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Soft-shell clams (Mya arenaria) have been dying in large numbers since the 1970s, from Canada to Maryland, due to a mysterious illness. Recently, scientists revealed the cause: clam leukemia.
Leukemia in clams?
Clams are basic organisms without legs, brains, or faces. However, they possess hearts that circulate hemolymph (their equivalent of blood) throughout their bodies. This hemolymph is where the cancer has established itself.
For decades, researchers believed a virus was responsible for the disease decimating clam populations. However, a team of experts in microbiology, cancer, and marine biology decided to analyze the clams' DNA for answers.
Their discovery raised more questions than it answered. The clams were suffering from cancer—and it was spreading between them.
Most cancers are isolated incidents and cannot be transmitted from one creature to another. Until recently, only two contagious cancers were known globally. One is a sexually transmitted disease in dogs called Canine Transmissible Venereal Tumor. The other is Devil Facial Tumor Disease, which spreads when Tasmanian devils bite each other on the face.
Clams, however, don’t engage in mating behaviors, and since they lack faces, they certainly aren’t biting one another. So how does the cancer spread? Researchers speculate that cancer cells might be released into the water. As filter feeders, clams process liters of water hourly. If cancer cells are present in the water, they could easily infect a new host.
The origin of this contagious clam leukemia remains a mystery. Where did these free-floating cancer cells originate? To investigate, scientists sequenced cancer cells from clams along the East Coast. The results were astonishing: all the cancer cells shared identical genes, indicating they originated from a single clam. This unfortunate clam developed cancer over 40 years ago, and its cancer cells spread into the ocean, infecting others. Over time, the cancer has traveled hundreds of miles, leaving countless clams affected.
Clams might not be the only ones affected. Mussels, oysters, and cockles are also suffering from similarly puzzling diseases. As long as there’s a demand for shellfish, these researchers will remain busy.
Fortunately, clam cancer is exclusive to clams. Consuming seafood and swimming in the ocean remain safe for humans—at least as safe as they’ve always been.
