Astaxanthin is a keto-carotenoid that can be found in most red-colored aquatic animals. Aquatic animals get Astaxanthin primarily from Algae. 

Astaxanthin can also be be produced synthetically via synthesis of Isophorone. 

As a food additive, classified under the E number E161j, has a intense red color, allowing it to be used a natural colorant for seafood and shellfish. It is also a powerful anti-oxidant which can lead to various different health benefits when consumed.