ABSTRACT
The subphylum Urochordata consists of three major classes namely the benthic Ascidiacea and pelagic Larvacea and Thaliacea, which includes Salpidae and Doliolidae. Being inhabitants of the oligotrophic oceanic surface waters, pelagic tunicates are exposed to relatively wider variations in algal density and temperature. In pelagic tunicates, semelparity imposes one of the two alternate reproductive strategies, either to manipulate generation time or clutch size. The benthic, sessile, suspension-feeding ascidians are more speciose. In colonial tunicates, the formative tissue(s) of a nascent clone is known to arise from different sources: the stoloneal septum in perophorids, atrial epithelium in polyclinids and clavelinids, palleal and vascular system. The thaliacean and colonial ascidian tunicates are metagenics, alternating between sexual and asexual reproduction. Hence, tracing the germ cell lineage through sexual and non-sexual stages in these metagenic tunicates may prove to be a fascinating field of research. The colonials have a source to generate germline cells during the adult stage.
