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# EFFECTS OF Ca++ AND K+ ON MOTILITY AND PHOTOMOTILITY OF THE MARINE CILIATE FABREA SALINA ### Spartaco Puntoni, Roberto Marangoni, Domenico Gioffrè and Giuliano Colombetti Istituto di Biofisica del C.N.R., Via S. Lorenzo 26, 56127 Pisa, Italy ## Abstract Fabrea salina is a motile ciliate marine protozoan belonging to the order Heterotrichida, that shows photomotile reactions. We report here the results of a study on motility and phototaxis of F. salina exposed to different Ca++ and K+ concentrations. Our findings show that concentrations of potassium ions greater than that of the growth medium and concentrations of calcium ions lower than that of the growth medium cause a significant increase in phototactic response, while the cell speed is not significantly affected. A more thorough analysis shows that the absolute concentrations of Ca++ or K+ are not important and that the phototactic responsiveness depends mainly on the ratio of the K+ concentration to the square root of the Ca++ (the so called Ja value), in agreement with the results obtained in other protozoa. Our results are tentatively discussed in terms of altered membrane potential or ion channel conductances.