Tunic morphology and cellulosic components of pyrosomas, doliolids, and salps (thaliacea, urochordata)

The morphology and cellulosic composition of the tunic was studied in pelagic tunicates (3 pyrosomas, 2 doliolids, and 13 salps). The tunic is transparent and gelatinous, consisting of an electron-dense cuticular layer with a fibrous tunic matrix. The thickness and density of the cuticular layer and of the tunic matrix differ from species to species. In some salps, the cuticular layer has numerous minute protrusions that are structurally identical to those found in several ascidians. Free mesenchymal cells (tunic cells) are distributed in the tunic. Whereas the number of tunic cells in the pyrosomas is similar to that in ascidians, there are many fewer tunic cells in doliolids and salps. These differences may be caused by the different functions of the tunic in each group. The existence of cellulose in the tunic was confirmed using electron diffraction in all of the species studied thus far. Their diffractograms indicate that the cellulose microfibrils consist of nearly pure I{beta} of the allomorph. These results show that tunic morphology and cellulosic composition are similar in ascidians and thaliaceans (pyrosomas, doliolids, and salps). The tunic is considered to be a homologous tissue in these animals, and their most recent common ancestor would have possessed this tissue.