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An investigation of the composition and hierarchical order in the skeletal elements of echinoderms

G. Subramanyam, R. Lakshminarayanan, S. Jegadesan and S. Valiyaveettil
National University of Singapore, SG

Keywords:
echinoderms, ossicles, high magnesium calcite, nano ridges

Abstract:
Biominerals occur in multifarious shapes and patterns and are adapted to perform a range of functions. Most of them possess a definitive inherent hierarchical order and pattern that starts from the atomic /molecular levels ranging through the nanoscales, microscales and extending to macroscales. The fabrication of the biomineral is assisted by specially designed biomacromolecules which may be proteins, proteoglycans or polypeptides. In our present study, we were interested in understanding the construction of the echinoderm skeletons. Echinoderms possess a discontinuous endoskeleton made of numerous small calcified plates (ossicles) and spines, linked by connective tissue providing mechanical support. We have chosen representative members from three classes for our investigations - the asteroid, (a seastar, Archaster typicus), the echinoid, (a sand dollar), the holothuroid, a sea cucmber, Holothuria leucospilota. We began our investigations by examining the surface morphology of the specimens using SEM and AFM. . The skeletons studied show a well-ordered and organized arrangement at the micron and submicron levels. At the microscale, the ossicles show a porous bicontinuous network of the mineral. Each micron-sized cavity on the ossicle is surrounded by nano-ridges decorating the exterior. The nano-plates (around 5-10 nm in diameter) are arranged in an iso-oriented fashion parallel to each other. The mineral compositions of the skeletal elements were investigated using FTIR, XRD and elemental analysis. The intramineral biomacromolecules were isolated from the 3 specimens under investigation and characterized using infrared spectroscopy and amino acid composition analysis. The echinoderm skeletons are found to be high-magnesium calcite possessing a small percentage of glycine-rich biomacromolecules occluded within the mineral phase. These biomacromolecules are comparable amidst the different classes of the phyla and exhibit strong similarities in terms of their amino acid composition.

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