The family of sea slugs known as Sacoglossans are actually - unlike any other animal on Earth - capable of producing their own food through photosynthesis.
Juveniles of the species are born as herbivores, consuming plant matter as food to survive. As they age, however, the sea slugs’ digestive tract begins to break down only most of the plant matter, while leaving the chloroplasts - the part of the plant that allows it to produce its own food via the process of photosynthesis - intact. Eventually, the intact chloroplasts become incorporated into the body of the sea slug, where its branched digestive tract allows the animal to spread them throughout its body evenly, permitting the animal to produce its own food using only sunlight.
After this process is complete, the sea slug never has to eat again.
(Source)