Astaxanthin is a xanthophyll carotenoid (C₄₀H₅₂O₄) naturally occurring in Haematococcus pluvialis microalgae and certain yeasts. It is one of the most potent antioxidants in nature, quenching singlet oxygen and reactive oxygen species. In shrimp aquaculture, dietary astaxanthin enhances shell pigmentation dramatically, improves immune gene expression, boosts reproductive success, and reduces oxidative tissue damage under stress.