Seaweed and sea slugs rely on toxic bacteria to defend against predators

Plants, animals and even microbes that live on coral reefs have evolved a rich variety of defense strategies to protect themselves from predators. Some have physical defenses like spines and camouflage. Others have specialized behaviors – like a squid expelling ink – that allow them to escape. Soft-bodied or immobile organisms, like sponges, algae and sea squirts, often defend themselves with noxious chemicals that taste bad or are toxic.

Some animals that can’t manufacture their own chemical weapons feed on toxic organisms and steal their chemical defenses, having evolved resistance to them. One animal that does this is a sea slug that lives on the reefs surrounding Hawaii and dines on toxic Bryopsis algae. Marine scientists suspected the toxin is made by a bacterium that lives within the alga but have only just discovered the species responsible and teased apart the complex relationship between slug, seaweed and microbe.

Ultimately, noxious chemicals allow predators and prey to coexist on coral reefs, increasing their diversity. This is important because diverse ecosystems are more stable and resilient. A greater understanding of the drivers of diversity will aid in reef management and conservation.

As marine scientists, we too study chemical defenses in the ocean. Our laboratory group at the Georgia Institute of Technology explores how marine organisms use chemical signaling to solve critical problems of competition, disease, predation and reproduction. That’s why we were particularly excited by the discovery of this new bacterial species.

Origins of a chemical defense

In a report published in the journal Science, researchers at Princeton University and the University of Maryland discovered that a group of well-studied toxic defense chemicals, the kahalalides, are actually produced by a bacterium that lives inside the cells of a particular species of seaweed.

The scientific community had long speculated that a bacterium might be responsible for producing the kahalalides. So the discovery of the kahalalide-producing bacteria – belonging to the class Flavobacteria – has solved a long-standing scientific mystery.

Bryopsis provides the bacteria with a safe environment and the chemical building blocks necessary for life and to manufacture the kahalalides. In return, the bacterium produces the toxins for the algae, which protect them from hungry fish scouring the reefs. But the seaweed isn’t the only organism that benefits from this arrangement.

The kahalalides, originally discovered in the early 1990s, also protect a sea slug, Elysia rufescens, that consumes it. The sea slugs accumulate the toxins from the algae, which then protects them from predators.