Krill is a collection of invertebrate animals (mostly crustaceans) included in the order Euphausiacea. It is zooplankton, which, along with phytoplankton and other microorganisms, forms PLANKTON. Plankton (including krill) is an essential food source for many marine species such as cetaceans, sharks, bluefish, and aquatic birds.
Krill represents the second level of the marine food chain. Krill oil is a source of essential polyunsaturated fatty acids from the omega-3 family.

Compared to vegetable-derived oils, krill oil mainly contains omega-3 Eicosapentaenoic Acid (EPA) and omega-3 Docosahexaenoic Acid (DHA), which are highly available and biologically active. Krill oil also contains a considerable amount of antioxidants, such as retinol (vitamin A), tocopherols (vitamin E), and astaxanthin (a specific carotenoid, a precursor of vitamin A).