The siliceous sponges form a major

clade In biology, a clade (), also known as a Monophyly, monophyletic group or natural group, is a group of organisms that is composed of a common ancestor and all of its descendants. Clades are the fundamental unit of cladistics, a modern approach t ...

of the phylum

Porifera Sponges or sea sponges are primarily marine invertebrates of the animal phylum Porifera (; meaning 'pore bearer'), a Basal (phylogenetics) , basal clade and a sister taxon of the Eumetazoa , diploblasts. They are sessility (motility) , sessile ...

, consisting of classes

Demospongiae Demosponges or common sponges are sponges of the class Demospongiae (from + ), the most diverse group in the phylum Porifera which include greater than 90% of all extant sponges with nearly 8,800 species worldwide (according to the World P ...

(common sponges) and Hexactinellida (glass sponges). They are characterized by spicules made out of

silicon dioxide Silicon dioxide, also known as silica, is an oxide of silicon with the chemical formula , commonly found in nature as quartz. In many parts of the world, silica is the major constituent of sand. Silica is one of the most complex and abundan ...

, unlike calcareous 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.

Systematics

Most studies support the monophyly of siliceous sponges. The group, as a part of the phylum Porifera, has been named ''Silicispongia'' Schmidt, 1862 and ''Silicea'' Bowerbank, 1864. ''Silicarea'' is a proposed new phylum based on molecular studies of the

phylum In biology, a phylum (; : phyla) is a level of classification, or taxonomic rank, that is below Kingdom (biology), kingdom and above Class (biology), class. Traditionally, in botany the term division (taxonomy), division has been used instead ...

. It consists of the Poriferan classes

and Hexactinellida. Some scientists believe that Porifera is

polyphyletic A polyphyletic group is an assemblage that includes organisms with mixed evolutionary origin but does not include their most recent common ancestor. The term is often applied to groups that share similar features known as Homoplasy, homoplasies ...

paraphyletic Paraphyly is a taxonomic term describing a grouping that consists of the grouping's last common ancestor and some but not all of its descendant lineages. The grouping is said to be paraphyletic ''with respect to'' the excluded subgroups. In co ...

, and that some sponges, the Calcarea, are a separate phylum which was the first to diverge from the main line of

kingdom Animalia Animals are multicellular, eukaryotic organisms in the Biology, biological Kingdom (biology), kingdom Animalia (). With few exceptions, animals heterotroph, consume organic material, Cellular respiration#Aerobic respiration, breathe oxygen, ...

. Silicarea is considered the next

to diverge from the primary animal lineage.

Ecology

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 A diatom (Neo-Latin ''diatoma'') is any member of a large group comprising several Genus, genera of algae, specifically microalgae, found in the oceans, waterways and soils of the world. Living diatoms make up a significant portion of Earth's B ...

became so successful that they significantly decreased the amount of silica present in sea water, after which "siliceous sponges could never again form reefs."

References

Sponge biology Silicon dioxide {{Poriferan-stub