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Phallusia nigra, a solitary ascidian, has no epibionts on the surface of tunic. Moreover, the tunic contains sulfuric acid. The pH inside the tunic was about 1 as measured with a needle-tip pH electrode, and the pH at the tunic surface was 4–5 as measured with a flat-surfaced pH electrode. The surface pH decreased to 2 when the tunic surface was gently wiped with tissue paper. Thus, the tunic seems to release acid in response to mechanical stimuli. The time course of the pH recording showed that the acidity is stable at pH 1–2 for more than five minutes after the needle-tip electrode is inserted, so neutralization of tunic acid with seawater would not occur rapidly. The acid is contained in highly vacuolated cells (tunic bladder cells), and most of these cells are located beneath the tunic surface (Stoecker, 1978; Hirose, 1999). The major anion in the tunic acid was sulfuric acid, and the SO42−/Cl− ratio was 4.63. In histological sections, the vacuolar lumen appear to occupy 25% of the total tunic and 75% of the tissue lying just beneath the surface of the tunic. The relationship between the properties of tunic acid and anti-predation, anti-fouling, and anti-infection is discussed.

Properties of Tunic Acid in the Ascidian Phallusia nigra (Ascidiidae, Phlebobranchia)