LIFE-CYCLE DEPENDENT PHOTOTACTIC ORIENTATION IN OPHRYOGLENA-CATENULA

The histophagous ciliate Ophryoglena catenula, which is characterized by a conspicuous ''watchglass organelle'', shows a pronounced orientat ion with respect to the direction of light. The five stages of the com mon reproduction cycle are all light sensitive: Positive phototaxis wa s regularly found in ''theronts'' exposed to unilateral white light at intermediate fluence rates (e.g. 10 klx), and also in ''trophonts'' a nd ''protomonts'' within 2.5 hours after feeding. Negative phototaxis was observed under identical experimental conditions in ''protomonts'' about 5 h after feeding as well as in ''tomonts'' and ''tomites'' whi ch had. settled down and were mechanically irritated. In contrast to t hese stages individuals of a rarely occurring ''migratory stage'' are insensitive to light. Theronts which were exposed to an attractive che mical stimulus immediately interrupted their oriented movement towards the direction of the light, showing near random orientation or follow ing the chemical gradient. Phototaxis in Ophryoglena is discussed in t erms of its significance for orientation in the natural environment of these ciliates.