The two main compounds having vitamin D activity are vitamin D2, or ergocalciferol, and vitamin D3, or cholecalciferol. Cholecalciferol has more vitamin D activity than ergocalciferol for some animals and is the form found in animal tissues. Cholecalciferol is involved in calcium and phosphorous metabolism. In particular, cholecalciferol aids in the absorption and deposition of calcium into the exoskeleton, absorption of phosphorous, and the conversion of organic phosphorous to inorganic phosphorous. Deficiency symptoms include poor growth, soft exoskeleton and lethargy. Sources of vitamin D include fish liver oils, liver meals and fish meal. Recommended supplementation level of vitamin D in commercial feeds is 5,000 IU/kg (Akiyama et al., 1992).