The Sea Slug Defying Biological Orthodoxy

Symbiosis may be more important to evolution than scientists once thought.

Excerpt: “The sea slug Elysia chlorotica is a small, unassuming creature that performs a remarkable feat: It eats algae and steals its chloroplasts, then incorporates them into its own body,” the horoscope explained. Years ago I had incorporated this fact into my own view of the world, and it had changed my understanding of the rules of biology.

This particular slug starts life a brownish color with a few red dots. Then it begins to eat from the hairlike strands of the green algae Vaucheria litorea: It uses specialized teeth to puncture the alga’s wall, and then it slurps out its cells like one might slurp bubble tea, each bright-green cellular boba moving up the algal straw.

The next part remains partially unexplained by science. The slug digests the rest of the cell but keeps the chloroplasts—the plant organelles responsible for photosynthesis—and distributes these green orbs through its branched gut. Somehow, the slug is able to run the chloroplasts itself and, after sucking up enough of them, turns a brilliant green. It appears to get all the food it needs for the rest of its life by way of photosynthesis, transforming light, water, and air into sugar, like a leaf.

The horoscope took this all as a metaphor: Something I’d “absorbed from another” is “integrating into your deeper systems,” it advised. “This isn’t theft, but creative borrowing.” And in that single line, the horoscope writer managed to explain symbiosis—not a metaphor at all, but an evolutionary mechanism that may be more prevalent across biology than once thought.

Elysia chlorotica is a bewitching example of symbiosis. It is flat, heart-shaped, and pointed at the tail, and angles itself toward the sun. Its broad surface is grooved by a web of veins, like a leaf’s is. Ignore its goatish head, and you might assume this slug was a leaf, if a particularly gelatinous one.

Sidney Pierce, a marine biologist retired from the University of South Florida, remembers his surprise when a grad student brought a specimen into his office in the Marine Biological Laboratory at Woods Hole, on Cape Cod, more than two decades ago. Photosynthesis requires specialized equipment and chemistry, which animals simply do not have—“yet here was an animal that’s figured out how to do it,” he told me. He spent the next 20-odd years trying to find the mechanism. “Unfortunately, I didn’t get all the way to the end,” he said.