Life is essentially about information, how information is perceived, how it is stored, passed on and used by organisms as they live and reproduce. In the world of photosynthetic microorganisms, where virtually all life depends on solar energy, light becomes also a source of information, used to orient microorganisms spatially and to guide their movements or growth. Responses using light as a sensory stimulus for orientation towards areas that best match their individual irradiation requirements are thus a virtually universal behavior among algae. The full exploitation of light information necessitates proper perceiving devices, able to change the small signal represented by the light falling upon them in a larger signal and response of an entirely different physical nature, that is, these devices, termed photoreceptors, must perform perception, transduction, amplification, and transmission.

The processing of a photic stimulus and its transformation into an oriented movement can be considered the “vision” phenomenon of motile algae. True vision involves production of a focused image of the external world, and the optical requirements for an eye probably cannot be satisfied by algae, requiring true multicellularity with cell specialization and division of labor. Still, algal “eyes” have many similarities with the complex vision systems of higher organisms, because they do possess optics, photoreceptors, and signal transduction chain components. The essential elements of these basic visual systems are the shading device(s), for example, the eyespot and the detector, that is, the true photoreceptor(s). When the eyespot is absent its function is performed by the whole algal body.