Most marine organisms make functional biomolecular materials that extend to varying degrees 'beyond their skins'. These materials are very diverse and include shells, spines, frustules, tubes, mucus trails, egg capsules and byssal threads, to mention a few. Because they are devoid of cells, these materials lack the dynamic maintenance afforded intra-organismic tissues and thus are usually assumed to be inherently more durable than their internalized counterparts. Recent advances in nanomechanics and submicron spectroscopic imaging have enabled the characterization of structure-property relationships in a variety of extra-organismic materials and provided important new insights about their adaptive functions and stability. Some structure-property relationships in byssal threads are described to show how available analytical methods can reveal hitherto unappreciated interdependences between these materials and their prevailing chemical, physical and ecological environments.