| Demosponge | |
|---|---|
 | |
| Included are the yellow tube sponge,Aplysina fistularis, the purple vase sponge,Niphates digitalis, the red encrusting sponge,Spirastrella coccinea, and the gray rope sponge,Callyspongia sp. | |
Scientific classification | |
| Kingdom: | Animalia |
| Phylum: | Porifera |
| Class: | Demospongiae Sollas, 1885 |
| Subclasses | |
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Demosponges orcommon sponges aresponges of theclassDemospongiae (fromAncient Greek:δῆμος,romanized: dêmos,lit. 'common people' +σπογγιά,spongiá, 'sponge'), the most diverse group in thephylumPorifera which include greater than 90% of allextant sponges with nearly 8,800species worldwide (according to the World Porifera Database).[5] Beingsiliceous sponges, they are predominantlyleuconoid in structure with anendoskeleton made of a meshwork ofspicules consisting of fibers of the proteinspongin, the mineralsilica, or both. Where spicules of silica are present, they have a different shape from those in the otherwise similarglass sponges.[6] Some species, in particular from theAntarctic, obtain the silica for spicule-building from the ingestion ofdiatoms.[7]
The many diverseorders in this class include all of the large sponges. About 311 million years ago, in theLate Carboniferous, the orderSpongillida split from the marine sponges, and is the only sponges to live in freshwater environments.[8] Some species are brightly colored, with great variety in body shape; the largest species are over 1 m (3.3 ft) across.[6] They reproduce both sexually andasexually. They are the only extant organisms thatmethylatesterols at the 26-position, a fact used to identify the presence of demosponges before their first known unambiguous fossils.[9][2]
Because of many species' long life span (500–1,000 years) it is thought that analysis of the aragoniteskeletons of these sponges could extend data regardingocean temperature,salinity, and other variables farther into the past than has been previously possible. Their dense skeletons are deposited in an organized chronological manner, in concentric layers or bands. The layered skeletons look similar toreefcorals. Therefore, demosponges are also calledcoralline sponges.
The Demospongiae have an ancient history. The first demosponges may have appeared during thePrecambrian deposits at the end of theCryogenian "Snowball Earth" period. Their presence has been indirectly detected by fossilized steroids, calledsteranes, hydrocarbon markers characteristic of the cell membranes of the sponges, rather than from direct fossils of the sponges themselves. They represent a continuouschemical fossil record of demosponges through the end of theNeoproterozoic.[2] The earliest Demospongiae fossil was discovered in the lowerCambrian (Series 2, Stage 3; approximately 515 Ma) of theSirius Passet Biota of North Greenland:[10] this single specimen had aspicule assemblage similar to that found in the subclassHeteroscleromorpha. The earliest sponge-bearingreefs date to the Early Cambrian (they are the earliest known reef structure built by animals), exemplified by a small bioherm constructed byarchaeocyathids and calcified microbes at the start of theTommotian stage about 530 Ma, found in southeast Siberia.[11] A major radiation occurred in the LowerCambrian and further major radiations in theOrdovician possibly from the middle Cambrian.[12]
TheSystema Porifera (2002) book (2 volumes) was the result of a collaboration of 45 researchers from 17 countries led by editors J. N. A. Hooper and R. W. M. van Soest.[13] This milestone publication provided an updated comprehensive overview of spongesystematics, the largest revision of this group (from genera, subfamilies, families, suborders, orders and class) since the start of spongiology in the mid-19th century. In this large revision, the extant Demospongiae were organized into 14 orders that encompassed 88 families and 500 genera. Hooper and van Soest (2002) gave the following classification of demosponges into orders:
However, molecular and morphological evidence show that theHomoscleromorpha do not belong in this class. TheHomoscleromorpha was therefore officially taken out of the Demospongiae in 2012, and became the fourth class of phylum Porifera.[14]
Morrow & Cárdenas (2015)[15] propose a revision of the Demospongiae higher taxa classification, essentially based on molecular data of the last ten years. Some demosponge subclasses and orders are actuallypolyphyletic or should be included in other orders, so that Morrow and Cárdenas (2015) officially propose to abandon certain names: these are theCeractinomorpha,Tetractinomorpha,Halisarcida,Verticillitida,Lithistida,Halichondrida andHadromerida. Instead, they recommend the use of three subclasses:Verongimorpha,Keratosa andHeteroscleromorpha. They retain seven (Agelasida,Chondrosiida,Dendroceratida,Dictyoceratida,Haplosclerida,Poecilosclerida,Verongiida) of the 13 orders from Systema Porifera. They recommend to resurrect or upgrade six order names (Axinellida,Merliida,Spongillida,Sphaerocladina,Suberitida,Tetractinellida). Finally, they create seven new orders (Bubarida,Desmacellida,Polymastiida,Scopalinida,Clionaida,Tethyida,Trachycladida). These added to the recently created orders (Biemnida andChondrillida) make a total of 22 orders in the revised classification. These changes are now implemented in the World Porifera Database[16] part of the World Register of Marine Species.
Sclerosponges were first proposed as a class of sponges,Sclerospongiae, in 1970 by Hartman and Goreau.[17] However, it was later found by Vacelet that sclerosponges occur in different classes ofPorifera.[18] That means that sclerosponges are not a closely related (taxonomic) group of sponges and are considered to be a polyphyletic grouping and contained within the Demospongiae. Likebats andbirds that independently developed the ability to fly, different sponges developed the ability to build a calcareous skeleton independently and at different times inEarth's history. Fossil sclerosponges are already known from theCambrian period.[19]
Chaetetids, more formally called "chaetetid hyper-calcified demosponges" (West, 2011), are common calcareousfossils composed of fused tubules. They were previously classified as extinctcorals,bryozoans,algae,stromatoporoids andsclerosponges. 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 ofchoanocytes andoocytes arise fromarcheocytes. Repeated cleavage of the zygote egg takes place in themesohyl and forms aparenchymella larva with a mass of larger internal cells surrounded by small, externallyflagellated 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 ofgemmules. In budding, aggregates of cells differentiate into small sponges that are released superficially or expelled through the oscula. Gemmules are found in the freshwater familySpongillidae. 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 cytological progression ofporiferaoogenesis andspermatogenesis (gametogenesis) shows great similarity to other metazoa.[20] Most of the genes from the classic set ofmeiotic genes conserved ineukaryotes are upregulated in the spongesGeodia hentscheli andGeodia phlegraei including genes for DNArecombination.[20] Since porifera are the earliest divergent animals, these findings indicate that the basic toolkit of meiosis and recombination were present early in eukaryote evolution.[20]
The most economically important group of demospongians to human are thebath sponges. These are harvested by divers and can also be grown commercially. They are bleached and marketed; thespongin gives the sponge its softness.
ExtantPorifera classes | ||
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