Many peridinium are photosynthetic and able to convert sunlight into food sources such as carbohydrates. Nonphotosynthetic species may be phagotrophic and digest bacteria or other single celled creatures after first engulfing them. Osmotrophs are the simplest forms and absorb nutrients released from organic matter directly through their membranes. Photosynthetic varieties are frequently studied by biologists that investigate the structure and function of cell nuclei and circadian rhythms. More advanced species of peridinium display plant and animal like characteristics and contain an eyespot or light sensitive area, which is composed of lipid droplets set between layers of membrane. These pigment proteins, termed carotenoids, respond to light and stimulate the minute organism to move toward the source. The membrane surface also contains vesicles called trichoysts, which discharge crystalline rods as a mechanism for self-defense.