Vitamin D (VD) is a steroid hormone, of which vitamin D 3 (VD 3 ) is the main form in fish. Fish obtain VD 3 majorly from diets, and VD 3 has been widely used in aquatic feed as feed additive. As one of the essential nutrients, VD 3 plays an indispensable role in the growth and health of fish. Due to the particularity of the fish living environment, the metabolic and physiological functions of VD 3 in fish have some difference...