Studying sea slugs in the group Sacoglossa can mean being on the receiving end of some very imaginative emails. Sidney K. Pierce, of the University of South Florida, retired a few years ago. “But to this day,” he told me, “I get questions from little kids in their science classes” who have stumbled upon the marvelous mollusks—and want to know if they could help “end world hunger.”
The answer, Pierce assured me, is no. But the proposal isn’t totally outlandish. Several sacoglossan sea slugs can harvest energy from the sun’s rays and, using only the contents of their cells, turn it into chemical packages of food. In other words, they photosynthesize—arguably the plantiest thing that earthly plants and algae do.
Except sea slugs are, of course, not plants or algae. They’re standard-issue animals that have blurred the boundaries between biological kingdoms, thanks to a spectacular act of thievery: They steal photosynthesizing machinery—in-cell structures called chloroplasts—from the algae they eat, and store the green, light-converting blobs in their body for extended periods. Some species can reap the nutritional benefits of these self-replenishing snack packs for months, perhaps for longer than a year. One sea slug that Pierce has studied extensively, Elysia chlorotica, can go the rest of its life without eating—moseying, mating, vibing—after just one algae-rich binge in its youth. “We collect them in the field,” he told me, “and we never feed them again.”
Humans do not do this; as far as we know, our bodies aren’t set up to carry out these grand heists. But oh, that we could. “What would it be like, if we just ate salad for a week, and then you don’t need to worry about where you’re getting your nutrients ever again?” Karen Pelletreau, of the University of Maine, told me. The slugs’ felonious feat, known as kleptoplasty, is so remarkable that it’s been held up by creationists as proof of intelligent design. (It is, to be clear, evolution.) And researchers still aren’t sure how the plant-pantomiming animals pull it all off.
“Spawning is a huge reproductive investment,” Sónia Cruz, also of the University of Aveiro, and another author on the new E. timida study, told me. “It takes a lot of energy out of them.” Each slug has to manufacture hundreds of eggs, each packed with enough nutrients to sustain its offspring during early development. The chloroplasts appear to offer an energetic boon, in some cases doubling the slugs’ output.
Anna Karnkowska, an evolutionary biologist at the University of Warsaw, in Poland, told me that lessons could be learned from the other members of the chloroplast-stealing club, most of which are single-celled creatures such as dinoflagellates (though at least a couple of marine worms seem to briefly hijack the structures too). These unicellular pirates are thought to have an especially intense relationship with their chloroplasts; for them, kleptoplasty might be an intermediate step toward fixing the structures permanently into their cells and making them heritable from generation to generation.
Sea slugs, with their multicelled anatomy and complex lifestyle, would have a much harder time passing pilfered chloroplasts down. As far as scientists can tell, what the slugs accomplish is akin to black-market organ theft, but little more: When the animal dies, its chloroplast cargo dies with it. But even if the chloroplasts’ tenancy is a dead end, it’s a fascinating push to rethink the strange and category-defying ways in which organisms interact, Karnkowska said. The slugs offer the chloroplasts a home, and get to, for a while, masquerade as pseudo-plants; the chloroplasts, in turn, become the sole survivors of carnage, enduring where the rest of their algal comrades could not.