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Meet the Costasiella sea slug, an animal that can photosynthesize

The Costasiella sea slug not only looks like a succulent—it acts like one, too. One of the few animals able to photosynthesize, this tiny invertebrate (also known as the leaf slug or leaf sheep) acquires chloroplasts by munching on Avrainvillea, a paddle-shaped seaweed with a velvety texture. It then stores those chloroplasts in its own body, which enables the slug to soak up sunlight and transform it into energy—a process that also gives the mollusk its green color.

A creature that resembles the base of a slug but with many green leaf-like parts growing off of it

GOOD NEIGHBORS These gastropods are usually found close to coral reefs, where they contribute to the local ecosystem by keeping algae populations in check. They often graze together, like a herd of minuscule sheep.

Costasiella Sea Slug

HABITAT Shallow tropical and subtropical coastal waters across Asia

DISCOVERED 1993, near Japan’s island of Kuroshima

SIZE Up to 0.3 inches long

THREATS Climate change and habitat loss EYES Dark and close together; give the animal a cartoonish appearance

RHINOPHORES Two dark antenna-like structures on the head that pick up chemical signals in the water so the slug can find food

BODY Covered in leaflike structures called cerata, which provide a larger surface area for photosynthesis

COLOR Vibrant green, with pink or purple tips and a white face

RADULA A single row of little teeth used for sucking up algae

1 Avrainvillea is ingested
2 Chloroplasts from the algae are absorbed and stored in the cerata
3 The chloroplasts turn sunlight, water, and CO2 into energy

The term for animals embedding chloroplasts from food into their own body is “kleptoplasty.” Other sea slugs, two species of marine flatworms, and yellow-spotted salamander embryos also exhibit this behavior.