Environmental factors and chemical agents affecting the growth of the pathogenic marine ciliate Uronema nigricans

The scuticociliate Uronema nigracans is an opportunistically parasitic marine ciliate known to cause disease in some aquacultural environments with epizootics documented from marine larval rearing systems, marine aquaria and in southern bluefin tuna Thunnus macoyii growout enclosures. This study examined growth responses of laboratory cultures of the ciliate and prey bacteria to variations in temperature and salinity, and the efficacy of potential chemotherapeutants for control of U. nigricans infections. Differences in ciliate growth responses were marginal at temperatures of 10 to 25 degrees C and at salinities between 15 and 35 ppt, though 3.5 ppt or less was lethal. Ciliates were found to be sensitive to fluctuations in bacterial densities, which may be a factor in the seasonal occurrence of the ciliate-related disease in tuna. Commonly used chemotherapeutants such as formalin, malachite green and hydrogen peroxide were all effective against the ciliate during in vitro trials.