PHOTOTAXIS IN THE STIGMA-FORMING CILIATE NASSULA-CITREA

The ciliated protozoan Nassula citrea responds to directional light of 10 klx showing negative phototaxis. The photobehaviour of swimming ce lls at different nutritional stages was analysed using image analysis methods. It was found that the phototactic response was most precise i n slightly starved cells. These are characterized by a well-developed stigma-like structure, which they expose at their anterior left side. The stigma-like spot, not clearly seen in recently fed cells, is forme d by almost 2000 yellow or orange vesicles. A streak of membranelles e xtends from the pigment spot to the cytopharyngeal basket. It is sugge sted that the stigma-like spot acts as a modulator of the incident irr adiance and that the membranelle cilia are involved in phototactic ori entation, e.g. representing the effector.