It has been previously reported that the tunic of Halocynthia roretzi, mainly composed of cellulose, is actively deformed with mass transfer by the mechanical stimuli.

In this study, how the tunic deforms in response to the mechanical environment was investigated.

The tunic specimen in the artificial seawater was still at 5˚C or underwent the mechanical stimuli at the temperature less than 10˚C. The mass and moisture content of the tunic, the concentrations of nitrate and dissolved organic matter in the artificial seawater used for the tunic, and the histological characteristics were evaluated.

The increase in mass of the tunic became lower as the region was closer to the bottom of Halocynthia roretzi. However, the decrease in mass caused by the mechanical stimuli was not different between the adjacent regions. Also, the tunic of the siphon, the tubular tissue for influx and efflux of the seawater, increased the mass more slowly after the stimuli. The size of the layer covering the outside of the tunic was inversely related to the increment in mass. The change in mass was corresponding to that in water content. The concentrations of nitrate and dissolved organic matter in the artificial seawater were enhanced 5 days after the stimuli while the concentration ratio of dissolved organic matter to nitrate was kept constant.

The water content in the tunic was used for controlling the mass response to the mechanical environment.