Smaller than a finger nail, the marine slug Elysia tuca may not seem like a formidable predator. Yet the tiny gastropod has an unwavering predilection for a rather intimidating prey. The slug hunts Halimeda incrassata, a species of seaweed that packs toxic defense compounds and is more stone than flesh—its body is 85% calcium carbonate, the same mineral found in limestone and coral.
Undaunted by the seaweed’s fortifications, the slug tracks it by sniffing out two chemicals produced by the alga, namely 4-hydroxybenzoic acid and halimedatetraacetate, report a team of researchers led by Julia Kubanek and Mark Hay at Georgia Institute of Technology, and Douglas Rasher at the University of Maine (Proc. Natl. Acad. Sci. USA 2015, DOI: 10.1073/pnas.1508133112).
After finding its prey, the slug pierces the seaweed with a sharp saw-like appendage called a radula, and sucks out the seaweed’s cytoplasm, including its chloroplasts, which the slug then uses to make its own energy from sunlight. “It’s a solar-powered slug,” Rasher says, explaining that it gets 60% of its fixed carbon from these stolen photosynthetic organelles. The slug also steals the seaweed’s toxic arsenal of halimedatetraacetate—one of the compounds used by the slug to track the seaweed—for use as its own defense.
Scientists have long studied the chemical-mediated relationships between terrestrial herbivores and their plant quarry. But this is one of the first such relationships reported from a marine environment. The new research “basically blows the notion that marine plant-herbivore interactions differ fundamentally from terrestrial associations out of the water, as it were,” comments May Berenbaum, a chemical ecologist at the University of Illinois, Urbana-Champaign.
For example, when the seaweed is attacked by the slug, the seaweed drops branches occupied by its predator, likely to avoid getting infected and killed by a fungus found on the slug’s radula. Terrestrial plants adopt a similar strategy during assaults from herbivorous insects. The parallel is fascinating, Rasher says, because herbivores and plants on land evolved independently for 400 million years from their marine counterparts, yet developed similar sorts of relationships involving chemical communication.
Henrik Pavia, a chemical ecologist at the University of Gothenburg in Sweden, thinks that chemical ecologists will see many more of these parallels in the future. “Marine chemical ecology has lagged for decades behind terrestrial research,” he says. Berenbaum agrees: “Observing organisms in their natural environment—the ecological dimension of chemical ecology—is orders of magnitude more difficult in marine ecosystems.”