Demospongiae

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Class Demospongiae includes approximately 4,750 species in 10 orders. Their geographic distribution in the marine environment is from the intertidal to the abyssal zone; some species inhabit freshwater.

Members of the Demospongiae are asymmetrical. Demospongians can range in size from a few millimeters to over 2 meters in largest dimension. They can form thin encrustations, lumps, finger-like growths, or urn shapes. Pigment granules in amoebocytes often make members of this class brightly colored, including bright yellow, orange, red, purple, or green.

(Note: Terminology is described in the Porifera account)

Within the mesohyl of demospongians may be spicules of both megascleres and microscleres with one to four rays, dispersed collagenous fibers (spongin), both, or neither. Members of Demospongiae are easily distinguished from those of the Hexactinellida because they have no six-rayed spicules. They possess a leuconoid structure, with a folded choanoderm . The pinacoderm is continuous, and the mesohyl is characteristically thickened. The thicker the mesohyl, the more diverse the forms of Demospongiae.

The Demospongiae can reproduce both sexually and asexually. In sexual reproduction, spermatocytes develop from the transformation of choanocytes, and oocytes arise from archeocytes . Repeated cleavage of the zygote egg takes place in the mesohyl and forms a parenchymula larva with a mass of larger internal cells surrounded by small, externally flagellated cells. The resulting swimming larva enters a canal of the central cavity and is expelled with the exhalant current. Methods of asexual reproduction include both budding and the formation of gemmules . In budding, aggregates of cells differentiate into small sponges that are released superficially or expelled through the oscula. Gemmules are found in the freshwater family Spongellidae. They are produced in the mesohyl as clumps of archeocytes, are surrounded with a hard layer secreted by other amoebocytes. Gemmules are released when the parent body breaks down, and are capable of surviving harsh conditions. In a favorable situation, an opening called the micropyle appears and releases amoebocytes, which differentiate into cells of all the other types.

Sponges are sessile, benthic organisms. However, larvae possess flagella and are capable of being free-swimming. All sponges are filter feeders, subsisting on bacteria and other small organisms. Water bearing these food particles enters through outer pores. Choanocytes capture most of the food; however pinocytes and amoebocytes can also digest food. Food particles may also be digested directly by cells of the mesohyl. Sponges are unlikely to be eaten by other animals due to their unpleasant taste. However, some organisms are able to subsist on sponges, and others live with them as symbionts. Some sponges harbor photosynthetic bacteria, while some serve as protection for other organisms.

The most economically important group of demospongians to human are the bath sponges. These are harvested by divers and can also be grown commercially. They are bleached and marketed; the spongin gives the sponge its softness and absorbency.

Although not all sponges preserve well, there is a fossil record. Several of the major lines of Demospongiae were already established in the lower Paleozoic. By the beginning of the Cretaceous, all orders of Demospongiae were represented.

The extent of organization is a reliable clue to phylogenetic relationships within the class Demospongiae. However, among the classes of phylum Porifera, it is difficult to distinguish evolutionary relationships. Organization does not necessarily relate to phylogeny; for example, the leuconoid structure was evolved independently several times.

References:

Bergquist, P. R. Sponges. Berkeley, CA: University of California Press; 1978.

Hickman, C. P. Pages 86-103 in Biology of the Invertebrates. Saint Louis, MO: C.V. Mosely Publishing.

Kozloff, E. N. Pages 74-91 in Invertebrates. Philadelphia, PA: Saunders College Publishing; 1990.

Contributors

Kristen Wheeler (author).

Glossary

sessile

non-motile; permanently attached at the base.

Attached to substratum and moving little or not at all. Synapomorphy of the Anthozoa