Sponge classes

Demospongiae Sollas (1885)

The class Demospongiae (D.) is the largest, morphologically most diverse, and successful group of the phylum Porifera. It is the only sponge class that is distributed in marine, brackish as well as in fresh water environments. It comprises almost 83% of all accepted living sponges, with 8076 (including 272 freshwater species) species worldwide (de Voogd et al., 2025). Demospongiae includes three subclasses and 22 extant orders (Morrow & Cárdenas, 2015). In the ocean, they are known worldwide from the hadal to the eulittoral zone (Van Soest et al., 2012). They are filter feeding animals as all other sponge groups, but also include some peculiar carnivorous representatives in the family Cladorhizidae (Hestetun et al., 2017; Vacelet & Boury‐Esnault, 1995).
Demosponges produce a skeleton of opaline spicules, frequently supplemented with spongin fibers. In some cases, they have only fibers of spongin or chitin (Ehrlich et al., 2018; Fromont et al., 2019). Solid, calcified (aragonite) skeletons, sometimes complemented by free‐spicules, also occur in various modern sponge groups (Clionaida, Haplosclerida, Merliida, and Agelasida), and in the fossil sphinctozoans which are actually a polyphyletic group (Senowbari‐Daryan & García‐Bellido, 2002; Vacelet, 1985). The demosponge spicules are mainly monaxons and tetraxons (triaenes) but never triaxons. Spicules in Demospongiae never fuse with each other. Only lithistids (DL.) have a solid hypersilicified skeleton formed by articulated siliceous spicules called desmas. The articulation means that there is always a thin space that separates particular desmas at places at which they connect. As such, in contrast to hexactinellids, no additional siliceous cement is involved. Desmas may have variable geometry (anaxial, monaxial, tetraxial, spheraxial) and shape being smooth or strongly sculptured. Lithistid sponges, due to their heavy silicification and articulation of desmas, dominate the demosponge fossil record. The earliest reliable demosponges are early Cambrian (515 Ma) disassociated fossil spicules (Botting et al., 2015 and discussion therein) but the origin of this class is estimated from sponge mitochondrial genomes to be in the Neoproterozoic (Dohrmann & Wörheide, 2017; Plese et al., 2021; Schuster et al., 2018).

Hexactinellida Schmidt (1870)

Only 712 hexactinellid (H.) species are known today, which comprises slightly over 7% of all known living sponges (de Voogd et al., 2025). Sponges of this class are characterized by the lack of dissolvable spongin as skeletal component, although the collagen matrix is seen on Transmission Electron Microscopy. This group is characterized by the presence of thin mesohyl and multinucleated syncytium which is a major tissue component (Leys, 2003). Hexactinellid sponges are inhabitants of waters with oceanic salinity. They are psychrophilic and live mainly in bathyal and abyssal ecosystems (down to about 7000 m; Tabachnick et al., 2017). In favorable conditions of low temperatures (below 14°C), high dissolved silicate and low light, they can inhabit shallow‐waters (about 8m depth; Goodwin et al., 2012). Loose spicules of some dead hexactinellids sometimes accumulate into spicule mats (Barthel, 1992), while hexactinellids with rigid frameworks may produce various reef constructions known in both recent and fossil state.
Siliceous (opaline) spicules may be loose or partially or completely fused. The latter condition (fused spicules) may result in the formation of a rigid choanosomal framework (Schulze, 1887). Such framework consists of specific dictyonal strands (Reid, 1963). Hexactinellids produce siliceous spicules of hexactinic or triaxonic (or cubic) symmetry or shapes clearly derived from such spicules by reduction of primary rays or branching of the primary rays. A loss of one or more rays results in pentactines, tetractines, triactines, diactines, and monactines. The earliest
reliable sponge fossils are hexactinellid spicules from Iran dated to c. 535 Ma. (Hamdi et al., 1989), but some hexacts are known also from Ediacaran‐Cambrian (541 Ma) cherts from South China (Chang et al., 2019).

Homoscleromorpha Bergquist (1978)

Homosclerophora (Ho.) is the smallest class of Porifera with a worldwide distribution in only marine environments, with three oceanic regions representing current hotspots of Homoscleromorpha occurrences: the Pacific Ocean, the Tropical Western Atlantic Ocean and the Mediterranean Sea (Lage et al., 2018). Homoscleromorphs are mainly distributed in cryptic semi‐dark and dark habitats, from intertidal to bathyal depths.
There are 136 valid species in this group representing about 1.4% of all sponges known so far (de Voogd et al., 2025). Although all Homoscleromorpha share common traits allowing to distinguish them from all other sponge classes (e.g., basal membrane under pinacoderm and choanoderm, flagellate pinacocytes, specific siliceous spicules when they exist; Gazave et al., 2012; Maldonado & Riesgo, 2007), the taxonomy and systematics within this class is a challenging issue because of the lack of obvious diagnostic characters and a rather high plasticity in some cases. However, this class has known one of the highest rates of new descriptions over the last two decades through the use of integrative taxonomy (e.g., Boury‐Esnault et al., 2013). Still containing only one order, this sponge class has seen its systematics overturned in the last 10 years, with its separation from the Demospongiae, the resurrection of one family (Oscarellidae) and the redefinition of the family Plakinidae (Ruiz et al., 2017). Among the two known families, all Oscarellidae are skeletal‐less, cytological traits being used for their identification: vacuolar and spherulous cells, organized in clusters in some cases, or harboring very peculiar paracristalline inclusions in some other cases.
Recently, it has been shown that the choanocyte kinetids structure strongly supported the division of the Oscarellidae family in two clades (Pozdnyakov et al., 2020). In Plakinidae, three genera are also known to be skeletal‐less (Ruiz et al., 2017; Ruiz et al., submitted), but in general this family lacks cytological descriptions. When present, spicules are rather small (100 μm on average) and not differentiated into mega‐ and microscleres (Van Soest et al., 2012). They are peculiar tetractines (calthrops, 10–300 μm) and derivatives after ramification (lophose spicules) or reduction of the number of actines (triods, 10–300 μm, diods 25–750 μm). Their ramifications can be homogeneous (homolophose spicules) or heterogeneous (heterolophose) in variable positioning along the actines. Homoscleromorph spicules are known in the fossil record from at least early Carboniferous (Mehl‐Janussen, 1999; Reid, 1970).

Calcarea Bowerbank (1862)

The class Calcarea (C.) corresponds to about 8.5% (837 species) of the phylum Porifera (de Voogd et al., 2025). The species within this class are all marine and can be found from the intertidal zone to, less frequently, the abyssal zone (some even in a depth range of 1120–4400 m; Janussen & Rapp, 2011). The skeleton in Calcarea is composed exclusively of calcium carbonate (spicules of Mg‐calcite) (Rossi et al., 2014).
Nowadays, the diversity of spicule types in this class is reduced, with only three basic forms: diactines, triactines, and tetractines (Manuel et al., 2002; Van Soest et al., 2012) with a variety of derivatives/variants within them. The plane of the actines (basal or apical), the angle between them (equiangular or sagittal), their shape (conical, slightly conical, or cylindrical), and the character of the tips (rounded, blunt, or sharp), are some of the characteristics that differentiate the spicules of Calcarea. Pentactines, which are mainly found in fossil Calcarea, have also been found in extant species, but they are rare (Chagas & Cavalcanti, 2017; Rossi et al., 2006). There is no differentiation in micro‐ and megascleres. Solid, hypercalcified (calcite) skeletons, complemented by free calcite spicules, also occur in some extant groups (orders Murrayonida, Lithonida) with affinities to fossil taxa (Vacelet et al., 2002).
The oldest fossils of Calcarea are specimens of Eiffelia Walcott, 1920 (Order Heteractinida) from the middle Cambrian Burgess Shale. However, some doubts remain if Eiffelia was indeed a genus of Calcarea (Botting & Butterfield, 2005). The first unchallenged fossil record of a calcareous sponge is that of Leucandra walfordi Hinde, 1893 from the Middle Jurassic, King’s Sutton, Northamptonshire (Pickett, 2002).