Two modes of tunic cuticle formation in a colonial ascidian Aplidium yamazii, responding to wounding.

The defensive responses in ascidian tunic were investigated in a colonial ascidian Aplidium yamazii by means of light- and electron-microscopy with special reference to the cuticle formation (restoration) of the tunic. When the tunic was wounded by cutting with a razor blade, the tunic around the wounds shrank to close the wound, and electron-dense fibers appeared and covered the exposed surface of the wound. The tunic shrinkage is probably promoted by the contraction of the cellular network in the tunic. On the other hand, when a part of the tunic was damaged by the injection of distilled water or 5% NaOH, a cuticular boundary appeared in the tunic, separating the damaged and undamaged tunic. Before the completion of the boundary formation, tunic cells gather around the boundary precursor that is formed from the electron-dense fibers. Thus, there are two different modes of defensive response involving cuticle formation in A. yamazii. Although the regulatory mechanism is still uncertain, one or other of the modes should be elicited in response to many damaging stimuli.