Projet
Broad comparison of echinoderm stereom structure and compositionEchinodermata ("spiny-skinned animals") is an ancient, exclusively marine phylum of uncertain origin. It is one of the most abundant of marine megafauna, characterized by endoskeletons of thousands of ossicles, composed of calcite with a high concentration of magnesium. Phylum Hemichordata is a minor group comprised mainly of acorn worms and are the sister group to Echinodermata together forming the ‘Ambulacraria’. The discovery of echinoderm-like ossicles in the trunk epidermis of ten acorn worm species has led to the hypothesis that echinoderm ossicles are an Ambulacraria plesiomorphy. Acorn worm ossicles are composed of calcium carbonate (calcite, vaterite or aragonite) and like those of echinoderms, develop in an extracellular occluded space bordered by a sheath of sclerocyte cells. The ossicles are microscopic, monotypic, with low to high Mg content, and lack intercalation with other ossicles. The function of acorn worm ossicles is unknown. Like those of echinoderms, some acorn worm ossicles are perforated by microscopic pores called stereom, but the links between them are tenuous because of our limited sampling of echinoderm stereom diversity. Furthermore, most of what we know of echinoderm ossicles is from adult urchins. For my MSc I will do a broad comparison of echinoderm ossicles and characterize the calcium polymorph, Mg content and stereom microfabrics, using scanning electron microscopy (SEM), micro-CT, X-ray microprobe analyses, and Confocal Raman spectroscopy. There are tens of thousands of different echinoderm ossicles, so my focus will be on microscopic ossicles to compare with those of hemichordates.
