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Structure and Composition of the Tunic in the Sea Pineapple Halocynthia Roretzi: A Complex Cellulosic Composite Biomaterial

Biological organisms produce high-performance composite materials, such as bone, wood and insect cuticle, which provide inspiration for the design of novel materials. Ascidians (sea squirts) produce an organic exoskeleton known as a tunic; however, currently, very little is understood about the detailed structure and composition of this cellulosic biocomposite. Here, we investigate the composition and hierarchical structure of the tough tunic from the species <i>Halocynthia roretzi</i>, through a cross-disciplinary approach combining traditional histology, immunohistochemistry, vibrational spectroscopy, X-ray diffraction, and atomic force and electron microscopies. The picture emerging is that the tunic of <i>H. roretz</i>i is a hierarchically-structured composite of cellulose and proteins with several compositionally and structurally distinct zones. At the surface is a thin sclerotized cuticular layer with elevated composition of protein containing halogenated amino acids and cross-linked via dityrosine linkages. The fibrous layer makes up the bulk of the tunic and is comprised primarily of well-ordered crystalline cellulose fibres with a lower protein content. The subcuticular zone directly beneath the surface contains much less organized cellulose fibres. Given current efforts to utilize biorenewable cellulose sources for the sustainable production of bio-inspired composites, these insights establish the tunic of <i>H. roretzi</i> as an exciting new archetype for extracting relevant design principles.