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> This involves a sea slug who eats algae that contain a bacteria that produces a useful set of poisons that protect the snail:
> 
> https://www.sciencedaily.com/releases/2019/06/190627164743.htm
> 
> 
> "The sea slug Elysia rufescens fights predators by wielding toxic chemicals that it acquires from eating algae. A team has discovered that these chemicals are made by bacteria living inside the algae, highlighting a surprising three-way dependence among sea slugs, algae and bacteria. 
> 
> "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 team found that the bacterial species, which they named Candidatus Endobryopsis kahalalidefaciens, produces about 15 or so different toxins, known as kahalalides. These chemicals are known to act as a deterrent to surrounding fish and other marine animals. At least one of the kahalalides has been evaluated as a potential cancer drug because of its potent toxicity.
> 
> "The researchers also discovered that the bacteria have permanently sacrificed their independence for a life of security, as they no longer possess the genes required for survival outside the algae. Instead, about a fifth of the bacteria's genome is directed toward pumping out toxic molecules that stop predators from eating the bacterium's home.
> One predator that can eat the toxins is the slug E. rufescens. The slug stores them, building up a chemical arsenal that is ten times more concentrated than the toxins in the algae.
> 
> "One of the questions the team asked was whether the slug acquires not just the chemicals but also the factory -- the bacteria -- itself. But they found that the slug doesn't retain the ingested bacteria but rather digests them as food, keeping just the chemicals.
> 
> ***
> 
> "The team compared the bacteria to a factory because the organism consumes raw materials in the form of amino acids supplied from the algae and releases a finished product in the form of toxic chemicals.
> 
> "This theme of specialized bacterial symbionts that have evolved to perform one function -- to make defensive molecules for the host in exchange for a protected living space -- appears to be surprisingly common in the marine environment, from algae to tunicates to sponges, Donia said.
> 
> "This is the second such relationship the team has identified. Their previous study, published April 1 in the journal Nature Microbiology, identified a bacterium that lives in symbiosis with marine sponges and produces toxins that protect the sponge from predation.
> 
> "'The weirdest thing is that the sponge has actually evolved a specialized type of cells, which we called 'chemobacteriocytes,' dedicated entirely to housing and maintaining a culture of this bacterium," Donia said. "This is very strange, given the small number of specialized sponge cells in general. Again, the bacterium cannot produce the substrates and cannot live on its own.'"
> 
> Comment: Once again it is difficult to understand how chance evolution  could create this scenario. How did the slug learn  to detoxify the poisons in the first place? And the algae had the same problem when they got together with the bacterium.