Pigmentation and acid storage in the tunic: protective functions of the tunic cells in the tropical ascidian Phallusia nigra

The tunic morphology of the tropical ascidian Phallusia nigra was studied w ith regard to its protective functions by means of light and electron micro scopy. The tunic has a smooth surface free of epibionts. Beneath the tunic surface, phagocytic cells containing pigment,granules (pigmentary tunic pha gocytes) are densely distributed and form the pigmentary layer. These pigme nts may protect the body from irradiation. When poorly pigmented specimens growing in a dim location are transplanted to a bright location, pigmentati on is induced and the tunic becomes black. In this process, tunic phagocyte s probably produce pigment granules and migrate to the pigmentary layer. Th e tunic under the pigmentary layer is packed with highly vacuolated cells ( tunic bladder cells). Vital staining with LysoSensor, a fluorescent pH indi cator, demonstrated that the vacuoles contain strong acid. Acidic fluid lea ks out when the tunic is injured. Leaking acid could serve to disinfect the injured area, be an irritant to potential predators, and/or be involved in anti-fouling.