PHOTOMOVEMENTS IN CILIATED PROTOZOA

Ciliates are unicellular, nonphotosynthetic organisms which show a num ber of light-induced responses. Orientation with respect to the direct ion of light, phototaxis, has been demonstrated in some species of cil iates. Most of these species bear conspicuous cell organelles such as subpellicular pigment granules, a colored stigma, a watchglass organel le, or a compound crystalline organelle. Several lines of evidence sug gest that these kinds of organelles are prerequisites for phototactic orientation of the cells. Photoreceptor molecules presumedly mediating the photobehavior of two species have been identified. The ecological advantage of light-induced responses in ciliated protozoa is still de bated. In some cases the organisms may utilize this behavior either to approach their potential prey, to escape their predators, to escape d amaging light, or to meet a mating partner. Several species of ciliate s display inverse phototactic behavior at different stages of their li fe cycle.