ESCAPE RESPONSE OF EUPLOTES-OCTOCARINATUS TO TURBELLARIAN PREDATORS

Physical contact between the freshwater ciliate Euplotes octocarinatus and its turbellarian predator, Stenostomum sphagnetorum, elicits a be havioural response of the potential prey which is different from the n ormal ''avoiding reaction'' of Euplotes. Because of its defensive char acter it is called ''escape response''. The response was analysed by t he use of a video camera and recorder system. It was found that the es cape response takes a regular course, beginning with sudden backward m oving of Euplotes, followed by a rapid turn around and a subsequent fo rward movement. Velocities during the escape response are 1 to 5 times higher than calculated for control cells. The whole reaction lasts ab out 2 s. Within this time a Euplotes cell moves more than 1,000 mum aw ay from its predator. While 90 % of well-fed Euplotes cells demonstrat ed the defensive response, starved cells cultivated separately from th eir predators revealed fewer escape responses, however, several of the starved cells regained the capacity for the escape response after exp osure to Stenostomum or Stenostomum-conditioned medium for 20 h. It is discussed whether or not the inducible behavioural response is trigge red by the same predator-released substances that are known to induce defensive morphological changes in Euplotes.