|Barrel sponge (Xestospongia testudinaria)|
The Demospongiae are the largest class in the phylum Porifera. 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 demosponges include 90% of all species of sponges and are predominantly leuconoid in structure.
The many diverse orders in this class include all of the large sponges. Most are marine dwellers, but several 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, with the first demosponge fossils appearing in Precambrian deposits at the end of the Cryogenian "Snowball Earth" period, where their presence has been 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 100-million-year-long chemical fossil record of demosponges through the end of the Neoproterozoic. 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 540–535 million years ago (Mya), 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
The extant Demospongiae have been organized into 14 orders that encompass 88 families, 500 genera, and more than 8000 described species.
Hooper and van Soest give the following classification of demosponges into orders:
- Subclass Homoscleromorpha Bergquist 1978
- Homosclerophorida Dendy 1905
- Subclass Tetractinomorpha
- Subclass Ceractinomorpha Levi 1953
However, molecular evidence suggests the Homoscleromorpha may not belong in this class and other classifications may need to be revised.
Demosponge systematics is an active area of research, and much is still to be learned. However, some rudimentary outlines can be made. The basal clade of the Demospongiae is the Homoscleromorpha, characterized by the possession of larvae more reminiscent of those of the Calcarea than those of the rest of the Demospongiae. Demosponges other than the Homoscleromorpha are split into two major groups, the Tetractinomorpha and the Ceractinomorpha. These two groups share characters that indicate common descent, such as a distinctive type and the presence of spongin. Currently, the two groups are each characterized by distinctive types of microscleres, though some doubt still remains as to whether the distinctive microsclere types evolved only once in each group. Fossils of each of these groups are known from the Cambrian, suggesting an early radiation of the major clades of demosponges. The Lithistida, a taxonomic grouping into which many of the fossil demosponges fall, is most certainly polyphyletic with members in both the Tetractinomorpha and the
- Homoscleromorpha: order Homosclerophorida
- Keratosa: orders Dendroceratida, Dictyoceratida and Verticillitida
- Myxospongiae: orders Chondrosida, Halisarcida and Verongida
- Haplosclerida (marine species)
- Remainder of the demosponges: orders Agelasida, Astrophorida, Hadromerida, Halichondrida, Poecilosclerida, Spirophorida and Haploscerida (freshwater species)
The branching order appears to be (Homoscleromorpha, (Keratosa, Myxospongiae)(Haplosclerida [marine species], remainder of the demosponges) ).
Chaetetids, more formally called "chaetetid hyper-calcified demosponges" (West, 2011), are common calcareous fossils composed of fused tubules. They were previously classified as extinct corals, bryozoans, algae, stromatoporoids and sclerosponges. The chaetetid skeleton has now been shown to be of polyphyletic origin and with little systematic value. Extant chaetetids are also described. This skeleton is now known from three demosponge orders (Hadromerida, Poecilosclerida, and Agelasida). Fossil chaetetid hyper-calcified demosponges can only be classified with information on their spicule forms and the original mineralogy of their skeletons (West, 2011).
Spermatocytes develop from the transformation of choanocytes and oocytes arise from archeocytes. Repeated cleavage of the zygote egg takes place in the mesohyl and forms a parenchymella larva with a mass of larger internal cells surrounded by small, externally flagellated cells. The resulting swimming larva enters a canal of the central cavity and is expelled with the exhalant current.
Methods of asexual reproduction include both budding and the formation of gemmules. In budding, aggregates of cells differentiate into small sponges that are released superficially or expelled through the oscula. Gemmules are found in the freshwater family Spongellidae. They are produced in the mesohyl as clumps of archeocytes, are surrounded with a hard layer secreted by other amoebocytes. Gemmules are released when the parent body breaks down, and are capable of surviving harsh conditions. In a favorable situation, an opening called the micropyle appears and releases amoebocytes, which differentiate into cells of all the other types.
The most economically important group of demospongians to human are the bath sponges. These are harvested by divers and can also be grown commercially. They are bleached and marketed; the spongin gives the sponge its softness and absorbency.
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- Gordon D, Love et al., "Fossil steroids record the appearance of Demospongiae during the Cryogenian period", Nature, 2009
- Robert Riding and Andrey Yu. Zhuravlev, "Structure and diversity of oldest sponge-microbe reefs: Lower Cambrian, Aldan River, Siberia", Geology 23.7 (July 1995:649-52) doi:10.1130/0091-7613(1995)023<0649:SADOOS>2.3.CO;2
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- ^ C. Borchiellini, C. Chombard, M. Manuel, E. Alivon, J. Vacelet, and N. Boury-Esnault (2004). "Molecular phylogeny of Demospongiae: implications for classification and scenarios of character evolution". Mol. Phylogenet. Evol. 32 (3): 823–37. doi:10.1016/j.ympev.2004.02.021. PMID 15288059.
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