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The calcareous sponges or calcereans are that make up the class Calcarea, characterized by spicules made of calcium carbonate in the form of high-magnesium calcite or aragonite. While the spicules in most species are triradiate (with three points in a single plane), some species may possess two- or four-pointed spicules. Treatise on Invertebrate Paleontology Part E, Revised. Porifera, Volume 3: Classes Demospongea, Hexactinellida, Heteractinida & Calcarea, xxxi + 872 p., 506 fig., 1 table, 2004, available here. . Treatise on Invertebrate Paleontology Part E, Revised. Porifera, Volumes 4 & 5: Hypercalcified Porifera, Paleozoic Stromatoporoidea & Archaeocyatha, liii + 1223 p., 665 figs., 2015, available here. . Unlike the far more common , calcareans lack microscleres, tiny spicules which reinforce the flesh. In addition, their spicules develop from the outside-in, mineralizing within a hollow organic sheath.
All three sponge body plans (asconoid, syconoid, and leuconoid) can be found within the class Calcarea. Typically, calcareous sponges are small, measuring less than in height, and drab in colour. However, a few brightly coloured species are also known.
Like the Homoscleromorpha, calcareous sponges are exclusively Viviparity. Symbiont transmission in marine sponges: reproduction, development, and metamorphosis
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. (2025). 9780030259821, Cengage Learning. ISBN 9780030259821 Some extinct species were hypercalcified, meaning that the spicule-based skeleton is cemented together by solid calcite.
Calcarean sponges likely first appeared during the Cambrian. The oldest putative calcarean genus is Gravestockia, from the "Atdabanian" (Cambrian Stage 3) of Australia. Calcareans are probably descended from "Heteractinida" sponges, which first appeared in the early Cambrian. Calcareans reached their greatest diversity during the Cretaceous.
Some molecular analyses suggest the class Calcarea is not exclusively related to other sponges, and should thus be designated as a phylum. This would also render Porifera (the sponge phylum) Paraphyly. Borchiellini et al. (2001) argued that calcareans were more closely related to Eumetazoa (non-sponge animals) than to other sponges. A few studies have also supported a sister group relationship between calcareans and Ctenophora (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.
Calcarea is divided into two subclasses (Calcinea and Calcaronea) and a number of orders. The two subclasses are mainly distinguished by spicule orientation, soft tissue and developmental traits. For example, calcineans develop from a parenchymella (a larva 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). Treatise on Invertebrate Paleontology Part E, Revised. Porifera, Volume 2: Introduction to the Porifera, xxvii + 349 p., 135 fig., 10 tables, 2003, available here. .
Class Calcarea
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