Demosponge

Demosponge
File:Barrel6.jpg
Barrel sponge (Xestospongia testudinaria, Haplosclerida)
Scientific classification
Kingdom:	Animalia
Phylum:	"Porifera"
Class:	Demospongiae Sollas, 1885
Subclasses
Heteroscleromorpha Keratosa Verongimorpha †Takakkawia

The Demospongiae are the largest class in the phylum Porifera. In fact, the demosponges include 81% 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.

Classification and Systematics

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 100-million-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 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 Systema 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. This milestone publication provided an updated comprehensive overview of sponge systematics, 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:

File:Monanchora arbuscula (Red encrusting sponge).jpg

Monanchora arbuscula (Poecilosclerida)

Geodia barretti (Tetractinellida)

• Subclass Homoscleromorpha Bergquist 1978
  • Homosclerophorida Dendy 1905
• Subclass Tetractinomorpha
  • Astrophorida Sollas 1888
  • Chondrosida Boury-Esnault & Lopès 1985
  • Hadromerida Topsent 1894
  • Lithistida Sollas 1888
  • Spirophorida Bergquist & Hogg 1969
• Subclass Ceractinomorpha Lévi 1953
  • Agelasida Verrill 1907
  • Dendroceratida Minchin 1900
  • Dictyoceratida Minchin 1900
  • Halichondrida Gray 1867
  • Halisarcida Bergquist 1996
  • Haplosclerida Topsent 1928
  • Poecilosclerida Topsent 1928
  • Verongida Bergquist 1978
  • Verticillitida Termier & Termier 1977

However, molecular and morphological evidence show that the Homoscleromorpha do not belong in this class. The Homoscleromorpha was therefore officially taken out of the Demospongiae in 2012, and became the fourth class of phylum Porifera.

Morrow & Cárdenas (2015) 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 actually polyphyletic or should be included in other orders, so that Morrow and Cárdenas (2015) officially propose to abandon certain names: these are the Ceractinomorpha, Tetractinomorpha, Halisarcida, Verticillitida, Lithistida, Halichondrida and Hadromerida. Instead, they recommend the use of three subclasses: Verongimorpha, Keratosa and Heteroscleromorpha. 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 and Chondrillida) make a total of 22 orders in the revised classification. These changes are now implemented in the World Porifera Database part of the World Register of Marine Species.

File:Chondrosia reniformis01.jpg

Chondrosia reniformis (Chondrosiida)

Spongia officinalis (Dictyoceratida)

Spongilla lacustris (Spongillida)

• Subclass Heteroscleromorpha Cárdenas, Pérez, Boury-Esnault, 2012
  • order Agelasida Verrill, 1907
  • order Axinellida Lévi, 1953
  • order Biemnida Morrow et al., 2013
  • order Bubarida Morrow & Cárdenas, 2015
  • order Clionaida Morrow & Cárdenas, 2015
  • order Desmacellida Morrow & Cárdenas, 2015
  • order Haplosclerida Topsent, 1928
  • order Merliida Vacelet, 1979
  • order Poecilosclerida Topsent, 1928
  • order Polymastiida Morrow & Cárdenas, 2015
  • order Scopalinida Morrow & Cárdenas, 2015
  • order Sphaerocladina Schrammen, 1924
  • order Spongillida Manconi & Pronzato, 2002
  • order Suberitida Chombard & Boury-Esnault, 1999
  • order Tethyida Morrow & Cárdenas, 2015
  • order Tetractinellida Marshall, 1876
  • order Trachycladida Morrow & Cárdenas, 2015
  • Heteroscleromorpha incertae sedis
• Subclass Verongimorpha Erpenbeck et al., 2012
  • order Chondrillida Redmond et al., 2013
  • order Chondrosiida Boury-Esnault et Lopès, 1985
  • order Verongiida Bergquist, 1978
• Sublclass Keratosa Grant, 1861
  • order Dendroceratida Minchin, 1900
  • order Dictyoceratida Minchin, 1900

Chaetetids

File:Chaetetid Bird Spring Upper Carboniferous Nevada.jpg

Fossil chaetetid from the Bird Spring Formation (Upper Carboniferous) of southern Nevada.

File:Chaetetid Bird Spring cross-section Upper Carboniferous Nevada.jpg

Cross-section of a fossil chaetetid (Bird Spring Formation, Upper Carboniferous, Nevada.

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).

Reproduction

File:Acarnus erithacus.jpg

Red volcano sponge (Acarnus erithacus, Poecilosclerida).

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.

Economic importance

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.

References

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• Barnes, R.S.K. et al. (2001). The Invertebrates: A Synthesis. Oxford: Blackwell Science. ISBN 0-632-04761-5
• Bergquist, P. R. Sponges. Berkeley, CA: University of California Press; 1978.
• Hickman, C. P. Pages 86–103 in Biology of the Invertebrates. Saint Louis, MO: C.V. Mosely Publishing.
• Kozloff, E. N. Pages 74–91 in Invertebrates. Philadelphia, PA: Saunders College Publishing; 1990.
• Kelly-Borges, M. and S. A. Pomponi. 1994. Phylogeny and classification of lithistid sponges (Porifera: Demospongiae): a preliminary assessment using ribosomal DNA sequence comparisons. Molecular Marine Biology and Biotechnology, 3(2): 87–103.
• Reitner, J. and D. Mehl. 1996. Monophyly of the Porifera. Verhandlungen des Naturwissenschaftlichen Vereins in Hamburg. 36: 5–32.
• West, R.R. 2011. Part E, Revised, Volume 4, Chapter 2A: Introduction to the fossil hypercalcified chaetetid-type Porifera (Demospongiae). Treatise Online 20: 1–79.