Phototactic Response of Euglena gracilis to Polarized Light

Abstract

PHOTOTAXIS (translational movement in response to a light stimulus) in the single cell micro-organism Euglena gracilis can be used as a means for studying sensory perception at the molecular level. This process seems to be controlled by a primitive visual system consisting of a photoreceptor, at the base of the flagellum, and a pigmented shading device called an eyespot^1–3. Euglena rotates about its long axis as it swims, and thus in the presence of light from one side the photoreceptor will be periodically shaded by the eyespot. It has been suggested¹ that this shading causes a succession of phobic responses (shock reactions) which act to point the organism towards the light source. Once the organism is properly oriented, a continuous shading of the photoreceptor results and no further phobic response occurs. (It is possible that the inverse is true, that is, that the shock response occurs on shading the photoreceptor. There is, however, some evidence against this type of mechanism, for example, negative phototaxis in eyespotless Euglena and the polarized light effect described in the present work.)