Based on a chain of conjugated double bonds, carotenoids are highly colored red-orange pigments in plants and fruits with antioxidant function. With few exceptions, animals cannot synthesize carotenoids de novo from basic biological precursors and must obtain carotenoids from their diet. In addition to increasing consumer acceptance by enhancing flesh color, which is not considered here, dietary carotenoids strengthen immunity and improve stress tolerance in finfish and invertebrates. In addition to increasing consumer acceptance by improving flesh color, which is not considered here, carotenoids enhance immunity and improve stress tolerance in finfish and invertebrates, with the immense importance of gut microbiota and epigenetics in the development of beneficial effects only beginning to be understood. From an ecological point of view, we discuss why some invertebrates store elevated levels of carotenoids in their tissues. Using the most commonly supplemented carotenoid, astaxanthin, as an example, we show that the structure and shape of the molecule is important for the development of effects. If the geometric isomers and stereoisomers of the carotenoid preparation used are not identified or recorded, different or conflicting results may be obtained for the same species. Finally, we point out topics that need to be intensified