ZOOLOGY-WEEK-5-PHYLUM-PORIFERA-LESSON-PROPER.pdf
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NSCELEC 2 WEEK 5 PHYLUM PORIFERA COMMON CHARACTERISTICS OF PHYLUM PORIFERA The invertebrates, or invertebrata, are animals that do not contain bony structures, such as the craniu...
NSCELEC 2 WEEK 5 PHYLUM PORIFERA COMMON CHARACTERISTICS OF PHYLUM PORIFERA The invertebrates, or invertebrata, are animals that do not contain bony structures, such as the cranium and vertebrae. The simplest of all the invertebrates are the Parazoans, which include only the phylum Porifera: the sponges (Figure 1). Parazoans (“beside animals”) do not display tissue-level organization, although they do have specialized cells that perform specific functions. Sponge larvae are able to swim; however, adults are non-motile and spend their life attached to a substratum. Phylum Porifera are the lowest multicellular animals belonging to the kingdom Animalia. The word “Porifera” mainly refers to the pore bearers or pore bearing species. Based on the embryological studies, sponges are proved as animals and are classified into a separate Phylum in the animals. This phylum includes about 5000 species. Poriferans are pore-bearing first multicellular animals. The pores are known as Ostia. The poriferans have a spongy appearance and are therefore called sponges. They are attached to the substratum and do not move. They have the ability to absorb and withhold fluids. They were initially regarded as plants due to the green colour and their symbiotic relationship with algae. Later, their life cycle and feeding system were discovered, and they were included in the animal kingdom. Some of the important characteristics of phylum Porifera are mentioned below. 1. The cells of Poriferans are loosely organized. 2. They are mostly found in marine water. Only a few are found in freshwater. 3. They are either radially symmetrical or asymmetrical. 4. Their body is usually cylindrical. 5. The scleroblast secretes spicules while spongin fibres are secreted by spongioblasts. 6. They have no organs in their body. 7. They depict cellular grade of organization. 8. The body comprises numerous pores known as Ostia and osculum. 9. The central cavity is called spongocoel or atrium which opens to the outside through the osculum. 10. They reproduce asexually by budding, and fragmentation. 11. The nutrition is holozoic. 12. They have neurosensory cells but are devoid of any specific nervous system. 13. They have the power to regenerate the lost parts. MORPHOLOGY OF SPONGES The morphology of the simplest sponges takes the shape of a cylinder with a large central cavity, the spongocoel, occupying the inside of the cylinder. Water can enter into the spongocoel from numerous pores in the body wall. Water entering the spongocoel is extruded via a large common opening called the osculum. However, sponges exhibit a range of diversity in body forms, including variations in the size of the spongocoel, the number of osculi, and where the cells that filter food from the water are located. While sponges (excluding the hexactinellids) do not exhibit tissue-layer organization, they do have different cell types that perform distinct functions. A. Pinacocytes, which are epithelial-like cells, form the outermost layer of sponges and enclose a jellylike substance called mesohyl. Mesohyl is an extracellular matrix consisting of a collagen-like gel with suspended cells that perform various functions. The gel-like consistency of mesohyl acts like an endoskeleton and maintains the tubular morphology of sponges. In addition to the osculum, sponges have multiple pores called ostia on their bodies that allow water to enter the sponge. In some sponges, ostia are formed by porocytes, single tube-shaped cells that act as valves to regulate the flow of water into the spongocoel. In other sponges, ostia are formed by folds in the body wall of the sponge. B. Choanocytes (“collar cells”) are present at various locations, depending on the type of sponge, but they always line the inner portions of some space through which water flows (the spongocoel in simple sponges, canals within the body wall in more complex sponges, and chambers scattered throughout the body in the most complex sponges). Whereas pinacocytes line the outside of the sponge, choanocytes tend to line certain inner portions of the sponge body that surround the mesohyl. The structure of a choanocyte is critical to its function, which is to generate a water current through the sponge and to trap and ingest food particles by phagocytosis. Note the similarity in appearance between the sponge choanocyte and choanoflagellates (Protista). This similarity suggests that sponges and choanoflagellates are closely related and likely share a recent common ancestry. The cell body is embedded in mesohyl and contains all organelles required for normal cell function, but protruding into the “open space” inside of the sponge is a mesh-like collar composed of microvilli with a single flagellum in the center of the column. The cumulative effect of the flagella from all choanocytes aids the movement of water through the sponge: drawing water into the sponge through the numerous ostia, into the spaces lined by choanocytes, and eventually out through the osculum (or osculi). In the meantime, food particles, including waterborne bacteria and algae, are trapped by the sieve-like collar of the choanocytes, slide down into the body of the cell, are ingested by phagocytosis, and become encased in a food vacuole. Lastly, choanocytes will differentiate into sperm for sexual reproduction, where they will become dislodged from the mesohyl and leave the sponge with expelled water through the osculum. C. The second crucial cells in sponges are called amoebocytes (or archaeocytes), named for the fact that they move throughout the mesohyl in an amoeba-like fashion. Amoebocytes have a variety of functions: delivering nutrients from choanocytes to other cells within the sponge, giving rise to eggs for sexual reproduction (which remain in the mesohyl), delivering phagocytized sperm from choanocytes to eggs, and differentiating into more-specific cell types. Some of these more-specific cell types include collencytes and lophocytes, which produce the collagen-like protein to maintain the mesohyl, sclerocytes, which produce spicules in some sponges, and spongocytes, which produce the protein spongin in the majority of sponges. These cells produce collagen to maintain the consistency of the mesohyl. The different cell types in sponges are shown in Figure 2. In some sponges, sclerocytes secrete small spicules into the mesohyl, which are composed of either calcium carbonate or silica, depending on the type of sponge. These spicules serve to provide additional stiffness to the body of the sponge. Additionally, spicules, when present externally, may ward off predators. Another type of protein, spongin, may also be present in the mesohyl of some sponges. Physiological Processes in Sponges Sponges, despite being simple organisms, regulate their different physiological processes through a variety of mechanisms. These processes regulate their metabolism, reproduction, and locomotion. Digestion Sponges lack complex digestive, respiratory, circulatory, reproductive, and nervous systems. Their food is trapped when water passes through the ostia and out through the osculum. Bacteria smaller than 0.5 microns in size are trapped by choanocytes, which are the principal cells engaged in nutrition, and are ingested by phagocytosis. Particles that are larger than the ostia may be phagocytized by pinacocytes. In some sponges, amoebocytes transport food from cells that have ingested food particles to those that do not. For this type of digestion, in which food particles are digested within individual cells, the sponge draws water through diffusion. The limit of this type of digestion is that food particles must be smaller than individual cells. All other major body functions in the sponge (gas exchange, circulation, excretion) are performed by diffusion between the cells that line the openings within the sponge and the water that is passing through those openings. All cell types within the sponge obtain oxygen from water through diffusion. Likewise, carbon dioxide is released into seawater by diffusion. In addition, nitrogenous waste produced as a byproduct of protein metabolism is excreted via diffusion by individual cells into the water as it passes through the sponge. Reproduction Sponges reproduce by sexual as well as asexual methods. The typical means of asexual reproduction is either fragmentation (where a piece of the sponge breaks off, settles on a new substrate, and develops into a new individual) or budding (a genetically identical outgrowth grows from the parent and eventually detaches or remains attached to form a colony). An atypical type of asexual reproduction is found only in freshwater sponges and occurs through the formation of gemmules. Gemmules are environmentally resistant structures produced by adult sponges wherein the typical sponge morphology is inverted. In gemmules, an inner layer of amoebocytes is surrounded by a layer of collagen (spongin) that may be reinforced by spicules. The collagen that is normally found in the mesohyl becomes the outer protective layer. In freshwater sponges, gemmules may survive hostile environmental conditions like changes in temperature and serve to recolonize the habitat once environmental conditions stabilize. Gemmules are capable of attaching to a substratum and generating a new sponge. Since gemmules can withstand harsh environments, are resistant to desiccation, and remain dormant for long periods, they are an excellent means of colonization for a sessile organism. Sexual reproduction in sponges occurs when gametes are generated. Sponges are monoecious (hermaphroditic), which means that one individual can produce both gametes (eggs and sperm) simultaneously. In some sponges, production of gametes may occur throughout the year, whereas other sponges may show sexual cycles depending upon water temperature. Sponges may also become sequentially hermaphroditic, producing oocytes first and spermatozoa later. Oocytes arise by the differentiation of amoebocytes and are retained within the spongocoel, whereas spermatozoa result from the differentiation of choanocytes and are ejected via the osculum. Ejection of spermatozoa may be a timed and coordinated event, as seen in certain species. Spermatozoa carried along by water currents can fertilize the oocytes borne in the mesohyl of other sponges. Early larval development occurs within the sponge, and free-swimming larvae are then released via the osculum. Locomotion Sponges are generally sessile as adults and spend their lives attached to a fixed substratum. They do not show movement over large distances like other free-swimming marine invertebrates. However, sponge cells are capable of creeping along substrata via organizational plasticity. Under experimental conditions, researchers have shown that sponge cells spread on a physical support demonstrate a leading edge for directed movement. It has been speculated that this localized creeping movement may help sponges adjust to microenvironments near the point of attachment. It must be noted, however, that this pattern of movement has been documented in laboratories, but it remains to be observed in natural sponge habitats. CLASSIFICATION OF PHYLUM PORIFERA Phylum Porifera is classified into three classes: 1. Class Calcarea They are found in marine, shallow, and coastal water. Their skeleton is composed of calcareous spicules made of calcium carbonate. The body is cylindrical and exhibits radial symmetry. The body organization is asconoid, syconoid, or leuconoid. Eg., Clathrina, Scypha 2. Hexactinellids They are found in marine and the deep sea. The skeleton is made up of six-rayed siliceous spicules. The body is cylindrical in shape and exhibit radial symmetry. The canal system is Sycon or Leucon. Eg., Euplectella, Hyalonema 3. Desmospongiae They are found in marine or freshwater. The body is asymmetrical and cylindrical in shape. The canal system is a leuconoid type. The skeleton comprises spongin fibres, siliceous spicules, which are monoaxon and triaxon. Eg: Spongia, Spongilla, etc Porifera Examples Some of the common Porifera examples are: Sycon These are solitary or colonial marine sponges found in shallow waters attached to the rocks. The body is cylindrical in shape with numerous spores. The radial canal is made up of flagellated cells. Water enters the body through Ostia and reaches the radial canals by prosopyles. These species undergo both sexual and asexual mode of reproduction. Hylonema These are also known as glass rope sponges found in marine water. The body is round or oval with twisted root tufts. Small amphidiscs are present in the skeleton. Cliona They are also known as Boring Sponges found in coral skeletons, mollusc shells, other calcareous objects. They are green, purple, or light yellow in colour. The canal system is the characteristic of the leuconoid type of sponges, and they reproduce asexually and sexually. Euplectella These are also known as Venus flower basket and are found in deep waters. The body is cylindrical, long and curved fastened in the mud at the bottom of the sea. The canal system is simple synconoid type. The skeleton consists of siliceous spicules fused at the tips forming a three-dimensional network with parietal gaps. Spongilla They are largely found in ponds, streams, lakes growing on submerged plants and sticks. The body wall consists of a thin dermis provided with pores called Ostia. They possess a rhagon type canal system. They reproduce sexually as well as asexually.
