|The Demospongia is by far the most diverse sponge group. Greater than 90 percent of the 5,000 known living sponge species are demosponges. This ratio is not maintained in the fossil record, where less than half of the known genera and families are demosponges. However, the vast majority of living demosponges do not possess skeletons that would easily fossilize, thus their fossil diversity, which peaks in the Cretaceous, is probably an enormous underestimate of their true diversity. As their great number of species would suggest, demosponges are found in many different environments, from warm high-energy intertidal settings to quiet cold abyssal depths. Indeed, all of the known freshwater poriferans are demosponges.
|Demosponge skeletons are composed of spongin fibers and/or siliceous spicules, though one genus (Oscarella) has neither. Demosponge spicules, if present, are siliceous, have one to four rays not at right angles, and have axial canals that are triangular in cross section. Demosponges take on a variety of growth forms from encrusting sheets living beneath stones to branching stalks upright in the water column. They tend to be large and only exhibit the leucon grade of organization.
Demosponge SystematicsDemosponge 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.