Phylum Porifera – Definition, General Characteristics, Classification, Structure, Reproduction

What is Phylum Porifera? – Definition of Phylum Porifera

Phylum Porifera refers to a group of simple, sessile aquatic animals commonly known as sponges. Sponges lack true tissues and organs and have a porous body structure with numerous channels and chambers that allow water to flow through them. They feed by filtering small particles from the water and are considered to be the simplest multicellular animals. Sponges are found in a variety of marine and freshwater environments and play important ecological roles in their ecosystems.

• Sponges are aquatic sessile (sedentary) invertebrates. They are a successful and diverse group that inhabits both marine and freshwater environments. R. E. Grant classified sponges in the phylum Porifera in 1830. (L. pore bearing animals).
• Due to the peculiar cellular organisation of these organisms, Huxely (1875) and Sollas (1894) considered classifying them separately from Metazoa. Tuzet (1963) noted that despite the fact that sponges had the most primitive characteristics, they were regarded diversified animals produced throughout the evolution of Metazoa and were classified as Parazoa.
• Numerous kinds of sponges have an endo skeleton formed of spicules composed of calcium carbonate or silica. Spicules appear in a variety of shapes and sizes; their kind, shape, combination, and arrangement aid in identifying a specimen.
• Sponges are ecologically, commercially, and evolutionarily significant. Sponges can function as bioremediators and indicators of environmental quality. In the past, sponges were utilised as household objects, for personal cleanliness, pain alleviation, disease treatment, and in art.
• Recent interest in sponges is largely attributable to their ability to produce novel bioactive chemicals, which may have medicinal and antifouling benefits.
• In addition, their skeletal architectures have sparked additional interest due to their exceptional optical and mechanical capabilities, which may enable the future production of new materials.

General Characteristics of Phylum Porifera

• Habitat: Poriferans are aquatic, with the majority being marine. A notable exception is the family Spongillidae, which inhabits freshwater environments.
• Lifestyle: Sponges are sessile and sedentary, anchoring themselves to substrates and resembling plants in their growth patterns.
• Body Structure: Their bodies can be vase-shaped or cylindrical, exhibiting either asymmetrical or radial symmetry. The surface is dotted with numerous small pores called ostia for water intake and one or more larger oscula for water expulsion.
• Organization: Porifera possess a cellular level of organization, lacking distinct tissues and organs, and are considered diploblastic with an outer ectoderm, an inner endoderm, and a mesenchyme layer in between.
• Internal Features: The interior, known as the spongocoel, may be hollow or filled with canals lined with choanocytes, specialized cells for feeding and maintaining water flow.
• Skeleton: Their support structure may consist of spongin fibers, siliceous spicules, or calcareous spicules, contributing to the diversity in their physical forms.
• Feeding and Waste Removal: Sponges lack a mouth; they feed and excrete waste through intracellular digestion and the flow of water through their bodies.
• Respiration and Excretion: These processes occur directly through their cell surfaces, as they lack specialized respiratory and excretory systems. Freshwater sponges may have contractile vacuoles for osmoregulation.
• Nervous System: Sponges do not have a differentiated nervous or sensory system, although some may possess a primitive network of neurons.
• Reproduction: Porifera can reproduce both sexually and asexually, often exhibiting hermaphroditism. Asexual reproduction can occur through budding and gemmules, while sexual reproduction involves internal fertilization, with development proceeding through distinct larval stages such as the amphiblastula or parenchymula.
• Regeneration: Sponges possess remarkable regenerative capabilities, allowing them to recover from injury and even reconstitute entire organisms from fragments.
• Body Forms: The structural organization within Porifera varies from the simple ascon type to the more complex sycon and leuconoid types.

Morphology of Sponges / Structure of Phylum Porifera

• The simplest sponges have the morphology of a cylinder with a big central hollow, the spongocoel, occupying the cylinder’s interior. Many pores in the body wall allow water to enter the spongocoel. Water entering the spongocoel is expelled through the osculum, a huge shared orifice.
• Nonetheless, sponges exhibit a variety of body morphologies, including differences in the size of the spongocoel, the number of osculi, and the location of the cells that filter food from the water.
• While sponges (with the exception of hexactinellids) lack tissue-layer organisation, they do possess diverse cell types with specific roles. Pinacocytes, which are epithelial-like cells, constitute the outermost layer of sponges and enclose a substance known as mesohyl that resembles jelly.
• Mesohyl is an extracellular matrix composed of a collagen-like gel containing suspended cells with a variety of activities. The gel-like consistency of mesohyl functions as an endoskeleton and preserves the tubular structure of sponges.
• In addition to the osculum, sponges have many pores on their bodies called ostia that allow water to enter. Porocytes, single tube-shaped cells that govern the flow of water into the spongocoel, generate the ostia in certain sponges. In other sponges, ostia are created by folds in the sponge’s body wall.
• Choanocytes (also known as “collar cells”) are found in a variety of locations depending on the type of sponge, but they invariably border the inner parts of a water-flowing area (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).
• Choanocytes tend to coat particular inner sections of the sponge body that encompass the mesohyl, whereas pinacocytes line the exterior of the sponge. The shape of a choanocyte is essential to its function, which is to generate a water current within the sponge and to capture and consume food particles by phagocytosis.
• See the visual similarities between sponge choanocytes and choanoflagellates (Protista). This resemblance shows that sponges and choanoflagellates share a recent common ancestor and are closely related. The cell body is encased in mesohyl and has all organelles necessary for normal cell operation. However, projecting into the “open space” within the sponge is a mesh-like collar formed of microvilli with a single flagellum at its centre.
• The cumulative action of the flagella from all choanocytes assists the passage of water through the sponge, bringing water in through the various ostia, into the choanocyte-lined spaces, and out through the osculum (or osculi).
• In the interim, food particles, such as watery bacteria and algae, are captured by the sieve-like collar of the choanocytes, slide down into the cell’s body, are ingested via phagocytosis, and are enclosed in a food vacuole. Choanocytes will then differentiate into sperm for sexual reproduction, dislodging from the mesohyl and exiting the sponge through the osculum as water is discharged.
• Amoebocytes (or archaeocytes) are the second essential cell type in sponges; they are so termed because they travel across the mesohyl in an amoeba-like manner. Amoebocytes have multiple purposes, including transporting nutrients from choanocytes to other cells in the sponge, producing eggs for sexual reproduction (which remain in the mesohyl), transporting phagocytosed sperm from choanocytes to eggs, and developing into more specialised cell types.
• Collencytes and lophocytes generate the collagen-like protein to preserve the mesohyl, sclerocytes produce spicules in some sponges, and spongocytes produce spongin in the majority of sponges. These cells create collagen to maintain the mesohyl’s consistency.
• Depending on the species of sponge, sclerocytes release tiny spicules made of calcium carbonate or silica into the mesohyl of certain sponges. These spicules assist to impart additional rigidity to the sponge’s body. Moreover, the exterior presence of spicules may deter predators. Spongin, an additional form of protein, may also be present in the mesohyl of some sponges.
• The presence and content of spicules/spongin are distinguishing characteristics of the three sponge classes: Class Calcarea has calcium carbonate spicules and no spongin, class Hexactinellida has six-rayed siliceous spicules and no spongin, and class Demospongia has spongin and may or may not have spicules; if they are present, they are siliceous. Spicules are particularly prevalent in the class Hexactinellida, which consists of glass sponges. Some of the spicules may acquire gigantic sizes (relative to the average size range of glass sponges, 3 to 10 mm), as shown in the 3 m-long Monorhaphis chuni.

Digestion in Phylum Porifera

• Sponges lack sophisticated digestive, respiratory, circulatory, and reproductive systems. When water travels through the ostia and out the osculum, their food is trapped.
• Bacteria less than 0.5 microns in size are consumed via phagocytosis by choanocytes, which are the major cells involved in nourishment. Pinacocytes are capable of phagocytosing particles larger than the ostium.
• Amoebocytes carry food from cells that have consumed food particles to cells that have not. With this form of digestion, in which food particles are digested within individual cells, the sponge pulls water through diffusion for this type of digestion. This method of digestion is limited by the fact that food particles must be smaller than individual cells.
• All other primary bodily functions of the sponge (gas exchange, circulation, and excretion) are carried out by diffusion between the cells lining the openings of the sponge and the water passing through those pores. All sponge cell types receive oxygen from water by diffusion.
• Carbon dioxide is also emitted into seawater by diffusion. In addition, nitrogenous waste created as a consequence of protein metabolism is expelled into the water as it travels through the sponge via diffusion by individual cells.

Reproduction in Phylum Porifera

• Sponge reproduction is both sexual and asexual. Fragmentation (when a piece of the sponge breaks off, settles on a new substrate, and develops into a new individual) or budding are typical forms of asexual reproduction (a genetically identical outgrowth grows from the parent and eventually detaches or remains attached to form a colony).
• The development of gemmules is an uncommon form of asexual reproduction that is exclusive to freshwater sponges. Gemmules are inverted sponge-like structures created by adult sponges that are resistant to the environment.
• An inner layer of amoebocytes is surrounded by a layer of collagen (spongin) that may be strengthened by spicules in gemmules. The collagen typically present in the mesohyl becomes the outermost layer of defence.
• Gemmules in freshwater sponges may endure unfavourable environmental conditions, such as temperature fluctuations, and serve to repopulate the area once environmental conditions settle.
• Gemmules are able to connect to a substratum and produce new sponges. Gemmules are a good method of colonisation for sessile organisms due to their ability to tolerate harsh conditions, desiccation resistance, and lengthy periods of dormancy.
• Sponges engage in sexual reproduction when gametes are produced. Sponges are monoecious (hermaphroditic), meaning that a single individual can concurrently generate both gametes (eggs and sperm).
• In some sponges, gamete generation can occur throughout the year, whereas in others, sexual cycles are dependent on water temperature. Moreover, sponges can become progressively hermaphroditic, generating oocytes first and spermatozoa subsequently.
• Oocytes develop from amoebocytes and are maintained in the spongocoel, while spermatozoa develop from choanocytes and are expelled through the osculum. As observed in certain species, spermatozoa ejection may be a coordinated and timed action.
• Spermatozoa conveyed by water currents are able to fertilise oocytes in the mesohyl of other sponges. Inside the sponge, early larval development takes place, and free-swimming larvae are eventually expelled via the osculum.

Locomotion of Phylum Porifera

• Adult sponges are generally sessile and spend their entire lives adhering to a stable substratum. They do not exhibit large-scale movement like other free-swimming sea invertebrates.
• Nonetheless, sponge cells are able to crawl along substrata due to organisational plasticity. In experimental conditions, scientists have demonstrated that on a physical support, distributed sponge cells have a leading edge for directed movement.
• It has been hypothesised that this restricted creeping movement aids sponges in adapting to microenvironments close to the attachment site. It must be noted, however, that this movement pattern has only been observed in laboratory settings and not in natural sponge environments.

Classification of Phylum Porifera

Class 1: Calcarea

Class Calcarea, also known as calcareous sponges, is distinguished by its members’ small size and the presence of calcium carbonate spicules in their skeletal structure. These sponges exhibit a variety of body plans and are exclusively marine, inhabiting various oceanic environments. Here are some key characteristics and the orders within this class:

Key Characteristics

• Size and Form: Calcareous sponges are generally small, usually not exceeding 10 cm in height, with solitary or colonial growth forms. Their body shapes range from vase-like to cylindrical.
• Body Structures: They can exhibit one of three types of body structures: asconoid, syconoid, or leuconoid, each representing different levels of complexity in their internal canal systems.
• Skeletal Composition: The skeleton consists of calcareous spicules, which can be monaxon (single axis), triaxon (three-rayed), or tetraxon (four-rayed), providing structural support.
• Habitat: These sponges are found solely in marine environments, contributing to the biodiversity of coral reefs and other marine ecosystems.

Orders within Class Calcarea

1. Order Homocoela (Asconosa)
   • Body Plan: Homocoelan sponges possess asconoid body plans, characterized by simple, cylindrical, and radially symmetrical forms.
   • Structure: The body wall of these sponges is thin and not folded, with choanocytes lining the spongocoel directly, facilitating water flow and nutrient capture.
   • Growth Form: They often adopt a conical shape, adapting to their specific marine surroundings.
   • Examples: Representative species include Leucosolenia and Clathrina.
2. Order Heterocoela (Syconosa)
   • Body Plan: Sponges in this order display syconoid and leuconoid structures, indicative of a more complex organization compared to asconoid forms.
   • Structure: These sponges have a thicker body wall with distinct folding. Choanocytes are present in the flagellated chambers or radial canals, rather than lining the spongocoel, which is instead covered by flattened endoderm cells.
   • Growth Form: Members of this order can be solitary or colonial, often presenting a vase-like appearance.
   • Examples: Well-known species include Sycon or Scypha and Grantia.

Class 2: Hexactinellids

Class Hexactinellida, also known as glass sponges, comprises a unique group within the phylum Porifera, distinguished by their silica-based skeletal structures and often intricate, lattice-like forms. Found primarily in the depths of tropical seas, these sponges exhibit a range of moderate sizes, with some species reaching lengths of up to one meter. Below are key characteristics and the orders within this class:

Key Characteristics

• Size and Form: Glass sponges can grow to moderate sizes, with certain species extending up to a meter. They typically display cup, urn, or vase-like shapes.
• Skeletal Structure: The skeletons of Hexactinellida are composed of siliceous spicules, which are triaxon and feature six rays. In some species, these spicules fuse together, creating a distinctive lattice-like skeleton.
• Body Composition: Unlike other sponges, Hexactinellids lack an epidermal epithelium. Instead, their inner chambers, lined with choanocytes, facilitate water flow and nutrient uptake.
• Habitat: These sponges are predominantly found in the deep waters of tropical seas, often attached to the seafloor.

Orders within Class Hexactinellida

1. Order Hexasterophora
   • Spicule Form: Hexasterophora are characterized by their hexaster spicules, which resemble stars, with axes branching into rays at their ends.
   • Chamber Arrangement: The flagellated chambers in these sponges are regularly and radially arranged, contributing to their unique internal structure.
   • Attachment: These sponges typically anchor directly to the substrate, securing their position in the deep-sea environment.
   • Examples: Notable species include Euplectella, also known as Venus’ flower basket, and Farnera.
2. Order Amphidiscophora
   • Spicule Form: Amphidiscophora sponges possess amphidisc spicules, featuring a convex disc with backwardly directed marginal teeth at both ends, providing structural support.
   • Chamber Variation: The flagellated chambers in Amphidiscophora show slight deviations from the typical structure found in other Hexactinellida.
   • Attachment: These organisms attach to the substrate using root tufts, ensuring stability in their deep-sea habitats.
   • Examples: Representatives of this order include Hyalonema and Pheronema.

Class Hexactinellida represents an intriguing and visually striking group of sponges within the marine ecosystem. Their unique silica-based skeletons not only contribute to their resilience in deep-sea environments but also add to the aesthetic and biological diversity of the oceanic world.

Class 3: Desmospongiae

Class Demospongiae represents the most diverse class within the phylum Porifera, encompassing a wide array of sponge species ranging from small to large in size. These sponges exhibit various forms, including conical, vase-like, cup-shaped, or cushion-like bodies, and inhabit mostly marine environments, with a few species found in freshwater. Their skeletal structure may consist of siliceous spicules, spongin fibers, both, or be completely absent. Here are the key characteristics and subclasses within Demospongiae:

Key Characteristics

• Species Diversity: Demospongiae contains the largest number of sponge species among all sponge classes.
• Size and Form: These sponges vary in size and can be solitary or colonial, with body shapes that include vase, cup, or cushion forms.
• Skeletal Composition: The skeleton is made up of siliceous spicules (monaxon or tetraxon), spongin fibers, or a combination of both, or it may be absent. Spicules are differentiated into large megascleres and small microscleres but are never 6-rayed.
• Body Structure: The canal system in Demospongiae is predominantly of the leucon type, with choanocytes confined to small rounded chambers.
• Habitat: While most species are marine, there are a few freshwater forms.

Subclasses and Orders within Demospongiae

Subclass I: Tetractinellida

• Form: Sponges are mostly solid, rounded, cushion-like, and flattened, usually without branches, and can range from dull to brightly colored.
• Skeleton: Primarily composed of tetraxon siliceous spicules, except in the Myxospongida order where spicules are absent.
• Habitat: Mostly found in shallow waters.

Orders within Tetractinellida:

1. Myxospongida: Characterized by a simple structure with an absence of spicules.
2. Carnosa: Features a simple structure without differentiated megascleres and microscleres.
3. Choristida: Contains both large and small spicules.

Subclass II: Monaxonida

• Variety: Occurs in various shapes, from rounded masses to branching types, and may be elongated or stalked with funnel or fan-shaped bodies.
• Spicules: Monaxon spicules are present, with spongin being either present or absent.
• Habitat: Widely distributed throughout the world, mostly in shallow waters, some in deep seas, and a few in freshwater.

Orders within Monaxonida:

1. Hadromerina: Features monaxon megascleres in the form of tylostyles.
2. Halichondrina: Characterized by monaxon megascleres, often of two types.
3. Poecilosclerina: Contains monaxon megascleres of two types, situated in different layers.
4. Haplosclerida: Has monaxon megascleres of only one type and typically includes spongin fibers.

Subclass III: Keratosa

• Appearance: Bodies are rounded and massive, often with conspicuous oscula.
• Composition: Known as horny sponges, their skeletons consist solely of spongin fibers without spicules.
• Habitat: Predominantly found in shallow and warm waters of tropical and subtropical regions.

Class Demospongiae encompasses a wide variety of sponges with diverse forms and structural complexities. Their adaptive strategies and ecological roles contribute significantly to marine biodiversity and ecosystem function.

Skeletal system in sponges

Sponges have an endoskeleton that gives their bodies structure and form. The skeletons of all sponges are implanted in the mesenchyme. The skeleton is composed of distinct spicules, interlacing spongy fibres, or both. The skeleton provides support and protection for the soft body portions of sponges. The skeleton is also the basis for classifying sponges into classes such as Calcarea, Hexactanellida, and Desmospongia.

Spongin

Spongin is an organic, sticky, elastic material whose chemical makeup mimics silk. It is a sulfur-containing scleroprotein that is chemically related to collagen. Several types of spongin fibres are found in Desmospongia. It may exist as a bonding agent between siliceous spicules. It can be found in the form of branching fibres that contain siliceous spicules. In Keratosa, spicules are missing and only spongin is produced.

Development of spongin

Spongin fibres are released by spongioblast cells, which are flask-shaped mesenchyme cells. During development, the spongioblast cells are organised in rows, and their spongin rods combine with neighbouring cells to create a fibre. After secreting a given amount of spongin, the spongioblasts vacuolate and are ultimately degraded.

Spicules

Structure and types

Spicules are small crystalline structures that provide stiffness and shape to sponges. Spicule consists of radiating spines or rays from a point. They are secreted by scleroblast cells, which are specialised mesenchymal amoebocytes. The following are examples of several types of spicules:

Based on the sort of deposit on the organic matter core: All types of spicules have an organic core that is surrounded by either calcium carbonate or colloidal silica. Consequently, there are two types of spicules:

1. Calcareous spicules: Calcareous spicules are composed of calcium carbonate or calcite as its organic component. This is a feature of the class Calcarea sponges.
2. Siliceous spicules: Colloidal silica or Silicon is the organic component of this form of spicules. These kinds of spicules are characteristic of the class Hexactanellida sponges.

Spicules can be large or little according to their size and function. Consequently, there are two types of spicules:

1. Megascleres: Megascleres are the largest spicules that make up the sponge’s primary structure.
2. Microscleres: They are the microscopic spicules that occur interstitially.

Based on the number of axes and rays: Spicules can appear in various shapes, including basic rods, forks, anchors, shovels, stars, plumes, etc. The morphologies of spicules depend on the number of axes and rays present. Thus, they can be categorised as follows:

1. Monaxon: These spicules are created by growth along a single axis in monaxon. They may be straight, resembling needles or rods, or curved. Their terminations might be pointy, hooked, or knobby. Both calcareous and siliceous monaxons exist.
2. Triaxon: These triaxon spicules contain three axes that intersect at right angles to create six rays. Thus, it is also known as a hexactinal spicule. These triaxon spicules are characteristic of the class Hexactanellida glass sponges.
3. Polyaxon: These are the spicules of a polyaxon, which have numerous equal rays emanating from a central point. They can be grouped to give the appearance of stars. Microscleres are seen alongside polyaxon spicules.

Development of spicules

Calcareous spicules are secreted by sclerocytes, a specialised type of cell. These sclerocytes are produced from mesenchymal scleroblasts with two nuclei. A set of two sclerocytes secretes a monaxon spicule or each ray of the triradiate spicule.

One of these sclerocytes functions as a thickening cell, while the other serves as the founder cell. The creation of the spicule is initiated by the deposition of a calcium carbonate particle between the two nuclei of binucleated mesenchymal cells. This particle grows, separating the two nuclei, resulting in the formation of two sclerocytes. Now, the thickening cell deposits an additional layer of calcium carbonate, increasing the spicule’s thickness. After the spicule is fully developed, both the thickening cell and founder cell migrate into the mesenchyme. The calcoblast is the scleroblast that secretes a calcareous spicule, while the silicoblast is the scleroblast that secretes a siliceous spicule.

Economic Importance of Phylum Porifera

The sponge phylum Porifera has great economic significance. Here are several examples:

• Bath sponges: Bath sponges are most frequently used for bathing and cleaning. Many commercial sponges are collected in the wild or cultivated in aquaculture facilities.
• Biomedical research: Several species of sponges produce bioactive substances with potential medical applications, including as anti-cancer, anti-inflammatory, and antimicrobial characteristics, according to biomedical studies. These chemicals are being studied for therapeutic development purposes.
• Food source: In some regions of the globe, certain types of sponges are consumed as a delicacy.
• Aquaculture: Certain species of sponges can be cultivated in aquaculture facilities and sold for a variety of applications, including aquarium filtration and water treatment.
• Habitat creation: Sponges provide essential habitat for a wide range of marine species, including fish and invertebrates. Moreover, they can prevent erosion by stabilising the seafloor.
• Tourism: Sponges are a great draw for divers and snorkelers, thereby contributing to the local economy through tourism.

The economic value of sponges underscores the need for sustainable management and conservation approaches to ensure their future availability and use.

Examples of Porifera

Sycon

• These are solitary or colonial marine sponges found adhering to rocks in shallow waters.
• Many spores cover the body’s cylindrical form. The radial canal is composed of cells with flagella.
• Water enters the body through the Ostia and travels through the prosopyles to the radial canals. These species are capable of both sexual and asexual reproduction.

Cliona

• They are commonly found in coral skeletons, mollusk shells, and other calcareous structures and are also known as Boring Sponges.
• They are coloured green, purple, or pale yellow.
• Leuconoid sponges are distinguished by their canal system, and they reproduce both asexually and sexually.

Hylonema

• These aquatic organisms are also known as glass rope sponges.
• The body is circular or oval with root tufts that are twisted.
• There are little amphidiscs in the skeleton.

Spongilla

• They are prevalent in ponds, streams, and lakes, where they grow on submerged vegetation and sticks.
• The body wall consists of a thin dermis with Ostia pores.
• They possess a canal system like a rhagon. They reproduce both sexually and asexually.

Euplectella

• These are located in deep waters and are also known as Venus flower baskets.
• The cylindrical, long, and curving body is embedded in the mud at the ocean floor.
• The canal system is a straightforward synconoid kind.
• The skeleton is composed of fused siliceous spicules that create a three-dimensional network with parietal gaps.

FAQ

References

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