Microstructure and mechanical design in the lantern ossicles of the regular sea‐urchin Paracentrotus lividusi A scanning electron microscope study

Abstract

Echinoids are the echinoderm group which has explored and exploited most efficiently the potential of the endoskeleton in a range of extremely advanced and sophisticated adaptive solutions. The most ingenious of these adaptations are employed in the dental apparatus, whose different elements represent a striking example of the versatility of the skeletal tissue. The dental ossicles (jaws, rotulae, compasses and teeth), though having the same basic organization, show a wide range of structural and functional solutions: inorganic phase and organic stroma are variously combined and integrated in a very plastic and adaptable tissue, which is able to fulfil very specific mechanical requirements. On the whole, all the different arrangements shown by the dental elements can be considered as differentiations of the two limit‐models of skeletal microarchitecture, represented respectively by classical porous stereom and a composite lamellar structure. The first structural model is by far the most common: all the lantern ossicles, except the tooth, show a rather complete range of variations on the theme of the three‐dimensional stereom, including either the usual labyrinthic pattern or other types of more specialized stereoms (laminar, galleried, fascicular, microperforate, imperforate), whose presence is closely related to the association with interacting elements (ligaments, muscles, other skeletal parts) and to specific functional requirements. In any case, remarkable differences can be observed easily between the superficial and the internal stereom microstructure, in parallel with as many conspicuous variations in the distribution and the organization of the associated stroma. The second model, on the contrary, can be detected only in the tooth and consists of a unique composite structure (primary and secondary elements), showing a number of specific differentiations in different zones: the primary structure consists of unique mineral elements (lamellar plates and prisms), which form the main framework of the tooth; the secondary structure, on the contrary, consists of stereom plates of more or less modified structure, which cement and reinforce the primary structure. The tooth is a true masterpiece of constructional design, where inorganic and organic components co‐operate to achieve a structure which is remarkably hard and resistant to different types of mechanical stress. The significance of these two different structural solutions and the functional implications of their numerous variations are discussed in the light of current knowledge of lantern mechanics.

Key words:

• Sea‐urchins
• Dental ossicles
• Microstructure ‐Mechanical design