Many nudibranchs appear to glow brilliantly with a variety of colors. Biophotonics is the study of light based technology in biological systems. These animals aren’t producing the light just reflecting it with specialized cells containing layers of reflecting crystals. Regardless of their color, these crystals are primarily made of mobile stacks of purine crystals. Guanine is a major component of these crystals and can also be found in fish scales. Guanine is widely used in the cosmetic industry to create iridescent effects in makeup, shampoo, and nail polish.