Abstract: Salinity is an important environmental factor that affects the growth and reproduction of aquatic organisms. To explore the effects of salinity changes on the marine ciliates, Trachelostyla pediculiformis was domesticated under variable salinities（5, 10, 15, 20, 25, 30, 35, 40, 45, 50, 55, 60, 65, 70, 75）. The purpose is to explore the population dynamics response and individual morphological changes of T. pediculiformis under different salinity. The results showed that the range of salinity adaption of T. pediculiformis was 5~75, and with the increase in salinity, the population growth rate first increased and then decreased. In addition, the population growth rate of T. pediculiformis cultured at 25~45 salinity was significantly higher than those of other salinity groups, and the population growth rate of T. pediculiformis was negative at 5, 10, and 75 salinity. Furthermore, under low salinity（10~20）and high salinity（50~70）, the cell length and width value of T. pediculiformis increased, and the length-width ratio decreased. In conclusion, the change in salinity affected the growth and development of T. pediculiformis. T. pediculiformis was in optimized condition under the cultural environment of salinity 25~45. Low（<25）or high（>45）salinity inhibits the growth of T. pediculiformis, resulting in cell expansion of T. pediculiformis. The essay provides a basis data for the research on the ciliate to the variation of marine salinity.

Key words: salinity, Trachelostyla pediculiformis, population growth, cell morphology