A long time ago, though we don’t know how long, a bay mussel somewhere in the Northern Hemisphere developed a leukemia-like cancer. It began as a mutation in a single rogue cell, which copied itself over and over and over again, and spread through the mussel’s hemolymph, the blood-like fluid in its body.

But then the cancer did something it was not supposed to do, or so we thought: It somehow spread to other mussels through the water. Cloning itself again and again in these new hosts, the malignancy continued to proliferate and infect new individuals.

Even stranger still, the disease didn’t remain in bay mussels alone: It’s now been found in two other shellfish species on opposite ends of