The ocean contains a vast number of living things, including many, many pathogens — from bacteria that thrive on coral to fungi that infect lobsters. A drop of seawater may hold 10 million viruses.

Recently, a team of scientists revealed a frightening member of this menagerie: free-floating cancer cells that cause contagious tumors in shellfish. Last year, they found one such cancer in a species of clam. On Wednesday, they reported that three more species were plagued with contagious cancers.

The cancers are specific to shellfish and do not appear to pose a danger to humans who eat them. But until now, infectious cancer was considered something of a fluke in the natural world, initially observed only in dogs and Tasmanian devils.

The latest research has made scientists wonder whether infectious tumors are actually more widespread.