The Light Eaters: How the Unseen World of Plant Intelligence Offers a New Understanding of Life on Earth

by Zoë Schlanger

Consider the emerald green sea slug. When I first read about it, I couldn’t stop talking about it with anyone who asked what was going on with me lately. The green sea slug, this whimsical thing that seemed to defy all boundaries between plant and animal, was going on with me. It was all I could think about. The slug, which lives in watery places all along the Atlantic coast of the United States, spends its early life a brownish color with a few red dots. It has one goal in those early moments: to locate the hairlike strands of the green algae Vaucheria litorea. When it finds them, it punctures the alga’s wall and begins to slurp out its cells as though through a straw, leaving the clear tube of empty algae behind. The algal cells are bright green, owing to the chlorophyll-filled chloroplasts inside them, which are responsible for photosynthesis. Under a microscope the whole exchange looks uncannily like the slug is drinking bubble tea, one bright green boba entering its mouth at a time. The slug digests the cells but keeps the chloroplasts within them intact, spreading them out through its branched gut. Now the slug itself has turned from brown to a brilliant green. After a few algal bubble teas, the slug never needs to eat again. It begins to photosynthesize. It gets all the energy it needs from the sun, having somehow also acquired the genetic ability to run the chloroplasts, eating light, exactly like a plant. How this is possible is still unknown. Remarkably, the now-...