Halimeda incrassata is a type of seaweed, and it’s pretty badass for an algae. It dominates the seagrass ecosystems around the Caribbean, defends itself both chemically and with a tough, calcified covering, and can self-amputate its branches if there’s something crawling around on them and bothering it. This doesn’t do much to deter the sea slug Elysia tuca, though, which prefers to feed on these seaweeds over all others. 

The slug has a few tricks for getting past the seaweed’s defenses. The tough exterior is no problem; the slug simply pierces it with a modified tooth and then sucks out nutrients. In a new study, researchers at the Georgia Institute of Technology showed that the slug also uses one of the seaweed’s chemical defenses against it, and steals another for its own use. 

After breaking down and isolating chemicals extracted from the seaweed, the researchers found that the slug detects two compounds—4-hydroxybenzoic acid (4-HBA) and halimedatetraacetate (HTA)—in the water and follows these to the seaweed. These compounds are part of the weed’s suite of defenses against other herbivores, but the slug isn’t bothered by them and instead uses them to lead them right to their prey. What’s more, they sequester HTA from the seaweed while feeding and then use it themselves, becoming unpalatable to fish that would otherwise eat them. 

The seaweed can still send the slugs packing with self-amputation, of course, and in the field, the researchers found that H. incrassata selectively loses branches inhabited by Elysia.  

It’s a cool set of behaviors—using defensive compounds to track a meal down, sucking nutrients out, and stealing defensive chemicals for yourself—and is previously unknown for marine herbivores. But it doesn’t make the slug all that unique. While its hunting, feeding, and defensive strategies are a novelty in the water, they’re similar to tricks that plant-eating insects have been using on land for a long time. The researchers say it's a neat example of marine and terrestrial animals converging on the same strategies, even after being separated by hundreds of millions of years of evolution in very different habitats.