Demosponges
| Demosponge | |
|---|---|
 |
|
| Barrel sponge (Xestospongia testudinaria, Haplosclerida) | |
| Scientific classification | |
| Kingdom: | Animalia |
| Phylum: | Porifera |
| Class: |
Demospongiae Sollas, 1885 |
| Subclasses | |
Heteroscleromorpha
Keratosa
Verongimorpha
†Takakkawia
Demospongiae is the most diverse class in the phylum Porifera. In fact, the demosponges include 90% of all species of sponges with nearly 7,000 species worldwide (World Porifera Database). They are predominantly leuconoid in structure. Their "skeletons" are made of spicules consisting of fibers of the protein spongin, the mineral silica, or both. Where spicules of silica are present, they have a different shape from those in the otherwise similar glass sponges.
The many diverse orders in this class include all of the large sponges. Most are marine dwellers, but one order (Spongillida) live in freshwater environments. Some species are brightly colored, with great variety in body shape; the largest species are over 1 m (3.3 ft) across. They reproduce both sexually and asexually.
The Demospongiae have an ancient history. The first demosponges may have appeared during the Precambrian deposits at the end of the Cryogenian "Snowball Earth" period. Their presence has been indirectly detected by fossilized steroids, called steranes, hydrocarbon markers characteristic of the cell membranes of the sponges, rather than from direct fossils of the sponges themselves. They represent a continuous -year-long chemical fossil record of demosponges through the end of the Neoproterozoic. The earliest Demospongiae fossil was discovered in the lower Cambrian (Series 2, Stage 3; approximately 515 Ma) of the Sirius Passet Biota of North Greenland: this single specimen had a spicule assemblage similar to that found in the subclass Heteroscleromorpha. The earliest sponge-bearing reefs date to the Early Cambrian (they are the earliest known reef structure built by animals, exemplified by a small bioherm constructed by archaeocyathids and calcified microbes at the start of the Tommotian stage about 530 Ma, found in southeast Siberia. A major radiation occurred in the Lower Cambrian and further major radiations in the Ordovician possibly from the middle Cambrian. (Finks, 1970)
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