Studies on the structure of the brooding organs of two botryllid ascidians, Botryllus delicatus and Botryllus sexiens

Abstract

The formative processes of the brooding organs of two botryllid ascidians, Botryllus delicatus and Botryllus sexiens, were observed. The structure of these organs was also studied in detail histologically. The brooding organs of these two species were formed from the branchial sac, in contrast to the brooding organs of other botryllids, which are formed from the peribranchial wall. The structures of the brooding organs of B. delicatus and B. sexiens were different, but the processes of their formation were almost the same. As the maturation of the oocytes occurred, a part of the epithelium of the branchial sac extended toward the peribranchial epithelium covering the ovary. The extension of the branchial epithelium looked like a bowl and made a surrounding space with the peribranchial epithelium in a peribranchial cavity. The oocyte was ovulated into this space and fertilized there. After ovulation, the extension of the branchial epithelium formed a sac-like brooding organ in combination with the peribranchial wall and held the embryo tightly, like the incubatory pouch of the genus Botrylloides. The unusual characteristics of the brooding organs in these two species lead to reconsideration of phylogenetic relationships and taxonomic criteria in botryllids.