Zooxanthella

Zooxanthellae are tiny, single-celled algae which are autotrophs: like plants, they can make their own energy from sunlight by photosynthesis. Nearly all of them live inside of animals like corals, sea anemones, sea slugs, and giant clams. Since they need light to live, they're always near the outside of their hosts, not too deep for light to penetrate. Organisms like these which live in cooperation (or "symbiosis") with, and inside of, other creatures are called endosymbionts.

You can think of zooxanthellae and coral as having made an evolutionary deal with each other— each organism gives the other something which it lacks and needs to survive. The relationship is a life-long one: new coral is "infected" with zooxanthallae as soon as it is born. The coral provides the soft zooxanthellae with shelter, nutrients, and a good supply of carbon dioxide for photosynthesis, and the zooxanthellae give the corals enough energy (in the form of carbohydrates) to build their massive reefs. They also manufacture some of the amino acids the corals use to build proteins. Over millions of years of co-evolution, the two creatures have become almost entirely dependent upon one other. Zooxanthellae are often ejected from coral in times of stress due to climate change, disease, or other damage, and neither organism survives for long when this happens. You can recognise coral that's lost its endosymbionts because of its colour— it goes a deathly white, in what's known as coral bleaching.