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The slug Elysia chlorotica has the appearance of a leaf. Because it can photosynthesize like a plant, it can survive for weeks or even months without feeding. The chloroplasts come from the algae that the slugs consume.

The slugs Elysia chlorotica, which exist off the coast of the United States, not only eat algae; they also take up the chloroplasts from algae, which color their skin an emerald green. Algal genes have been integrated and are present in the DNA of these slugs, having been transferred from the nucleus of an algal cell to the nucleus of an animal cell. This is an example of kleptoplasty or chloroplastic symbiosis.

A slug, which can photosynthesize like a plant, can survive without eating for months — A slug, which looks like a leaf, can survive for months without eating because it can photosynthesize like a plant.
PHOTOGRAPH BY PATRICK J. KRUG

Experiments have demonstrated that when Elysia chlorotica is exposed to light and CO₂, it can use photosynthesis to incorporate CO₂ into its organic matter. Elysia chlorotica appears to feed on algae just during the start of its life, after which it draws all of its energy from photosynthesis. Chloroplasts are functioning throughout the rest of the sea slug’s life.

Experiments have revealed that this sea slug, which resembles a little leaf about an inch or two in length, can live for nine months or more without feeding. They can survive by photosynthesizing the stolen algae parts under the sunlight.