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Concept
Sponge spicule
Summary
Spicules are structural elements found in most sponges. The meshing of many spicules serves as the sponge's skeleton and thus it provides structural support and potentially defense against predators.
Sponge spicules are made of calcium carbonate or silica. Large spicules visible to the naked eye are referred to as megascleres, while smaller, microscopic ones are termed microscleres. The composition, size, and shape of spicules are major characters in sponge systematics and taxonomy.
Sponges are a species-rich clade of the earliest-diverging (most basal) animals. They are distributed globally, with diverse ecologies and functions, and a record spanning at least the entire Phanerozoic.
Most sponges produce skeletons formed by spicules, structural elements that develop in a wide variety of sizes and three dimensional shapes. Among the four sub-clades of Porifera, three (Demospongiae, Hexactinellida, and Homoscleromorpha) produce skeletons of amorphous silica and one (Calcarea) of magnesium-calcite. It is these skeletons that are composed of the elements called spicules. The morphologies of spicules are often unique to clade- or even species-level taxa, and this makes them useful in taxonomic assignments.
In 1833, Robert Edmond Grant grouped sponges into a phylum he called Porifera (from the Latin porus meaning "pore" and -fer meaning "bearing"). He described sponges as the simplest of multicellular animals, sessile, marine invertebrates built from soft, spongy (amorphously shaped) material.
Later, the Challenger expedition (1873–1876) discovered deep in the ocean a rich collection of glass sponges (class Hexactinellida), which radically changed this view. These glass sponges were described by Franz Schulze (1840–1921), and came to be regarded as strongly individualised radially symmetric entities representing the phylogenetically oldest class of siliceous sponges. They are eye-catching because of their distinct body plan (see lead image above) which relies on a filigree skeleton constructed using an array of morphologically determined spicules.
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