Demosponge
Encyclopedia
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.
There are many diverse orders
in this class, including all of the large sponges. Most are marine dwellers, but several live in freshwater environments. Some species are brightly colored, and there is great variety in body shape; the largest species are over 1 metres (3.3 ft) across. They reproduce both sex
ually and asexually
.
s appearing in Precambrian
deposits at the end of the Cryogenian
"Snowball Earth" period, where their presence has been detected by fossilized steroids, called sterane
s, hydrocarbon markers that are characteristic of the cell membranes of the sponges, rather than from direct fossils of the sponges themselves. They represent a continuous 100-Myr-long chemical fossil record of demosponges through the end of the Neoproterozoic
. The earliest sponge-bearing reef
s date to the Early Cambrian, exemplified by a small bioherm constructed by archaeocyathids and calcified microbes at the start of the Tommotian stage (about 540–535 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
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:
However, molecular evidence suggests that the Homoscleromorpha may not belong in this class and that other classifications may need to be revised.
more reminiscent of that of the Calcarea than that of the rest of the Demospongia. 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 larval 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 is 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 Ceractinomorpha.
A molecular study of the mitochondrial genome suggests that five major clades exist in the Demospongiae. These clades are
The branching order appears to be ( Homoscleromorpha, ( Keratosa, Myxospongiae )( Haplosclerida [marine species], Remainder of the demosponges) )
and forms a parenchymula 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.
gives the sponge its softness and absorbency.
Class (biology)
In biological classification, class is* a taxonomic rank. Other well-known ranks are life, domain, kingdom, phylum, order, family, genus, and species, with class fitting between phylum and order...
in the phylum
Phylum
In biology, a phylum The term was coined by Georges Cuvier from Greek φῦλον phylon, "race, stock," related to φυλή phyle, "tribe, clan." is a taxonomic rank below kingdom and above class. "Phylum" is equivalent to the botanical term division....
Porifera. Their "skeletons" are made of spicules consisting of fibers of the protein spongin
Spongin
Spongin, a modified type of collagen protein, forms the fibrous skeleton of most organisms among the phylum Porifera, the sponges. They are secreted by sponge cells known as spongocytes...
, 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.
There are many diverse orders
Order (biology)
In scientific classification used in biology, the order is# a taxonomic rank used in the classification of organisms. Other well-known ranks are life, domain, kingdom, phylum, class, family, genus, and species, with order fitting in between class and family...
in this class, including all of the large sponges. Most are marine dwellers, but several live in freshwater environments. Some species are brightly colored, and there is great variety in body shape; the largest species are over 1 metres (3.3 ft) across. They reproduce both sex
Sex
In biology, sex is a process of combining and mixing genetic traits, often resulting in the specialization of organisms into a male or female variety . Sexual reproduction involves combining specialized cells to form offspring that inherit traits from both parents...
ually and asexually
Asexual reproduction
Asexual reproduction is a mode of reproduction by which offspring arise from a single parent, and inherit the genes of that parent only, it is reproduction which does not involve meiosis, ploidy reduction, or fertilization. A more stringent definition is agamogenesis which is reproduction without...
.
Classification
The Demospongia have an ancient history with the first demosponge fossilFossil
Fossils are the preserved remains or traces of animals , plants, and other organisms from the remote past...
s appearing in Precambrian
Precambrian
The Precambrian is the name which describes the large span of time in Earth's history before the current Phanerozoic Eon, and is a Supereon divided into several eons of the geologic time scale...
deposits at the end of the Cryogenian
Cryogenian
The Cryogenian is a geologic period that lasted from . It forms the second geologic period of the Neoproterozoic Era, preceded by the Tonian Period and followed by the Ediacaran...
"Snowball Earth" period, where their presence has been detected by fossilized steroids, called sterane
Sterane
Steranes are a class of 4-cyclic compounds derived from steroids or sterols via diagenetic and catagenetic degradation and saturation. Steranes have an androstane skeleton with a side chain at carbon C-17. The sterane structure constitutes the core of all sterols. Steranes are sometimes used as...
s, hydrocarbon markers that are characteristic of the cell membranes of the sponges, rather than from direct fossils of the sponges themselves. They represent a continuous 100-Myr-long chemical fossil record of demosponges through the end of the Neoproterozoic
Neoproterozoic
The Neoproterozoic Era is the unit of geologic time from 1,000 to 542.0 ± 1.0 million years ago. The terminal Era of the formal Proterozoic Eon , it is further subdivided into the Tonian, Cryogenian, and Ediacaran Periods...
. The earliest sponge-bearing reef
Reef
In nautical terminology, a reef is a rock, sandbar, or other feature lying beneath the surface of the water ....
s date to the Early Cambrian, exemplified by a small bioherm constructed by archaeocyathids and calcified microbes at the start of the Tommotian stage (about 540–535 Ma), found in southeast Siberia. A major radiation occurred in the Lower Cambrian
Cambrian
The Cambrian is the first geological period of the Paleozoic Era, lasting from Mya ; it is succeeded by the Ordovician. Its subdivisions, and indeed its base, are somewhat in flux. The period was established by Adam Sedgwick, who named it after Cambria, the Latin name for Wales, where Britain's...
and further major radiations in the Ordovician
Ordovician
The Ordovician is a geologic period and system, the second of six of the Paleozoic Era, and covers the time between 488.3±1.7 to 443.7±1.5 million years ago . It follows the Cambrian Period and is followed by the Silurian Period...
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 HomoscleromorphaHomoscleromorphaHomoscleromorpha is a subclass of marine demosponges containing a single order, Homosclerophorida and a single family, Plakinidae.-Taxonomy:This class has recently been recognised as the fourth major line of sponges....
Bergquist 1978- Homosclerophorida Dendy 1905
- Subclass Tetractinomorpha
- AstrophoridaAstrophoridaAstrophorida is an order of sea sponges under the class Demospongiae.Some of the members of this order are known to be eaten by hawksbill turtles....
Sollas 1888 - Chondrosida Boury-Esnault & Lopès 1985
- HadromeridaHadromeridaHadromerida is an order of sea sponges belonging to the class Demospongiae.The order includes such species as Cliona celata and some members of the order are a targeted prey for hawksbill turtles....
Topsent 1894 - Lithistida Sollas 1888
- SpirophoridaSpirophoridaSpirophorida is an order of sea sponges belonging to the class Demospongiae.Members of this order are known to be eaten by hawksbill turtles....
Bergquist & Hogg 1969
- Astrophorida
- Subclass Ceractinomorpha Levi 1953
- Agelasida Verrill 1907
- DendroceratidaDendroceratidaIn taxonomy, the Dendroceratida are an order of sponges of the class Demospongiae. They are typically found in shallow coastal and tidal areas of most coasts around the world. They are generally characterized by concentric layers of spongin fibers, and large flagellated chambers that open...
Minchin 1900 - Dictyoceratida Minchin 1900
- HalichondridaHalichondridaHalichondrida is an order of marine demosponges. Demosponges with simple spicules and a generally confused skeletal arrangement are presently grouped into this order although its monophyly is considered suspect and further taxonomic revision can be expected....
Gray 1867 - Halisarcida Bergquist 1996
- HaploscleridaHaploscleridaHaplosclerida is a order of demosponge. This order includes fresh water sponges....
Topsent 1928 - PoeciloscleridaPoeciloscleridaPoecilosclerida is an order of mostly marine demosponges though there are a few freshwater species. It is the most speciose of the orders and contains about 25 recognised families...
Topsent 1928 - Verongida Bergquist 1978
- Verticillitida Termier & Termier 1977
However, molecular evidence suggests that the Homoscleromorpha may not belong in this class and that other classifications may need to be revised.
Systematics
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 Demospongia is the Homoscleromorpha, characterized by the possession of a larvaLarva
A larva is a distinct juvenile form many animals undergo before metamorphosis into adults. Animals with indirect development such as insects, amphibians, or cnidarians typically have a larval phase of their life cycle...
more reminiscent of that of the Calcarea than that of the rest of the Demospongia. 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 larval type and the presence of spongin
Spongin
Spongin, a modified type of collagen protein, forms the fibrous skeleton of most organisms among the phylum Porifera, the sponges. They are secreted by sponge cells known as spongocytes...
. 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 is 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 Ceractinomorpha.
A molecular study of the mitochondrial genome suggests that five major clades exist in the Demospongiae. These clades are
- 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) )
Reproduction
Spermatocytes develop from the transformation of choanocytes and oocytes arise from archeocytes. Repeated cleavage of the zygote egg takes place in the mesohylMesohyl
The mesohyl, formerly known as mesenchyme, is the gelatinous matrix within a sponge. It fills the space between the external pinacoderm and the internal choanoderm...
and forms a parenchymula 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
Gemmules
Gemmules were imagined particles of inheritance proposed byCharles Darwin as part of his Pangenesis theory. This appeared in his book The Variation of Animals and Plants under Domestication, published in 1868, nine years after the publication of his famous book On the Origin of Species.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 sponginSpongin
Spongin, a modified type of collagen protein, forms the fibrous skeleton of most organisms among the phylum Porifera, the sponges. They are secreted by sponge cells known as spongocytes...
gives the sponge its softness and absorbency.