Acid Containers and Cellular Networks in the Ascidian Tunic with Special Remarks on Ascidian Phylogeny

Abstract

Tunic is a unique integument that contains cellulosic components and various types of free cells (tunic cells), and this tissue is exclusively found in the subphylum Urochordata (= Tunicata). In order to discuss the ascidian phylogeny, the presence or absence of the two characteristic types of tunic cells (tunic bladder cells and the tunic net cells) are examined in 65 ascidian species covering 11 families out of the 15 recognized ones. The tunic bladder cells are exclusively distributed in the species of Didemnidae, Holozoinae, Diazoninae, and Ascidiidae. The species of Corellidae have hemocytes, giant cells, that are very similar to the bladder cells, but these cells are not distributed in the tunic. The tunic pH of these species is usually acidic, because the tunic bladder cells contain strong acid in the vacuoles. The tunic net cells are found in Polyclinidae, Polycitorinae, and Didemnidae. The tunic rounding assay suggests that the tunic net cells may be involved in the contraction of the tunic at least in the species of Polyclinidae and Polycitorinae. If these types of tunic cells can be considered as synapomorphic characters, the character-state distribution suggests that the suborders Phlebobranchia and Aplousobranchia are not monophyletic groups.