The Characteristics of the Classes of Sponges
Investigating the Specific Traits of Members of Phylum Porifera
Sep 24, 2009
The 15,000 species of living sponges are grouped taxonomically into three classes based primarily on the structure of their skeletal elements (support structure) – class Calcarea, class Hexactinellida, and class Demospongia.
Class Calcarea
(L. calis, lime): The calcareous sponges are so named for their calcium carbonate (lime) spicules (splinters). The spicules, laid down as calcite, may have two to four points and lack a hollow axial canal.
All the sponges in this class are marine dwellers most commonly found in shallow tropical waters. They are found from the intertidal zone down to around 200 meters (660 feet), although one species is known from a depth of 4,000 meters (13,200 feet).
Calcareous sponges are typically small, measuring about 8 to 10 cm (3-4 inches) and they demonstrate relatively simple shapes, either purse, vase, pear, or cylinder-shaped.
This class is considered to be the ancestral group of sponges and it is the only class demonstrating all three sponge body plans – asconoid, syconoid, and leuconoid. Calcareous sponges lack the hollow canals that mark the morphology of other sponge types. Instead, their skeleton has either a mesh or honeycomb structure making them structurally stronger than other sponges.
Class Hexactinellida
(Gr. hex, six + aktis, ray): The skeleton of a hexactinellid is composed entirely of silica in the form of six-pointed siliceous spicules. Hence, their common name of glass sponges. The spicules are composed of three perpendicular rays giving them six points.
Furthermore, the spicules of hexactinellid sponges are fused together imparting a rigidity of structure not found in other sponge types. When the living tissue is removed, the cylindrical skeletons often have the appearance of spun glass and resemble ornate jewel-like objects.
Hexactinellid sponges occur worldwide, mostly at depths between 200 and 1000 meters (650-3,300 feet). Although they are more abundant and diverse at shallower depths of polar regions.
They are especially abundant and diverse in Antarctic waters. Frequently, they are the most conspicuous form of benthic life in these chilly waters and it appears that they may even be important in structuring biodiversity on the continental slopes as well as the continental shelf of Antarctica.
All glass sponges are upright and most appear to be radially symmetrical. They are pale in color and typically cylindrical but they may also be cup-shaped, urn-shaped, or branching. The average height of a hexactinellid is 10-30 cm (4-12 inches), but some can grow to be much larger. They possess a cavernous central cavity (atrium) through which water passes and while they superficially resemble the syconoid body plan, they differ too much internally to be considered truly syconoid.
Hexactinellids are completely sessile (immobile), even the larvae seem to display no movement outside of their ability to disperse small distances within currents. Furthermore, unlike other sponges, hexactinellids do not contract when stimulated.
Class Demospongia
(Gr. demos, people + spongos, sponge) The Demospongia is not only the most diverse group of sponges; it is also the largest class of sponges containing over 90% of all the known living sponge species.
Demospongia can range in size from a few millimeters to giants over 2 meters (6-7 feet) in their largest dimension and they come in a bright pallet of colors, including bright yellow, orange, red, purple, and green. Produced by pigment granules in amoebocyte cells, the significance of such bright hues is uncertain but protection from solar radiation and warning coloration have been suggested for some species.
Marine Demospongia are found from the warm, shallow intertidal zone to dark, cold abyssal depths where they grow as lumps, finger-like projections, or urn shapes.
Only this class of sponges also includes freshwater species. These delicate freshwater sponges are often colored green from the algae within them and live as branching masses or as thin encrustations on plants, submerged logs and twigs, and rocks. Only one species, Spongilla lacustris, has been found growing out of soft bottom sediments in standing water.
Freshwater sponges (genus Spongilla) are found in clean lake waters and slow-moving streams throughout North America. However, they are more common in northern and eastern lakes and streams than in southern and western ones. In general, freshwater sponges will not be found in areas with frequent physical disturbances, high pollution levels, or turbid waters containing high levels of silt.
The skeletal elements of Demospongia consist of siliceous spicules having one to four rays and/or coarse spongin fibers. Demospongia predominantly exhibit the leuconoid body structure.
Recent molecular analysis of sponge phylogeny suggests that the traditional phylum designation of Porifera should be eliminated. In its place the class Calcarea should be delegated as a separate phylum with the other sponge classes placed in phylum Silicarea.
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