Minute protrusions of the cuticle: Fine surface structures of the tunic in ascidians.

The fine structure of the cuticular surface of the tunic was studied by scanning and transmission electron microscopy in 25 species belonging to 9 families of ascidians. The cuticular surface is ornamented with numerous minute protrusions in some species, but not in others. The minute protrusions are usually papillate in shape, and less than ∼0.05 μm high (in some species of the families Polyclinidae and Polycitoridae), or ∼0.1 μm high (in all species of Botryllidae, all colonial species of Styelidae and one solitary species of Pyuridae). In Clavelina miniata (Polycitoridae), the tunic is provided with parallel ridges on the surface of which papillate protrusions are distributed. The minute protrusions of Halocynthia roretzi (Pyuridae) are irregularly shaped, giving an appearance of paving stones. No protrusions are found in the families Didemnidae, Cionidae, Perophoridae, and Ascidiidae, and in one solitary species of Styelidae. At least in some colonial species, the density of the minute protrusions is lower at the edge of the colony than elsewhere. It is very probable that minute protrusions are found shortly after the secretion of tunic, growing larger both in size and in number as the tunic becomes older.