2 MODES OF TUNIC CUTICLE FORMATION IN A COLONIAL ASCIDIAN APLIDIUM-YAMAZII, RESPONDING TO WOUNDING

The defensive responses in ascidian tunic were investigated in a colon ial ascidian Aplidium yamazii by means of light- and electron-microsco py with special reference to the cuticle formation (restoration) of th e 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 tun ic shrinkage is probably promoted by the contraction of the cellular n etwork in the tunic. On the other hand, when a part of the tunic was d amaged by the injection of distilled water or 5% NaOH, a cuticular bou ndary appeared in the tunic, separating the damaged and undamaged tuni c. 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 invo lving cuticle formation in A. yamazii. Although the regulatory mechani sm is still uncertain, one or other of the modes should be elicited in response to many damaging stimuli. (C) 1997 Elsevier Science Ltd.