| Calcareous sponges | |
|---|---|
 | |
| "Calcispongiae" fromErnst Haeckel'sKunstformen der Natur, 1904 | |
Scientific classification | |
| Domain: | Eukaryota |
| Kingdom: | Animalia |
| Phylum: | Porifera |
| Class: | Calcarea Bowerbank, 1864 |
| Subclasses | |
Thecalcareoussponges[2][3] (classCalcarea) are members of the animal phylumPorifera, the cellular sponges. They are characterized byspicules made ofcalcium carbonate, in the form of high-magnesiumcalcite oraragonite. While the spicules in most species are triradiate (with three points in a single plane), some species may possess two- or four-pointed spicules.[4][5] Unlike other sponges, calcareans lack microscleres, tiny spicules which reinforce the flesh. In addition, their spicules develop from the outside-in, mineralizing within a hollow organic sheath.[6]
All sponges in this class are strictly marine, and, while they are distributed worldwide, most are found in shallow tropical waters. Like nearly all other sponges, they are sedentaryfilter feeders.
All threesponge body plans (asconoid,syconoid, andleuconoid) can be found within the class Calcarea. Typically, calcareous sponges are small, measuring less than 10 cm (3.9 in) in height, and drab in colour. However, a few brightly coloured species are also known.
Like theHomoscleromorpha, calcareous sponges are exclusivelyviviparous.[7]
Calcareous sponges vary from radially symmetrical vase-shaped body types to colonies made up of a meshwork of thin tubes, or irregular massive forms. The skeleton has either a mesh or honeycomb structure of interlocking spicules.[3] Some extinct species were hypercalcified, meaning that the spicule-based skeleton is cemented together by solid calcite.[5]
Of the approximately 11,000 living species of Porifera, only around 800 are calcareans. Some older studies applied the nameCalcispongiae to the class, though "Calcarea" is much more common in modern nomenclature.
Calcarean sponges likely first appeared during theCambrian Period. The oldest putative calcarean genus isGravestockia, from the "Atdabanian" (Cambrian Stage 3) of Australia.[4] Calcareans are probably descended from "heteractinid" sponges, which first appeared in the early Cambrian.[6][8] Calcareans reached their greatest diversity during theCretaceous period.
Some molecular analyses suggest the class Calcarea is not exclusively related to other sponges, and should thus be designated as aphylum. This would also render Porifera (the sponge phylum)paraphyletic. Borchiellini et al. (2001) argued that calcareans were more closely related toEumetazoa (non-sponge animals) than to other sponges.[9] A few studies have also supported a sister group relationship between calcareans andCtenophora (comb jellies). Many authors have strongly doubted the hypothesis of sponge paraphyly, arguing that genetic studies have incomplete sampling and are incompatible with the unique anatomical traits shared by living sponges.[10]
Calcarea is divided into two subclasses (Calcinea and Calcaronea) and a number of orders.[4][5] The two subclasses are mainly distinguished by spicule orientation, soft tissue and developmental traits. For example, calcineans develop from aparenchymella (alarva with a solid center and radial symmetry). Calcaroneans, on the other hand, develop from an amphiblastula (a larva with a hollow center and semi-bilateral symmetry).[11][4]
ClassCalcarea
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Thecalc sponges are exclusively marine and mostly live in shallow water.
