| Siliceous sponges | |
|---|---|
 | |
| Hexactinellid spicule | |
Scientific classification | |
| Domain: | Eukaryota |
| Kingdom: | Animalia |
| Phylum: | Porifera |
| Clade: | Silicispongia Schmidt, 1862 |
| Classes | |
Thesiliceous sponges form a majorclade of the phylumPorifera, consisting of classesDemospongiae (common sponges) andHexactinellida (glass sponges). They are characterized byspicules made out ofsilicon dioxide, unlikecalcareous sponges.
Individual siliachoates (silica skeleton scaffolding) can be arranged tightly within the sponginocyte or crosshatched and fused together. Siliceous spicules come in two sizes called megascleres and microscleres.
Most studies support the monophyly of siliceous sponges.[1]
The group, as a part of the phylum Porifera, has been namedSilicispongia Schmidt, 1862 andSilicea Bowerbank, 1864.[2]Silicarea is a proposed new phylum based on molecular studies of thephylumPorifera.[citation needed] It consists of the Poriferan classesDemospongiae andHexactinellida. Some scientists believe that Porifera ispolyphyletic/paraphyletic, and that some sponges, theCalcarea, are a separate phylum which was the first to diverge from the main line ofkingdom Animalia. Silicarea is considered the nextphylum to diverge from the primary animal lineage.
Siliceous sponges are usually found in the marine ecosystem but they are occasionally found in freshwater.
During the Triassic, siliceous sponges grew reefs similar to calcarea of the modern era. During the Cretaceous period,diatoms became so successful that they significantly decreased the amount of silica present in sea water, after which "siliceous sponges could never again form reefs."[3]
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