The class Demospongiae, contains the vast majority (~85%) of all sponge species currently living in our seas and oceans. The name of this class derives from the Greek ‘demos‘ meaning person and the term ‘spongia‘ meaning sponge, that is to say the ‘people sponge’ or ‘common sponge’. This name is due to the fact that all those sponges that do not belong to the group of Calcarea, Hexactinellida or Homoscleromorpha, are considered Demospongiae, and therefore as a class they are a kind of catch-all. As a consequence, a great morphological diversity exists within the Demospongiae group. We are dealing with a class of mostly marine species, with the sole exception of the species of the family Spongillidae, which inhabit highly oxygenated fresh waters.
Demospongiae may present spicules of siliceous nature, or they may lack them. The spicules are usually supplemented or even replaced by a protein spongin network. This spongin network, and the absence of siliceous spicules, has meant that some species have historically been used as bath sponges, to the point of endangering certain species in certain areas due to harvesting pressure.
As previously mentioned, the Demospongiae are a very heterogeneous class, both in terms of shape and coloration. We can find from species that are encrusting, to others whose bodies are tall and slender, or low and extensive, or even fan-shaped, vase-shaped or spherical. The only common feature within the class Demospongiae is that their bodies adopt a leuconoid-like morphology, in orther words, they have a reduced atrial cavity, and therefore the appearance of these sponges is considerably massive. They may have more than one opening for the outflow of filtered water. By filtering the water, Demospongiae capture the organic particles present in the water column, on which they feed.
The approximately 6,900 species of Demospongiae are classified into three subclasses (Heteroscleromorpha, Keratosa and Verongimorpha), in which there are 22 orders, comprising 88 families and just over 1,000 genera.