Astaxanthin is the main carotenoid pigment found in aquatic animals. It is also found in some birds, such as flamingoes, quails, and other species. This carotenoid is included in many well-known seafoods such as salmon, trout, red seabream, shrimp, lobster, and fish eggs. Astaxanthin, similar to other carotenoids, cannot be synthesized by animals and must be provided in the diet. Mammals, including humans, lack the ability to synthesize astaxanthin or to convert dietary astaxanthin into vitamin A. Astaxanthin belongs to the xanthophyll class of carotenoids. It is closely related to beta-carotene, lutein, and zeaxanthin, sharing with them many of the general metabolic and physiological functions attributed to carotenoids. In addition, astaxanthin has unique chemical properties based on its molecular structure. The presence of the hydroxyl (OH) and keto (CdO) moieties on each ionone ring explains some of its unique features, namely, the ability to be esterified and a higher antioxidant activity and a more polar nature than other carotenoids. In its free form, astaxanthin is considerably unstable and particularly susceptible to oxidation. Hence it is found in nature either conjugated with proteins (e.g. salmon muscle or lobster exoskeleton) or esterified with one or two fatty acids (monoester and diester forms) which stabilize the molecule. Various astaxanthin isomers have been characterized on the basis of the configuration of the two hydroxyl groups on the molecule. The geometrical and optical isomers of astaxanthin are distributed selectively in different tissues and levels of free astaxanthin in the liver are greater than the corresponding concentration in the plasma, suggesting concentrative uptake by the liver. Astaxanthin, similar to other carotenoids, is a very lipophilic compound and has a low oral bioavailability. This criterion has limited the ability to test this compound in well-defined rodent models of human disease (PMID: 16562856). Astaxanthin is a carotenoid widely used in salmonid and crustacean aquaculture to provide the pink colour characteristic of that species. This application has been well documented for over two decades and is currently the major market driver for the pigment. Additionally, astaxanthin also plays a key role as an intermediary in reproductive processes. Synthetic astaxanthin dominates the world market but recent interest in natural sources of the pigment has increased substantially. Common sources of natural astaxanthin are the green algae Haematococcus pluvialis (the red yeast), Phaffia rhodozyma, as well as crustacean byproducts. Astaxanthin possesses an unusual antioxidant activity which has caused a surge in the nutraceutical market for the encapsulated product. Also, health benefits such as cardiovascular disease prevention, immune system boosting, bioactivity against Helicobacter pylori, and cataract prevention, have been associated with astaxanthin consumption. Research on the health benefits of astaxanthin is very recent and has mostly been performed in vitro or at the pre-clinical level with humans (PMID: 16431409). Astaxanthin is used in fish farming to induce trout flesh colouring.