Organic & inorganic skeletons
In between the inner and outer “skin” cells (ectopinacocytes and basipinacocytes), the body of a sponge contains a matrix of protein (striated collagen) that forms the background substance, called the mesohyl. It is through this protein matrix that individual sponge cells rove and perform different functions. To give this matrix some structure, most sponges also have an organic and inorganic skeleton as well. This skeleton may consist of spongin fibres (thick strands of smooth collagen deposited in concentric layers that may form very complex meshes), collagen filaments (which are microscopic protein strands often in tangled bundles), and a mineral skeleton composed of spicules (made of silicon dioxide or calcium carbonate). This skeleton gives structure to the water canal system in particular, such that the meshes of fibres or spicule tracts contain the water canals. Many sponges are also able to incorporate foreign particles into their skeletons, like bits of coral, rock or spicules form other sponge in the sediment, confusing the identifier completely.
Sponge skeleton from scanning electron microscopy of Cymbastela stipitata, with all the soft tissue dissolved away.
Sponge skeleton from scanning electron microscopy of Spongia sp., showing spongin fibres composed of collagen protein.
Coscinoderma mathewsi, one of the few commercial bath sponge species native to eastern Australia and showing real commercial potential, based on its fine quality spongin fibre skeleton.
Skeleton of Dysidea sp. seen under scanning electron microscopy after acid treatment to dissolve all the soft tissues, showing the fragments of coral and other foreign detritus incorporated into the collagen sponge fibres.
Sponge skeleton seen under transmitted light microscopy of Clathria (Microciona) sp., showing transparent spicules (megascleres) made of silica dioxide in amongst the soft tissue and foreign detritus such as sand grains.Spicules are secreted by special cells (sclerocytes). These cells extract silica or calcium from the seawater, and deposit these minerals on top of an organic thread (axis) made of collagen protein. This is all done within the cell, and therefore the method of manufacturing sponge spicules is unique (biomineralisation within highly specialized cells). In other animals with spicules, like octocorals for example, their spicules (or sclerites) are produced outside the cells.
Spicules show a tremendous range of geometric shapes across the phylum Porifera, and these geometric structures are used to classify sponges. However, some sponges have neither fibres nor mineral skeletons (including spicules), with only roving cells in the body’s protein matrix, challenging even experts to correctly identify them.
Put very simply, spicules are conveniently classified as megascleres (large spicules) or microscleres (small spicules). Terminology and classification of spicules, however, is far more complex.
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