The animal that steals solar power from plants
The emerald green sea slug, Elysia chlorotica, can survive for months without eating. It achieves this by consuming algae and stealing their chloroplasts—the cellular engines of photosynthesis. The slug embeds these stolen chloroplasts into its own digestive cells, turning sunlight directly into usable energy in a process called kleptoplasty.
The Leaf-Like Mollusk of the Salt Marshes
In the shallow salt marshes and tidal pools along the eastern coast of North America, from Nova Scotia down to Florida, lives a small marine mollusk named Elysia chlorotica. At first glance, a mature individual can easily be mistaken for a submerged leaf. Its body is flattened, bright green, and bordered by two wing-like flaps called parapodia that fold over its back. Running along these flaps is a network of branching veins, heightening the plant-like illusion. Yet this organism is a true animal—a sacoglossan sea slug.
The slug is not born green. In its juvenile stage, Elysia chlorotica is a translucent, brownish creature with tiny red or grayish spots. It only develops its vibrant green coloration after it encounters and feeds on a specific species of yellow-green intertidal alga, Vaucheria litorea. Once it ingests this alga, the animal undergoes a profound transformation, both in its physical appearance and in how it fuels its metabolism.
The Mechanics of Kleptoplasty
Animals typically consume food to break down complex organic molecules into simpler components through digestion. Elysia chlorotica takes a fundamentally different path. The slug possesses a specialized feeding apparatus equipped with a single row of radular teeth. It uses these microscopic, blade-like structures to puncture the outer cell walls of the filamentous Vaucheria litorea without obliterating the internal contents. Applying suction, the slug draws out the algal cytoplasm and its organelles intact.
Instead of digesting all the cellular material, the slug's gut sorts the ingested components. While other organelles and nutrients are broken down for immediate energy, the chloroplasts—the cellular structures responsible for photosynthesis—are preserved. The epithelial cells lining the slug's extensively branched digestive diverticula engulf these intact chloroplasts through phagocytosis. The stolen organelles are then nestled directly inside the animal's own digestive cells. This biological phenomenon of capturing and utilizing functional plastids from prey is known as kleptoplasty.