The Tunic

The tunic is a distinguishing feature within the tunicates, hence the name. It is a dynamic exoskeleton, which uniquely is never molted but enlarges with zooid growth (Ruppert et al., 2004). It is external to the epidermis but contains free cells, fibres forming a matrix and three histological components of connective tissue, and is covered by a cuticle (Harrison and Ruppert, 1991). It plays many roles such as anchoring adults to the substrate, supporting and protecting underlying tissues, participating in immunological functions and in the elimination of nitrogenous wastes in some species (Harrison and Ruppert, 1991). Often the tunic contains living organisms on the surface or within the matrix (Harrison and Ruppert, 1991), this has not been studied in E. diaphanis. 

The tunics of the Ecteinascidians are generally thick, translucent and gelatinous (Harrison and Ruppert, 1991).They are made up of various proteins and carbohydrates as well as structural fibres composed of a cellulose known as tunicin (Ruppert et al., 2004). These tunicin fibres occur in successive thin sheets, each sheet has its fibres orientated at a different angle and are stacked within the tunic providing toughness and strength (Ruppert et al., 2004). 

The free living cells found within the tunic derive from the hemocoel, during the larval and juvenile stages, leaving the epidermis and colonising the tunic, where they acquire distinctive characteristics (Harrison and Ruppert, 1991). It is thought that these cells contribute to fibre formation and defensive activity (Harrison and Ruppert, 1991).

The tunics of acsidians also contain hemocytes and tunic vessels which develop as tubular outgrowths of the hemal system and body wall (Ruppert et al., 2004). These vessels are unique in that they are lined with epidermis and epidermal basal lamina(Ruppert et al., 2004). Tunic growth is not yet fully understood however, it is thought to be the responsibility of the epidermis and tunic hemocytes to ensure tunic growth is consistent with zooid growth (Ruppert et al., 2004).