Reference: Bio/. Bull 196: 113-120. (February, 1999) Tunic Morphology and Cellulosic Components of Pyrosomas, Doliolids, and Salps (Thaliacea, Urochordata) EUICHI HIROSE 1 *, SATOSHI KIMURA 2 . TAKAO ITOH 2 . AND JUN NISHIKAWA 3 1 Department of Chemistry, Biology and Marine Science, Faculty of Science, University of the Ryukyus, Nishihara, Okinawa 903-0213, Japan: 2 Wood Research Institute, Kyoto University, Uji, Kyoto 611. Japan; and 3 Ocean Research Institute, University of Tokyo, Nakano, Tokyo 164-8639, Japan Abstract. 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 gelat-inous, consisting of an electron-dense cuticular layer with a fibrous tunic matrix. The thickness and density of the cu-ticular 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 1/3 of the allomorph. These results show that tunic morphology and cellulosic composition are similar in ascidians and thali-aceans (pyrosomas, doliolids, and salps). The tunic is con-sidered to be a homologous tissue in these animals, and their most recent common ancestor would have possessed this tissue. Introduction Members of the phylum Chordata are characterized by having a notochord during some stage of development. Urochordata (also called Tunicata) is one of three subphyla Received 8 June 1998; accepted 13 October 1998. * To whom correspondence should be addressed. E-mail: euichi @sci.u-ryukyu. ac.jp in the phylum Chordata. The name Tunicata is derived from the unique integumentary tissue, called the tunic, that en-tirely covers the epidermis. The Urochordata includes three classes; all of the species possess tunic in the classes As-cidiacea and Thaliacea, whereas the presence of tunic is not well documented in the class Appendicularia. The tunic is a peculiar tissue among metazoans because of its cellulosic components (De Leo et al., 1977) and the presence of free-living cells (tunic cells) in the tunic, that is, outside the epidermis. To date, the biology and biochemistry of the tunic have been studied mainly in ascidians, sessile forms of tunicates, but they have not been well investigated in pe-lagic tunicates. In ascidians, many types of tunic cells have been de-scribed, and they are involved in various biological func-tions, such as phagocytosis (De Leo et al., 1981; Hirose et al., 1994), conduction of impulses (Mackie and Singla. 1987), contractility of the tunic (Hirose and Ishii, 1995). bioluminescence (Aoki et al., 1989; Chiba et al., 1998), photosynthetic symbiosis (Hirose et al.. 1996b), and al-lorecognition (Hirose et al.. 1997c). The tunic is overlaid by a cuticular layer that sometimes has a subcuticular layer beneath it. In several ascidian species, the cuticular surface has numerous minute protrusions that are 100 nm high or less. Descriptions of the cuticular fine structures in 116 ascidian species indicate that the presence or absence of cuticular protrusions has phylogenetic significance (cf. Hi-rose et al, 1997b). Little information has been accumulated on the tunic of pelagic tunicates, such as pyrosomas, do-liolids. and salps (reviewed in Welsh, 1984, and Bone, 1998). In this study, we investigated the tunic morphology of thaliaceans, with special attention to the distribution of the tunic cells and the fine structure of the cuticule. 113
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