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Phylum Porifera: Sponges
Phylum Porifera: Sponges. Source: Earthlife.net

Phylum Porifera: An Introduction To The Sponges

Phylum Porifera: Sponges

Most animals move to search for food, but a sessile sponge draws food and water into its body instead. The entrance of water through myriads of tiny pores is reflected in the phylum name, Porifera. A sponge uses a flagellated “Collar cell,” the choanocyte, to move water. The beating of many tiny flagella, one per choanocyte, draws water past each cell, bringing in food and oxygen, as well as carrying away wastes. The sponge body is designed as an efficient aquatic filter for removing suspended particles from the surrounding water. Sponges clear many liters of water each day and are important primary consumers within their ecosystems. Biologists are often fascinated by sponges because they function so well with so few body parts.

Some growth habits and forms of sponges.
Some growth habits and forms of sponges. Source: Integrated Principles of Zoology, 17th Edition, Hickman et al.

Most of the approximately 8600 sponge species are marine; a few inhabit brackish water, and some 150 species live in fresh water. Marine sponges are abundant in all seas and at all depths. Sponges vary in size from a few millimeters to over 2 m in diameter; the latter size characterizes the great loggerhead sponges. Recent studies of the size and growth rate of the Caribbean reef sponge Xestospongia muta suggest that this sponge may be an astonishing 2300 years old. Many sponge species are brightly colored because of pigments in their dermal cells. Red, yellow, orange, green, and purple sponges are not uncommon. 

Although their embryos are free-swimming, adult sponges are always attached, usually to rocks, shells, corals, or other submerged objects. Some bore holes into shells or rocks; others even grow on sand or mud. Some sponges, including the simplest, appear radially symmetrical, but many are quite irregular in shape. Some stand erect, some are branched or lobed, and others are low, even encrusting, in form. Their growth patterns often depend on shape of the substratum, direction and speed of water currents, and availability of space, so that the same species may differ markedly in appearance under different environmental conditions. Sponges in calm waters may grow taller and straighter than those in rapidly moving waters.

Sponge choanocytes have a collar of microvilli surrounding a flagellum. Beating of the flagellum draws water through the collar (blue arrows) where food is trapped on microvilli (red arrows).
Sponge choanocytes have a collar of microvilli surrounding a flagellum. Beating of the flagellum draws water through the collar (blue arrows) where food is trapped on microvilli (red arrows). Source: Integrated Principles of Zoology, 17th Edition, Hickman et al.

Many animals, such as crabs, nudibranchs, mites, bryozoans, and fishes, live as commensals or parasites in or on sponges. Larger sponges particularly tend to harbor a great variety of invertebrate commensals. Sponges also grow on many other living animals, such as molluscs, barnacles, brachiopods, corals, or hydroids. Some crabs attach pieces of sponge to their carapace for camouflage and for protection against predators. Some reef fishes do graze shallow water sponges and sponges are an important part of the diet of hawksbill turtles. Surprisingly, dorid nudibranchs eat glass sponges. Sponges, and the microorganisms that live in or on them, produce a wide variety of bioactive chemicals. An extract from a marine sponge appears effective against leishmaniasis, a disease caused by a kinetoplastan parasite and another shows promise for treating herpetic infections. Many bacteria isolated from marine taxa also have antimicrobial or antiviral effects: for example, some inhibit Staphylococcus aureus infections and others are active against Escherichia coli, some strains of which may cause sickness due to food poisoning. These and other results have increased interest in sponge culturing as a source of valuable pharmaceuticals.

The skeletal framework of a sponge can be fibrous and/or rigid. When present, the rigid skeleton consists of calcareous or siliceous support structures called spicules. The fibrous part of the skeleton comes from collagen protein fibrils in the intercellular matrix of all sponges. Collagen comes in several types differing in chemical composition and form. One form of collagen is traditionally called spongin. Sponges harbor microalgae and cyanobacteria on the body surface and deep inside the body. The presence of photosynthetic organisms inside the sponge led some to propose that spicules were able to transmit light into the body. The fiber optics of siliceous spicules have now been confirmed. This has sparked interest among materials scientists and engineers in the enzymatic machinery needed to form silica nanoparticles and to fuse these particles into spicules inside and outside sponge cells. In many cases, the exterior simplicity of a sponge masks chemical and functional sophistication.

Sponges are an ancient group, with an abundant fossil record extending back to the early Cambrian period and even, according to some claims, the Precambrian. Classification is based on spicule form and chemical composition. Living poriferans traditionally have been assigned to three classes: Calcispongiae, Hexactinellida, and Demospongiae. Members of Calcispongiae typically have spicules of crystalline calcium carbonate with one, three, or four rays. Hexactinellids are glass sponges with six-rayed siliceous spicules, where the six rays are arranged in three planes at right angles to each other. Members of Demospongiae have a skeleton of siliceous spicules that develop around an axial filament, or spongin fibers, or both. A fourth clade, Homoscleromorpha, contains sponges that lack a skeleton or have siliceous spicules without an axial filament.

Diverse forms of spicules, many amazingly complex and beautiful, support a sponge’s body. Spongin fibers provide support in some sponges.
Diverse forms of spicules, many amazingly complex and beautiful, support a sponge’s body. Spongin fibers provide support in some sponges. Source: Integrated Principles of Zoology, 17th Edition, Hickman et al.

Characteristics of Phylum Porifera

  1. Multicellular; body an aggregation of several types of cells differentiated for various functions, some of which are organized into incipient tissues with some integration. However, the pinacoderm approaches a true tissue epithelium in homoscleromorph sponges.
  2. Body with pores (ostia), canals, and chambers that form a unique system of water currents on which sponges depend for food and oxygen.
  3. Mostly marine; all aquatic.
  4. Radial symmetry or none.
  5. Outer surface of flat pinacocytes; most interior surfaces lined with flagellated collar cells (choanocytes) that create water currents; a gelatinous protein matrix called mesohyl contains amebocytes of various types and skeletal elements.
  6. Skeletal structure of fibrillar collagen and calcareous or siliceous crystalline spicules, often combined with variously modified collagen (spongin); type IV collagen, characteristic of other animals, occurs only in homoscleromorph sponges.
  7. No organs or true tissues; digestion intracellular; excretion and respiration by diffusion.
  8.  Reactions to stimuli apparently local and independent in cellular sponges, but electrical signals in syncytial glass sponges; nervous system probably absent.
  9. All adults sessile and attached to substratum.
  10.  Asexual reproduction by buds or gemmules and sexual reproduction by eggs and sperm; free-swimming flagellated larvae in most.

About Samiha Nazibath Owishe

Hello everyone. You can call me Owishe. I love bioscience and conservation and like to write about them. Currently, I am head of Project Goodall & Project Darwin in Society & Science Foundation. Knock me about my article anytime at owisheadit@gmail.com

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