Halocynthia roretzi is a solitary ascidian, whose body is entirely covered with the tissue called the 'tunic'. While the tunic has cellulose Iβ, chitin sulfate-like polysaccharide, blood vessels, nerve cells and hemocytes, it also has the components contributing to mechanical properties, including α-smooth muscle actin. The previous reports indicated that the tunic of Halocynthia roretzi responded to mechanical stimuli and deformed itself. In this study, the mechanism of responding the mechanical stimuli in the tunic was investigated. When the tunic was just put into the artificial seawater without the mechanical stimuli at 5 °C, an increment in the mass of the tunic, corresponding to that in the water content of the tunic, was observed. Also, the increment per day became higher at the position closer to the siphon, where the seawater flows in and out. When the mechanical stimuli were given to the tunic at the temperature less than 10°C, the mass was decreased for all the positions. In addition, the increment of the mass per day at the siphon was reduced after the mechanical stimuli were given while those at other positions were not. The distribution of the layer at the outermost surface of the tunic, positive for the Hematoxylin-Eosin stain, was varied in each position. The tunic position with larger distribution size and thickness of the layer showed smaller increment in the mass of the tunic. The tunic discharged nitrate and dissolved organic matter, the ratio of which was kept constant whether the mechanical stimuli were given to the tunic or not. The concentrations of nitrate and dissolved organic matter increased not just after giving the mechanical stimuli to the tunic, but 5 days later.
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