Delicate yet voracious, the sea slug Elysia rufescens grazes cow-like on bright green tufts of algae, rooting around to find the choicest bits.

But this inch-long marine mollusk gains not only a tasty meal — it also slurps up the algae’s defensive chemicals, which the slug can then deploy against its own predators.

In a new study, a Princeton-led team has discovered that these toxic chemicals originate from a newly identified species of bacteria living inside the algae. The team found that the bacteria have become so dependent on their algal home that they cannot survive on their own. In turn, the bacteria devote at least a fifth of their metabolic efforts to making poisonous molecules for their host.

The intertwined story of these three characters — the sea slug E. rufescens, marine algae of the genus Bryopsis, and the newly identified bacteria — form a three-way symbiotic relationship. A symbiotic relationship is one in which several organisms closely interact. In this example, the slug gets food and defensive chemicals, the algae get chemicals, and the bacteria get a home and free meals for life in the form of nutrients from their algae host.

“It’s a complicated system and a very unique relationship among these three organisms,” said Mohamed Donia, assistant professor of molecular biology at Princeton University and senior author on the study. “The implications are big for our understanding of how bacteria, plants and animals form mechanistic dependencies, where biologically active molecules transcend the original producer and end up reaching and benefitting a network of interacting partners.”