Giant clams are keystone species of coral reef ecosystems. Like coral, giant clams harbor a vibrant community of algae and bacteria. But unlike coral, little is known about how these microorganisms help giant clams thrive or cause them to perish. To find out, authors of a new study placed giant clams in aquariums with one or two species of coral. DNA profiling revealed that the clams were home to three distinct varieties of microbial communities, or “microbiotypes”. Interestingly, these microbiotypes weren’t altered by changes in water temperature or by the type of coral species placed near clams. But dying clams did show one trait in common—an overwhelming presence of bacteria from the vibrionaceae family, which in humans are usually associated with infection from eating undercooked seafood. What’s more, clams died most frequently around coral of the species Acropora cytherea. That suggests that Acropora cytherea could make giant clams susceptible to infection by vibrionaceae bacteria. While the mechanism of this crippling action remains unknown. the study’s findings could help researchers understand how best to protect marine creatures and their environment.