The effects of heating on the behaviour of Oxytricha bifaria (Ciliophora, Hypotrichida)

Experimental populations of the ciliate Oxytricha bifaria grown at 24 degre es C were exposed to progressively higher temperatures in a thermally isotr opic environment. Their behaviour was monitored and an ethogram was drawn f or each step of the warming process (24, 29, and 34 degrees C), as well as for three further periods of time after the temperature was brought back to the initial 24 degrees C. As the temperature increased, (i) the percentage of mobile cells decreased (significantly at 34 degrees C), (ii) their peri ods of immobilisation were more frequent; (iii) their average creeping velo city did not change; (iv) the radius of the arcs and the length of both the linear segments and the arcs decreased; (v) the frequency of the "side-ste pping reaction" increased progressively, and (vi) it was often performed as bursts of reactions. Moreover, (vii) a new behavioural pattern, the "maxim um-rotation reaction," was performed. When the ciliates were brought back t o 24 degrees C, their behavioural parameters, except for general mobility, did not return to normal values during the first 90 min. Linear heating of the experimental populations of O. bifaria induced nonlinear and time-stabl e effects on their behaviour (12 out of 17 parameters actually changed at 3 4 degrees C).