Researchers have identified a range of chemical cues that govern the lives of foraging insects. Now, for the first time, scientists have located an herbivore's foraging cues in underwater environs.

Specific chemical signals in seaweed, scientists at the University of Maine have found, guide the underwater sea slug's foraging behaviors and defensive strategies.

"Specialized herbivores on land and sea appear to make a living in similar ways," researcher Doug Rasher said in a press release.

In a new paper published in the journal Proceedings of the National Academy of Sciences, Rasher and his colleagues show that the sea slug (Elysia tuca) is drawn to the 4-hydroxybenzoic acid produced by its preferred food source. Once the sea slug locates the seaweed, it uses a saw-like tool to reach in and steal the plant's chloroplasts.

These chloroplasts continue to synthesize the sun's energy once they're inside the slug, granting the sea slug the ability to harvest energy from the sun.

The seaweed (Halimeda incrassata) also produces halimedatetraacetate, a toxin used to discouraged the predation of other larger herbivores. But the slug is immune to the toxin, and instead uses the chemical for its own defense.

"A sea slug seeks out and eats a toxic seaweed," explained marine scientist Bob Steneck. "It uses the seaweed's toxins to deter its predators and obtains energy from the plants' solar-powered chloroplasts. Who says there's no free lunch? The sea slug's lunch is not only free -- it generates metabolic power for them via photosynthesis."

The combination of protective toxins and chloroplasts are unique to these reproductive seaweeds, and they're not easy to find.

"It's really impressive that hordes of Elysia are able to rapidly sniff out and swarm reproductive seaweeds, considering Halimeda reproduction is rare, patchy and only 36 hours long," Rasher said.