Properties of tunic acid in the ascidian Phallusia nigra (Ascidiidae, Phlebobranchia)

Phallusia nigra, a solitary ascidian, has no epibionts on the surface of tu nic. Moreover, the tunic contains sulfuric acid. The pH inside the tunic wa s about 1 as measured with a needle-tip pH electrode, and the pH at the tun ic surface was 4-5 as measured with a flat-surfaced pH electrode. The surfa ce pH decreased to 2 when the tunic surface was gently wiped with tissue pa per. Thus, the tunic seems to release acid in response to mechanical stimul i. 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 insert ed, 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 th e SO42-/Cl- ratio was 4.63. In histological sections, the vacuolar lumen ap pear to occupy 25% of the total tunic and 75% of the tissue lying just bene ath the surface of the tunic. The relationship between the properties of tu nic acid and anti-predation, anti-fouling, and anti-infection is discussed.