Tunic morphogenesis in the sea peach (Halocynthia papillosa, Ascidiacea, rochordata): Cellular events, the initiation of twisted fibrous arrangements and spine formation

Abstract

The adult tunic of the sea peach ( Halocynthia papillosa ) shows a high degree of organisation. Tunic morphogenesis was monitored from the onset of tunic secretion until juveniles reached the age of 3 months. While some characteristics of the adult tunic are still missing, like certain types of intratunical cells and striated bodies, its main features have already developed by this time. Crucial events take place at or soon after the onset of metamorphosis (stage M 0). Cuticular spines cover the external surface of the juvenile. At least two types of intratunical cells enter the tunic and the fibrous material adopts a three-dimensional twisted helicoidal architecture. The initiation of this helicoidal arrangement of fibrils directly after stage M 0 is discussed regarding accompanying developmental events. The existence of cells that penetrate the outer compartment of the tunic at the end of larval life is reported for the first time.