In this study, we investigated the changes in the physiological and histological traits ofa sea squirt (Halocynthia roretzi) with the emergence of the soft tunic syndrome induced by thewater temperature control (6, 9, 12, 15, 18, 21, 24 and 27??C). It was observed that the inductionrate of the soft tunic syndrome was highest at 15??C, but lowest at 24??C. Based on the tunic colorcondition and contraction strength, the whole process were classified into 4 stages as S0, S1, S2and S3. Interestingly, there were significant differences in oxygen consumption and filtration ratewere observed during S0-S3. The most distinctive aspects were change of blood cell compositionat stage S3, whereas multi-vacuole cell ratio was decreased by 1/2 and morula cell ratio expandedabout 10 times during S0-S3. Further, change of organ structure started following the syndromesuch as degeneration of epithelial cells, microfilaments, increment in hemocytes and damage inmuscle fiber have been detected in tunic, siphon, branchial sac, body wall musculature andpyloric gland. Briefly, our study results indicated that the normal physiological functions of thesea squirt can be affected due to the soft tunic syndrome induced by water temperature.