Classification of Phylum Porifera
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The porifera or sponges are marine animals and they consist of loosely organized cells.While all animals have unspecialized cells that can transform into specialized cells, sponges are unique in having some specialized cells that can transform into other types, often migrating between the main cell layers and the mesohyl in the process. Sponges do not have nervous, digestive or circulatory systems. Instead most rely on maintaining a constant water flow through their bodies to obtain food and oxygen and to remove wastes, and the shapes of their bodies are adapted to maximize the efficiency of the water flow. All are sessile aquatic animals and, although there are freshwater species, the great majority are marine (salt water) species.
The outer surface of a sponge is lined with thin flat cells called the pinacocytes. these cells are mildly contractlie and due to this the shapes of some sponges changes.In many sponges pinacocytes are specialised into tubelike contractile porocytes. Openings in the porocytes act as pathways for water through the body wall and in this way they can regulate water circulation.
Just below the pinacocyte layer of a sponge is a jelly like layer called the mesophyl. The mesenchyme cells also known as amoebiod cells move about the mesophyl and are specialised for reproduction, secreting skeletal elements, transporting and storing food and forming contractile rings around the openings in the sponge wall.
Choanocytes also called the collar cells which are below the mesophyl line the inner chamber (s), these cells are flagelatted cells that have a collar like ring of microvilli, surrounding a flagellum.a net like mesh is formed in the collar when the microfilaments connect the microvilli the flagelum creates water currents through the sponge, and the collar filter microscopic food paqrticals from the water.
Sponges are supported by a skeleton that may consist of microscopic needlelike spikes called spicules.amoebiod cells form spicules.these spicules are made of calcium carbonate or silica and may take on a variety of shapes.Alternatively, the skeleton may be made of spongin (a fibrous protien made of collagen).
The nature of the skeleton is an important characteristic in the sponge taxonomy.
Water Flow and Body Types
A spong's life is dependent on the water currents the choanocytes create. The flow of the water brings food and oxygen to a sponge and carries away the metabolic and digestive wastes. The way the food is filtered and how the water circulates causes the phylum to have a certain body type. Three types of bodies have been described by the zoologists.
Ascon: these are vase like sponges. Ostia are the outer openings of porocytes and lead directly to a chamber called spongocoel. Choanocytes line the spongocoel and water is drawn into it by the flagellar movement of choanocytes through the ostia. Water exits through a single large opening at the top of the sponge called the Osculum.
Sycon: in this body form, the sponge wall appears to be folded. Water enters a Sycon sponge through openings called dermal pores. Dermal pores are the openings of invaginations of the body wall, called incurrent canals. Pores in the body wall connect incurrent canals to radial canals and the radial canals lead to spongocoeal. Radial canals are lined by the choanocytes, and the beating of the choanocyte flagella moves water from the ostia, through incurrent and radial canals, to the spongocoel, and out the Osculum.
Leucon: sponges have an extensively branched canal system. Water enters the sponge through ostia and moves through branched incurrent canals. Canals leading away from the chambers are called the excurrent canals. Proliferation of chambers and canals has resulted in the absence of a spongocoeal, and often, multiple exits (oscula) for water leaving the sponge.
Maintenance and Vital Functions
Sponges consume particles that range in size from 0.1 to 50um. Their food consists of bacteria, microscopic algae, protest, and other suspended organic manner. The pray is slowly drawn into the sponge and consumed.
Sponges help in reducing the turbidity of coastal waters. A single Leucon sponge, 1 cm in diameter and 10 cm in height can filter in excess of 20 litres of water everyday.
Small suspended food particles are filtered by the choanocytes. Water passes through their collar near the base of the cell and moves into a sponge chamber at the opening end of the collar. Suspended food is trapped on the collar and moved along the microvilli to the base of the collar, where it is incorporated into a food vacuole. With pH changes and lysosomal enzyme activity the food is digested. Partially digested food is passed to amoeboid cells, which distribute it to other cells.
Sponges are not limited to feed by the filtration method. Pinococytes lining the incurrent canals may phagocytize larger food particles up to 50um. Nutrients dissolved in the sea water can be actively transported by the sponge.
Sponges don't have nerve cells to coordinate body functions. Most reactions occur due to individuals responding to a stimulus e.g. water circulation in some sponges is minimum at sunrise and at a maximum just before sunset because light inhibits the constriction of porocytes and other cells surrounding ostia keeping incurrent canals open. Other reactions however suggest some communication among cells. For example the rate of water circulation through the sponge can drop suddenly without and apparent external cause. This reaction can be due only choanocytes ceasing activities more or less simultaneously, and this implies some form of internal communication. The nature of this communication is unknown. Amoeboid cells transmitting chemicals messages and ion movement over cell surfaces are possible control mechanisms.
Due to the presence of an extensive canal system and circulation of large volumes of water through sponges, all sponge cells are in close contact with water and so the nitrogenous waste removal and gaseous exchange occurs by diffusion
Some sponges host photosynthesizing micro-organisms as endosymbionts and this coalation often results in the production of more food and oxygen than can be consumed. Freshwater sponges often host green algae as endosymbionts within archaeocytes and other cells, and benefit from nutrients produced by the algae. Many marine species host other photosynthesizing organisms.The spicules made of silica conduct light into the mesohyl, where the photosynthesizing endosymbionts live. Sponges that host photosynthesizing organisms are commonest in waters with relatively poor supplies of food particles, and often have leafy shapes that maximize the amount of sunlight they collect.
Few sponges are carnivorous. They can capture small crustaceans using spicule-covered filaments. In most cases little is known about how they actually capture prey. Most known carnivorous sponges have completely lost the water flow system and choanocytes.
Sponges do not have the complex immune systems of most other animals. However they reject grafts from other species but accept them from other members of their own species. In a few marine species, grey cells act as the guards for the sponges. When invaded, they produce a chemical that stops movement of other cells in the affected area, thus preventing the intruder from using the sponge's internal transport systems. If the intrusion persists, the grey cells accumulate in the area and release toxins that kill all cells in the area. The immune systems stay activated for up to 3 weeks or so.
Most sponges are monoecious but do not usually self fertilise because individual produce eggs and sperms at different times. Certain choanocytes lose their collar and their flagella undergo meiosis and form flagellated sperms. Other choanocytes and amoeboid cells in some sponges probably undergo meiosis to form eggs. Sperm and eggs are released from sponge oscula. Fertilisation occurs in the ocean water resulting in planktonic larvae development. In some sponges the eggs are retained inside the mesophyl of the parent. Sperm cells exit one sponge through the Osculum and enter another sponge with the incurrent water. Choanocytes trap the sperms and incorporate them into vacuole. The choanocytes lose their collar and flagellum; they become amoeboids, and transport the sperm to the eggs.
In some sponges, early development occurs in the mesophyl. Cleavage of a zygote results in the formation of a flagellated larval stage. The breaks free and is carried the water carries it away from the parent sponge. After about two days the larva settles in a suited environment and starts to mature into and adult.
Asexual reproduction also occurs in some sponges. This involves the formation of resistant capsules, called gemmules which contain masses of amoeboid cells. At the death of the parent sponge in winter, gemmules are released from them which can survive adverse conditions. When favourable condition are observed in spring time the amoeboid cells stream out of a tiny opening, called the micropyle, and organise into a sponge.
Some sponges have the remarkable power of regeneration. Though this is possible if the right cells are present in the sponge. A few species reproduce by budding
Sponges are very competitive for living space .Many sponges shed spicules, forming a dense carpet several meters deep that keeps away organism which would otherwise prey on the sponges. They also produce toxins that prevent other sessile organisms such as bryozoans or sea squirts from growing on or near them.
Sponges are important ecological constituents of reef communities, but they do not commonly contribute to the construction of reef frameworks.
Sponges are worldwide in their distribution, from the Polar Regions to the tropics. Most are found to be in quiet and clear waters because sediment stirred up by waves or currents block their pores, making it difficult for them to feed and breathe. The greatest numbers of sponges are usually found on firm surfaces such as rocks, but some sponges are found on soft sediment they attach themselves by means of a root-like base.
Sponges are more abundant but less diverse in temperate waters than in tropical waters, possibly because organisms that prey on sponges are more abundant in tropical waters.
The calcium carbonate or silica spicules are too rough for most uses, but two genera, Hippospongia and Spongia, have soft, entirely fibrous skeletons. Early Europeans used soft sponges for many purposes including padding for helmets, portable drinking utensils and municipal water filters. Sponges were used as cleaning tools, applicators for paints and ceramic glazes and discreet contraceptives.
The luffa "sponge", which is commonly sold for use in the kitchen or the shower, is not derived from an animal but from the fibrous "skeleton" of a gourd.
Sponges have medicinal potential due to the presence in sponges themselves or their microbial symbionts of chemicals that may be used to control viruses, bacteria, tumours and fungi.
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