ORIGINAL PAPER Forming a tough shell via an intracellular matrix and cellular junctions in the tail epidermis of Oikopleura dioica (Chordata: Tunicata: Appendicularia) Keisuke Nakashima & Atsuo Nishino & Euichi Hirose Received: 11 March 2011 /Revised: 31 May 2011 /Accepted: 1 June 2011 /Published online: 12 June 2011 # Springer-Verlag 2011 Abstract A postanal tail is a major synapomorphy of the phylum Chordata, which is composed of three subphyla: Vertebrata, Cephalochordata, and Tunicata (Urochordata). Among tunicates, appendicularians are the only group that retains the tail in the adult, and the adult tail functions in locomotion and feeding in combination with a cellulose- based house structure. Given the phylogenetic position of tunicates, the appendicularian adult tail may possess ancestral features of the chordate tail. We assess the ultrastructural development of the tail epidermis of the appendicularian Oikopleura dioica. The epidermis of the larval tail is enclosed by the larval envelope, which is a thin sheet similar to the outer tunic layer of ascidian larvae. The epidermis of the adult tail seems to bear no tunic-like cellulosic integuments, and the tail fin is a simple folding of the epidermis. Every epidermal cell, except for the triangular cells at the edge of the tail fin, has a conspicuous matrix layer of fibrous content in the apical cytoplasm without enclosing membranes. The epidermis of the larval tail does not have a fibrous matrix layer, suggesting the production of the layer during larval development and metamorphosis. Zonulae adhaerentes firmly bind the epidermal cells of the adult tail to one another, and the dense microfilaments lining the cell borders constitute a mechanical support for the cell membranes. The intracellu- lar matrix, cell junctions, and cytoskeletons probably make the tail epidermis a tough, flexible shell supporting the active beating of the oikopleuran adult tail. Keywords Epidermal cell . Fin . Larval envelope . Lifelong tail . Urochordates Introduction The subphylum Tunicata (Urochordata) is one of the three subphyla of the phylum Chordata, and recent molecular phylogenies indicate that the Tunicata is the closest sister group to ours (i.e., subphylum Vertebrata) among the extant taxa (Delsuc et al. 2006; Putnam et al. 2008). All tunicates produce a cellulosic extracellular matrix outside the epidermis; this is one of the most remarkable synapomor- phies of this taxon because cellulose synthesis has never been found in any other metazoans to date. The ability to make cellulose arose once in the tunicate ancestor via horizontal gene transfer from a bacterial lineage (Matthysse et al. 2004; Nakashima et al. 2004; Sagane et al. 2010). Ascidians and thaliaceans (salps, doliolids, and pyrosomes) have a gelatinous, leathery, or cartilaginous matrix called the tunic that covers their epidermis. Cellulosic components in the tunic have been well documented in ascidians (e.g., Van Daele et al. 1992; Helbert et al. 1998; Nakashima et al. 2008), and the presence of cellulose was also demonstrated Communicated by: Sven Thatje K. Nakashima Marine Genomics Unit, Okinawa Institute of Science and Technology Promotion Corporation, 1919-1 Tancha, Onna, Okinawa 904-0412, Japan A. Nishino Department of Biological Sciences, Graduate School of Science, Osaka University, 1-1 Machikaneyama, Toyonaka, Osaka 560-0043, Japan E. Hirose (*) Department of Chemistry, Biology and Marine Science, Faculty of Science, University of the Ryukyus, 1 Senbaru, Nishihara, Okinawa 903-0213, Japan e-mail: euichi@sci.u-ryukyu.ac.jp Naturwissenschaften (2011) 98:661–669 DOI 10.1007/s00114-011-0815-y