M27.htm

Phylum Porifera
(sponges)

The porifera (sponges ) are multicellular organisms found in a variety of body forms. The plasticity of body shapes and sizes within a species is often determined by numerous environmental factors, making it difficult to generalize about the typical body morphology of any particular species.

Although multicellular, sponges lack true tissues and embryological germ layers (such as endoderm , mesoderm , and ectoderm ), so are considered to be at the cellular grade of organization. They also have no organ systems or specialized sensory structures. Although sponge larvae are typically motile, adult sponges are usually benthic sessile organisms, which feed on particles (usually plankton and organic detritus ) suspended in water currents created by flagellated cells known as choanocytes , which line internal chambers within the sponge . Many sponges have a skeleton consisting of either calcium carbonate , silicon dioxide , or collagen fibers, - or a combination of these substances.

	Unidentified sponge from the Caribbean , photo by T. Popin.
	Sections of body walls for sponges with the three main types of body architecture .

Although sponges are found in a wide variety of body forms, they have three basic types of body construction. Cross sections of the body walls of these three forms are shown above. The simplest is the asconoid type, where there is typically a central spongocoel lined by choanocytes , and a single osculum where water exits from each spongocoel . These are usually smaller, simple sponges of the Class calcarea , and tend to be radially symmetrical. In the Syconoid condition, the body wall folds to form secondary choanocyte chambers, which then empty into the spongocoel through a system of canals. Many calcareous sponges have a syconoid plan of construction. The leuconoid condition arises from even more complex folding of the body wall, and the central spongocoel is replaced by a complex system of excurrent canals. Most of the Calcarea , and all species of demospongiae , have a leuconoid plan of construction.

There are about 9,000 species of sponges alive today. Sponges are a very old group, having evolved in the pre-cambrian era, so their earliest affinities are unknown, although sponges have left a good fossil record since the Cambrian . Most sponges are marine, however there are about 200 species that inhabit freshwaters. Sponges of the class Calcarea are almost all found in shallow waters (less than 100 meters depth), and are common in intertidal habitats. The class Demospongiae , which contains about 95% of all sponge species, has representatives that can be found at almost all depths, from the intertidal to the abyssal zone.

The class Sclerospongiae (coralline sponges ) have skeletons of spongin , silica , and aragonite calcite , and are usually found only in crevices, caves , and under ledges of coral reefs (some researches consider the sclerospongiae a subclass of the Demospongiae ).

	Leucetta losangelensis, a calcareous sponge from the upper Gulf of California.
	Glass sponge skeleton, modified from BIODIDAC, Univ. of Ottowa.

Members of the class Hyalospongia (or Hexactinellida ) are usually found only in deeper waters (over 200 meters in depth), although a relatively shallow water species (Rhabdocalyptus dawsoni ) has been recently discovered. Calcareous sponges are usually attached to hard substrates (such as rock), but the Demospongiae and Hexactinellida can live on soft sediments as well as hard substrates.

Sponges are found worldwide in most marine environments, and are often very abundant and ecologically important. Although they are preyed on by a number of different animals, sponges deter most predators, as well as many competitors for limited living space in shallow waters, with an array of chemical defenses, or biotoxins . These chemicals include anti-microbial compounds as well as chemicals toxic to many invertebrates and vertebrates. The wide variety of biochemical compounds produced by sponges make them a good source of pharmaceutical compounds, such as antibiotics , antiviral agents, anti-tumour compounds, anti-inflammatories , and many others.

Classes of the Phylum Porifera:

Calcarea
Demospongiae
Sclerospongiae
Hyalospongia (or Hexactinellida )

REFERENCES

Barnes, 1980
Brusca & Brusca, 1990
Barnes, Calow, and Olive, 1993
Meglitsch & Schram, 1991

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