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Porifera

Porifera (pōrĬf´ərə) [Lat.,=pore bearer], animal phylum consisting of the organisms commonly called sponges. It is the only phylum of the animal subkingdom Parazoa and represents the least evolutionarily advanced group of the animal kingdom. All adult sponges are sessile (nonmotile), and nearly all are marine; there are six families of freshwater sponges. Sponges are subdivided into three classes.

Anatomy

Sponges lack organs and tissue, and all the cells exhibit considerable independence. The sponge is made up of two single-cell-deep layers and an intermediate mesohyl (mobile cells plus extracellular matrix). The outer (sac) layer consists of flattened polygonal cells called pinacocytes. The middle (mesohyl) layer consists of gelatinous protein/carbohydrate material, a range of mobile cells, and a skeleton of calcareous or siliceous spicules, or of elastic proteinaceous fibers called spongin fibers. The inner layer consists of flagelled cells called collar cells, or choanocytes.

The body is permeated by numerous pores called ostia that open into inhalant canals that lead to the feeding chambers, which are made up of choanocytes; here also are large openings, termed oscules, fed by exhalant canals, that carry the water current from the choanocyte chambers to the exterior. The concerted whipping action of the choanocyte flagella creates a current of water from ostia through the sponge body oscules. The choanocytes filter plankton and small bits of organic detritus from the water and, like the pinacocytes, absorb oxygen. Food is digested in ameboid archaeocytes that pick up food vacuoles from the choanocytes, which ingest the mainly particulate food. Waste products are carried out through the osculum.

Different types of amoebocyte spongiocytes and sclerocytes are responsible for secreting the skeletal material. Achaeocytes give rise to egg cells and sperm derive from choanocytes. The body of most sponges is irregular in form, although an almost radial symmetry is displayed by some. Three types of sponge structure are recognized: the asconoid, the most primitive, is regular, tube-shaped, and radially symmetrical; the syconoid is a more irregular structure that displays some degree of folding of the body wall while still maintaining a basic radial symmetry; the leuconoid is highly irregular, displays the greatest degree of folding of the body wall, and has lost radial symmetry. In the leuconoid sponges choanocytes line the pockets formed by the convoluted body wall.

Sponges are limited in size by the rate at which water can flow in and out of the spongocoel, bringing in food-bearing water and oxygen and removing waste products. Because the asconoid type has the smallest surface area, sponges of this structure are among the smallest in the phylum; leuconoid sponges, with a large amount of surface area, represent some of the largest members of the phylum.

Reproduction

Pieces of sponge are able to regenerate into whole new sponges. Asexual reproduction occurs by budding or by fragmentation. The buds may remain attached to the parent or separate from it, and each bud develops into a new individual. Freshwater sponges, as well as several marine species, form resistant structures called gemmules that can withstand adverse conditions such as drying or cold and later develop into new individuals. Gemmules are aggregates of sponge tissue and food, covered by a hard coating containing spicules or spongin fibers. Sexual reproduction also occurs. Most sponges are hermaphroditic, the same individual producing eggs and sperm, but in some species the sexes are separate. The larvae are flagellated and swim about freely for a short time. After settling and attaching to a suitable substrate, the larvae develop into young sponges.

Class Calcarea (Calcareous Sponges)

Sponges in this class are typified by skeletal spicules composed of calcium carbonate. The spicules often protrude through the epipinecodermal covering of the body wall, giving the organism a rough texture. Calcareous sponges are small, usually only a few inches high, and are generally dull in appearance, although several species are brightly colored. Members of this class are among the simplest sponges, and all three morphological types—asconoid, syconoid, and leuconoid—are represented. There are approximately 150 known species, exclusively marine and shallow-water dwellers.

Class Hexactinellida (Glass Sponges)

These are deep-sea sponges. They lack an epidermal covering, and their skeletons are composed of spicules of silica. The spicules, which often form a latticework, have six points or some multiple thereof. Glass sponges are pale in color and are cup- or basket-shaped. The spongocoel is large, and the osculum is covered by a grillwork of fused spicules. When the living tissue is removed, the cylindrical skeletons often have the appearance of spun glass. The glass sponge known as Venus's-flower-basket (Euplectella) supplies a home for certain shrimps that become trapped by the lattice of spicules. The body plan of Hexactinellida is between syconoid and leuconoid.

Class Demospongiae (Demosponges)

Most sponges belong in this class. It includes sponges with a skeleton made up of silicon-containing spicules or spongin fibers or both. In the latter case, the spongin provides a matrix in which the spicules are embedded. The Demospongiae vary in size from small, encrusting forms to very large, irregular masses. All are leuconoid; many are brightly colored. The freshwater sponges belong to this class; they are frequently green because of symbiotic algae that live in the amoebocytes. The fibrous sponges also belong to this class; they include the common bath sponges, Hippospongia communis and Spongia officinalis, and most of the other sponges used commercially. The boring sponges (family Clionidae) are extremely interesting because of their ability to bore into calcareous rocks and mollusk shells. They begin their boring as larvae and spend their lives in the tunnels they form. Sulfur sponges (Cliona species) are bright yellow boring forms inhabiting shallow waters on the east and west coasts of the United States.

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Porifera

Porifera

The phylum Porifera contains all the species of sponges. Phylogenetically, Porifera is most closely related to Protista, making it the first animal phylum to have evolved to be multicellular. This also makes Porifera the simplest in form and function. Sponges arose 550 million years ago in the pre-Cambrian period, evolving from colonial protists, groups of identical single cell organisms that live together. Evidence for this comes from specialized cells called choanocytes which sponges use in feeding. Although sponges are made up of many cells with specialized functions, their cells are not organized into true tissues. This lack of true tissue layers makes sponges different from all other animals except protozoans, which are not multicellular. Sponges also lack symmetry, true organs, a digestive or respiratory system, a nervous system, muscles, and a true mouth.

Sponges are sessile ; they are attached to one place and do not move around. They range in size from over 1 meter (3 feet) long to 2 millimeters (less than 1/8 of an inch) long. All sponges live in water, from the deepest seas to the shallow coastal waters. Most species are marine and can be found in all the oceans; only 3 percent live in fresh water. All sponges have the ability to completely regenerate an adult from fragments or even single cells. Sponges reproduce sexually, with one sponge producing both sperm and eggs from the choanocytes at different times, giving rise to a larvae that is free living (not sessile). A very few species reproduce asexually by budding . Some of the first naturalists like Aristotle mistakenly thought sponges were plants because they do not move and can regenerate.

Sponges depend on the water currents flowing through them for food and gas exchange. Sponges have specialized cells for gathering small particles of food from the water and distributing the food around the organism. Water comes in through pores along the body wall into the spongocoel , the main cavity of a sponge, and flows out a large opening in the top called an osculum. Choanocytes, also called collar cells, are specialized feeding cells which line the spongocoel. Choanocytes have a flagellum that extends out of the cell and sweeps food particles into a sticky, collarlike opening. They are similar in shape and function to certain colonial protists, such as the choanoflagellates. Amoebocytes, which digest food and transport it around the sponge, are specialized cells that move around the sponge's body under the epidermis , the outer layer of cells, through a jellylike middle cell layer. Amoebocytes move in a way that is similar to how amoebae move. Amoebocytes secrete hard structural fibers called spicules, which are made of calcium carbonate or silica. In some sponges, amoebocytes secrete other materials that make up the skeleton called spongin which are flexible fibers made of collagen. Only sponges have spicules. This structural feature is part of what divides sponges into different classes.

There are over nine thousand identified species of sponges, and more are identified all the time. These species are classified into three classes: Demospongiae, Calcarea, and Hexactinellida.

Most species of sponges are in the class Demospongiae. Sponges in this class are mostly marine, but the class also contains the few species that do live in fresh water. Because the materials that make up the skeleton and spicules of these sponges are so varied; the overall sizes and shapes of the sponges are also varied. The amoebocytes of the sponges in Demospongiae contain pigment, giving these sponges many different bright colors.

Sponges within the class Calcarea are characterized by spicules made of calcium carbonate. All species in Calcarea have spicules of a similar size and shape. Most species are not colored. Calcarea sponges are usually less than 15 centimeters (6 inches) tall, and live in the shallow ocean waters along coasts.

Glass sponges make up the class Hexactinellida. They are unique because their spicules have six points and a hexagon shape. The spicules fuse together to form elaborate lattice skeletons which make the sponges look as if they are made of glass. Most Hexactinellida live in the Antarctic Ocean and are found in deep waters, from 200 meters (650 feet) down.

see also Phylogenetic Relationships of Major Groups.

Laura A. Higgins

Bibliography

Anderson, D. T., ed. Invertebrate Zoology. Oxford: Oxford University Press, 1998.

Barnes, Robert D. Invertebrate Zoology, 5th ed. New York: Saunders College Publishing, 1987.

Campbell, Neil A., Jane B. Reece, and Lawrence G. Mitchell. Biology, 5th ed. Menlo Park, CA: Addison Wesley Longman, Inc., 1999.

Purves, William K., Gordon H. Orians, H. Craig Heller, and David Sadava. Life: The Science of Biology, 5th ed. Sunderland, MA: Sinauer Associates Inc. Publishers, 1998.

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Porifera

Porifera

Porifera, or sponges, are the simplest and oldest of the multicelled animals, with fossils dating back to Precambrian times. They are aquatic and sessile , living permanently attached to submerged objects. More than 5,000 species are known, most of which occur in shallow coastal waters and in the deep sea. About 150 species live in fresh water. Sponges are found at all latitudes, even in polar regions.

Sponges are unique among animals because they lack a brain, nerves, muscles, organs, and specialized tissues. They rely upon highly specialized, but poorly coordinated cells. As the name Porifera ("pore bearers") suggests, the body is perforated. Numerous small pores (ostia) convey water into an internal canal system lined with flagellated collar cells (choanocytes). The flagella of these cells beat synchronously to produce currents that pump water through the sponge. Choanocytes filter water through their sievelike collars to remove suspended food particles (bacteria, protozoans, microscopic algae, organic particles). The particles are digested by wandering amoeboid cells (amoebocytes), which carry nutrients to various parts of the sponge. Filtered water and waste products are expelled through large vents (oscula).

The skeleton supporting these canals and chambers is composed of needlelike spicules and/or elastic protein fibers (spongin). The spicules are made of silica or calcium carbonate and occur in various shapes and sizes characteristic of each species.

Sponges can reproduce both sexually and asexually. Clouds of sperm expelled into the water by one sponge are drawn into other sponges with water currents. Specialized cells (modified choanocytes) carry sperm to the eggs. Zygotes develop into ciliated larvae that are released into the water, where they are planktonic for a short period before settling onto a suitable substrate to become adult sponges. Asexual reproduction occurs by fragmentation and/or budding; for example, freshwater sponges use resistant buds (gemmules) for surviving winter or periods of drought.

see also Animalia; Coral Reef

Anthony Ricciardi

Bibliography

Barnes, Robert D. Invertebrate Zoology, 5th ed. Philadelphia, PA: Saunders College Publishing, 1987.

Margulis, Lynn, and Karlene V. Schwartz. Five Kingdoms: An Illustrated Guide to the Phyla of Life on Earth, 3rd ed. New York: W. H. Freeman, 1998.

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Porifera

Porifera (Spongiaria, sponges) A phylum of aquatic Animalia, most of which are marine. They lack definite tissues and organs, but have a filter-feeding system composed of flagellate cells, pores, and canals. Most are covered externally by either a cortex or a dermis. The soft body may be supported by a skeleton composed of collagen, with or without siliceous or calcareous spicules, protein spongin fibres, or a combination of the last two. The body shape is very variable, being influenced by the substratum and water currents (e.g. where the water movement is very strong, sponges often grow as rounded clumps, but in calm water they may assume branching shapes). Sponges have remarkable powers of regeneration, small, broken pieces being capable of growing into a whole animal. Larvae are motile and produced by sexual reproduction. The phylum ranges from the Cambrian to the present.

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Porifera

Porifera (Spongiaria, sponges) A phylum of multicellular animals that are not included in the Animalia. Sponges are sessile, benthic, filter feeders with a bag-like body, a central cavity, and an outer surface pierced with tiny openings (ostia), through which water enters, and an upper, larger opening (osculum), through which it leaves. Many sponges have a skeleton that is either horny, or composed of spongin, or consists of calcareous or siliceous spicules. They may be fossilized as a complete skeleton or as isolated spicules. Their fossil record dates from the Cambrian (or possibly from the Precambrian) and they are a common source of biogenic silica.

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Porifera

Porifera The phylum of marine and freshwater invertebrates that comprise the sponges, which live permanently attached to rocks or other surfaces. The body of a sponge is hollow and consists basically of an aggregation of cells between which there is little nervous coordination. The body is supported by an internal skeleton of spicules of chalk, silica, or fibrous protein (bath sponges have protein skeletons). Undulipodium-bearing (flagellated) cells (choanocytes) cause water to flow in through openings (ostia) in the body wall and out through openings (oscula) at the top; food particles are filtered from the water by the choanocytes. See also Placozoa.

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