to descend from a single cell, probably an immune cell, that mutated and multiplied and has been picking up new mutations ever since. The original cell of the soft-shell clam cancer is at least 200 years old, while the original cockle cancer cell probably first mutated thousands of years ago. The substantial changes these cancer cells have undergone since then stunned scientists. They’re continuing to investigate just how many shellfish species are vulnerable to these ancient cancers.