Somewhere in the ocean, a tiny sea anemone called Nematostella vectensis is quietly doing something scientists thought only animals with backbones could do. It can tell helpful bacteria from harmful ones. Now, researchers have figured out how.
A team at Heinrich Heine University Düsseldorf and Kiel University in Germany discovered that this sea anemone uses special cells called nematosomes to sort through bacteria. The nematosomes swallow and destroy unfamiliar, potentially dangerous bacteria while leaving the helpful bacteria alone—the ones that naturally live inside the anemone and keep it healthy.
Before this study, scientists believed only vertebrates—animals like fish, birds, and humans with spines—had this selective ability. That kind of careful discrimination was thought to require an adaptive immune system, which builds up memories of germs over an animal's lifetime. The new research, published in the journal Nature Communications, shows that even simple sea anemones, which sit near the base of the animal family tree, can make these distinctions.
Dr. Sebastian Fraune led the study. His team focused on the nematosomes—tiny, moving cell clusters floating inside the anemone. The researchers found these structures act like bouncers at a club, deciding which bacteria get to stay and which ones get kicked out. To prove their point, the scientists used a gene-editing tool called CRISPR/Cas to switch off a gene called cJun. Without this gene working properly, the anemones produced fewer nematosomes and lost their ability to tell friend from foe. The result: the animals' internal microbial community went out of balance, and they became sick more easily.
The implications reach far beyond the ocean. "The ability to identify microorganisms on a selective basis is thus likely to be significantly older than assumed to date," Dr. Fraune said. His colleague Dr. Nida Kaya, the study's lead author, put it another way: invertebrates—the vast group of spineless creatures including insects, worms, and shellfish—already possess sophisticated mechanisms for working with beneficial bacteria and fighting harmful ones.
This discovery could change how scientists understand the evolution of immune systems. If sea anemones have been balancing helpful and harmful microbes for hundreds of millions of years, then animals have had to manage these microscopic relationships far longer than anyone realized. Understanding that history might help researchers develop new approaches to human health, since our own bodies rely heavily on the bacteria living inside us.
The findings also open doors to studying something called trained immunity—how the innate immune system might remember past invaders without antibodies. The nematosomes, with their ability to distinguish between closely related bacterial strains, offer a promising model for figuring out exactly how that cellular memory works.