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