| Demosponge | |
|---|---|
| Barrel sponge (Xestospongia testudinaria) | |
| Scientific classification | |
| Kingdom: | Animalia |
| Phylum*: | "Porifera" |
| Class: | Demospongiae Sollas, 1885 |
| Subclasses | |
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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.[1] 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 metre (3.3 ft) across.[1] They reproduce both sexually and asexually.
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The Demospongia 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 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.[2] The earliest sponge-bearing reefs date to the Early Cambrian,[3] 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.[4] 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:[1]
However, molecular evidence suggests that the Homoscleromorpha may not belong in this class and that other classifications may need to be revised.[2]
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 larva 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.[5] These clades are
The branching order appears to be ( Homoscleromorpha, ( Keratosa, Myxospongiae )( Haplosclerida [marine species], Remainder of the demosponges) )
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 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.
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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