The slugs, Elysia chlorotica,

A slug... or is it a leaf?
The slugs, Elysia chlorotica, which exist off the coast of the United States, can live for almost a year by surviving off sunlight and CO2 just like plants do. Experiments revealed this sea slug which starts out its life journey primarily eating algae, eventually evolves to survive mainly thru photosynthesis and can live for nine months or more without traditional feeding. This is an example of kleptoplasty or chloroplastic symbiosis.
Connect with Nature! A green sea slug that inhabits the saltwater marshes of Eastern Canada is being hailed as the first animal to feed itself using photosynthesis, the sunlight-derived energy source that sustains plants.

Research led by University of Florida biologist Sidney Pierce shows the emerald marine creature Elysia chlorotica -- which shares North Atlantic coastal habitats with a species of algae called Vaucheria litorea -- has evolved, over countless millennia, to absorb some of the plant's photosynthesis-enabling genes into its own DNA.