Cold microgradients elicit adaptive behavior in isotropically cooled, inert populations of Oxytricha bifaria (Ciliophora, hypotrichida)

To complete our investigations on the oriented behavioral response of isotr opically cooled, inert populations of Oxytricha bifaria to a warm thermal g radient, their physiological potentialities under cold microgradient condit ions arising at 8.5 degrees C were studied. We monitored the behavior of th e experimental populations, both at the level of the passing cold wave fron t, and afterwards when the thermal gradient stabilized, evaluating (i) thei r distribution in general, (ii) their relative centroids, (iii) the percent age of both backward creeping and immobile ciliates, and (iv) the numerical indices and rates of their creeping tracks. At the arrival of the cold wav e front, the oxytrichas react immediately to the thermal stimulus, creep ba ckwards at very high velocity along uninterrupted linear tracks, and thus m ove away from the cooling source. No specific behavioral response was ever observed in the static microgradient conditions. At 8.5 degrees C, despite their inertness, the ciliates are still able to behave adaptively, reacting immediately and orientatedly, once a directional factor (the cold-repellin g thermal gradient) arises in an isotropic environment. This is similar to their behavior in the symmetric warm attracting thermal gradient.