Introduction
Phylum Porifera, also known as sponges, is a group of simple aquatic animals that lack true tissues and organs. Porifera phylum animals are the lowest multicellular animals belonging to the animal kingdom. Porifera word is taken from Latin which means pore-bearing species. Around 5000 species are included in the phylum Porifera. Porifera animals have pores which are known as Ostia. These are the first organism Pore-bearing multicellular animals.
Poriferans are spongy in appearance that’s why they are known as sponges. Porifera is attached to the substratum. They get their food via absorption. Firstly due to their green appearance, they are included in the plant kingdom but later on, due to their symbiotic association with algae and life cycle, they are included in the animal kingdom.
The phylum Porifera comprises the sponges. Sponges are simple invertebrate animals that live in aquatic habitats. Although the majority of sponges are marine, some species live in freshwater lakes and streams. They are found in shallow ocean environments to depths as great as five kilometers (km).
All adult sponges are sessile, meaning they live permanently attached to rocks or other submerged objects and do not move about on their own. Some sponges grow in thin encrusting layers over surfaces.
A few species can even bore into hard surfaces like clam shells, coral skeletons, and rock. Many sponge species grow upright in branching tree-like or tubular vase-like forms. While some sponges, like the giant barrel sponges of the Caribbean, reach several meters in diameter, most sponges are small organisms that often go unnoticed on the reef or seafloor because they don’t look like other, more familiar, animals or noticeably move.
Many of the small sponges that hide under rocks or live on coral reefs are colored in vivid hues of red, yellow, orange, purple, crimson, sky blue, and ultramarine.
Anatomy
The phylum name Porifera means pore-bearing. Sponges take their name from small holes that cover their bodies. The history of life is written on the bodies of the animals that previously inhabited and continue to inhabit earth.
As we learn about the different invertebrate phyla, we will be able to chart some of this history. Sponges are considered to be one of the simplest animals, primarily because their bodies are not organized in organ systems or even tissues. Rather, sponges are made up of a grouping of cells that work together to contribute to meeting the daily needs of the sponge.
Epithelial cells form a skin-like layer on the outer surface of a sponge . These cells protect and enclose the sponge; they can contract and shorten, moving the sponge body slightly. All sponges are filled with the pores that give the phylum its name.
The tiny holes are actually the hollow insides of porocyte cells, a special type of epithelial cell. Porocytes are narrow and elongated cells that connect the outside of the sponge to the inside cavity.
These pores are sometimes also referred to as ostia, and they provide openings for water, which carries planktonic food and oxygen, to enter the sponge body.
Simple vase-like sponges have a single large top opening, called the osculum through which water leaves the sponge. Most compound sponges have many oscula all over the body of the sponge.
The oscula are surrounded by cells and are bigger than the ostia. Epithelial cells around the osculum can contract enough to close the opening, but the process is slow (up to several minutes).
The inner surface of the sponge is lined with cells called collar cells, also known as choanocytes. The collar is made of fine tubes surrounding a long whiplike thread called a flagellum. As flagella (plural of flagellum) in the collar cells move back and forth, they create a current of water that moves into the ostia and out the osculum.
Several gallons of water can circulate through a fist-sized sponge in a single day, bringing in tiny food particles such as suspended bacteria, bits of plant and animal matter, and tiny drifting planktonic organisms.
As the water circulates, the fine tubes of the collar cells filter out the food particles and take them into the cells for digestion. For this reason sponges are described as filter feeders.
Between the outer surface of epithelial cells and the inner surface of collar cells is a jellylike material. In this jelly are the structures that support the sponge. There are also free-moving cells called amoebocytes, which can move throughout the jelly layer.
During feeding, some of the particles taken in by the collar cells are passed on to amoebocytes, which carry them to other cells of the sponge. Several kinds of amoebocytes serve special functions, like producing the sponge skeleton, digesting and transferring nutrients, or reproducing themselves.
The skeletal elements of the sponge are produced by the amoebocytes. The amoebocytes produce spongin, the soft fiber that forms natural bath sponges. These sponges feel soft and springy to the touch because they have soft skeletons made of flexible fibrous spongin.
Other sponges have a stiff skeleton that feels prickly because it is made of hard, sliver-like spicules, which are also built by the amoebocytes. Some sponges have both spicules and spongin and feel both prickly and flexible.
Many species of sponges can be identified by the shape and composition of their spicules. Siliceous sponges have spicules made of silicon. Calcareous sponges have spicules made of calcium.
Spicules also have many shapes and sizes. While some sponges have no spicules, others have so many that they look and feel like lacy skeletons of glass.
All cells in a sponge are in contact with or near to seawater. Because each cell exchanges oxygen and carbon dioxide and discharges waste products into the seawater, a sponge has no respiratory, circulatory, or excretory system.
Reproduction
Sponges can reproduce either asexually or sexually. Asexually reproduction (without eggs and sperm) often occurs by budding, similar to growing a new branch on a tree. Cells on the side or base of the parent begin to bulge out and form a new organism. The buds may remain attached to the parent, or they may detach and settle down nearby to form a separate organism.
Sponges also reproduce sexually when specialized gametocyte cells produce sperm and eggs. Sponges undergo synchronous spawning and eject sperm and egg cells into the water. If gametes (sex cells; either sperm or egg) from the same species meet, they form a larval sponge.
After a period of planktonic drifting, the larva settles to a suitable location on the bottom and grows into an adult sponge. The drifting larval stage means that sponges can colonize new locations, even though as adults they remain attached in a sessile lifestyle.
Freshwater sponges can live in areas that are subject to cyclical wet and dry periods. They have a special strategy to help them deal with these harsh conditions. Freshwater sponges can produce a “resting” stage called a gemmule.
A gemmule is a small, encysted bud that can tolerate being dried out for a long period of time. When the gemmule is exposed to water, it can resume development as a sponge.
Organisms that can undergo a phase where they are dormant to survive harsh conditions are said to be in cryptobiosis (from the root words crypto meaning hidden and bio meaning life), because they do not appear to be living. In reality, these organisms are in a state of suspended animation.
Characteristics of Phylum Porifera
The following are some characteristics of phylum Porifera:
1. They are present mostly in marine water but also present in freshwater.
2. Their bodies are uneven and loosely arranged.
3. Porifereans can either be symmetrical or asymmetrical.
4. No organs are present inside the body.
5. They are cylindrical.
6. They have a cellular level of organization.
7. Sponginin fibers are secreted by spongioblast and spicules by scleroblast.
8. Porifera reproduces asexually via fragmentation or budding.
9. Numerous pores are present over the body which is known as Ostia.
10. The cavity present inside the body is known as spongocoel which is open to the surrounding via osculum.
11. They take food via a Holozoic mode of nutrition.
12. Porifera animals do regenration.
13. Indirect development and holoblastic cleavage.
14. Respiration and excretion did via diffusion.
Classification of Phylum Porifera
Every one of the types of this phylum is assembled into three classes, depending mostly upon the idea of the skeleton. They are both marine and freshwater animals.
Calcarea
They all are present in both marine and freshwater.
Their skeleton comprises calcareous spicules which are forms of calcium carbonate.
The body is cylindrical
They show radial symmetry.
Body organization of calcera are asconoid, leuconoid and syconoid.
Examples: Clathrina, Leucosolenia, Scypha etc.,
Hexactanellida
They are present in marine and also in the deep sea.
Their skeleton shows six-rayed siliceous spicules.
The canal system is sycon and leucon type
The body is cylindrical and has radial symmetry.
Example: Euplectella, Hyalonema etc.,
Demospongiae
Present in both freshwater and marine water.
Leuconoid type canal system is present
The skeleton system comprises spongin fibers and siliceous spicules.
They are asymmetrical
The cylindrical body is present.
Example: Cliona, Spongia, Spongilla, Chalina etc.,
Symbiosis in Sponges
Many species of plants and animals live on or in some other organism in a close association of symbiosis (from Greek root words meaning “living together”). Large sponges have many small chambers where other organisms can live symbiotically.
Although the sponges rarely benefit from this arrangement, they do not seem to suffer harm, and their symbionts, the organisms that live in them, do gain benefits. This type of symbiosis is called commensalism.
For example, certain species of shrimp live in the chambers of sponges and feed on the particles that are flowing through the chambers.
Porifera Examples
Some the example of the Phylum Porifera are:
Sycon
These are marine sponges present either solitary or colonial which are attached to rocks. Cylindrical shape and have numerous spores. The flagellated cell is made from a radial canal. From the Ostia, water enters the body and reached radial canals via prosopyles. Sycon reproduces both sexually and asexually.
Hylonema
These are also called glass rope wipes present in marine water. It is round or oval with twisted tufts.
Cliona
They are also called Boring Sponges, found in coral skeletons, mollusk shells, and other calcareous articles. They are green, purple, or light yellow in variety. The canal system is one of the main characteristics of leuconoid sponge. They reproduce both sexually and asexually.
Euplectella
These are also called Venus flower baskets present in marine water. These are long cylindrical and curved present on the mud of the upper surface of the sea. Simple synconoid canal systems are present. Their skeleton consists of silicious spicules which are fused around their tips and form a 3-D structure.
Spongilla
They are majorly found in ponds, lakes, streams. The body is covered via the thin dermis, and small pores i.e., Ostia are present on the surface. Rhagon-type canal systems are present in Spongilla. They reproduce via both modes.
Sycon
Sycon is a marine sponge widely distributed in shallow water. Typically, it is found attached to shells, rocks and is sedentary. Predominantly, they harbor in temperate regions.
Systematic Position of Sycon
Kingdom : Animalia
Phylum : Porifera
Class. : Calcarea
Order. : Heterocoela
Family. : Sycettidae
Genus. : Sycon
Phylum Porifera
The phylum includes nearly 5000 species that are known.
Typically characterized by the presence of pores all through the body justifying the name Porifera, the ones who bear pores and are also referred to as sponges.
Generally, they are represented by Sycon, Leucosolenia, Euplectella, Hyalmonema, etc.
Sycons are typically small, they range from 2.5cm to 7.5cm in height. These tube-shaped structures resemble water vessels.
In some instances, they are also referred to as crown sponges because of the spicules they possess, arranged in the form of a crown while in some other instances, they are referred to as Q-tip or “Pineapple” sponges based on their resemblance.
Typically, Sycons are delicate in appearance, organized in an asconoid system wherein the body has a simple tube with a lack of foldings of the outer body wall.
Habit and Habitat
Sycons lead a solitary/colonial life. These marine entities are found attached to the solid substrate such as shells and rocks of corals and molluscs. Various species of sponges under this genus do not have the tendency to thrive deeper in the ocean. Sycons seem to thrive well in slow-wave action and low tides. They are found in the underside of rocks in comparatively protected regions amongst hydroids, bryozoans, and other entities. Some also grow as seaweeds.
Structure
There is a great diversity observed in the form of sponges ranging from simple to complicated entities. However, they still can be categorized as intermediate when it comes to structural diversities. It has branched cylinders which are connected at the basal end, intact to the substratum.
The body is flexible though it is secured firmly. A closer look at these entities reveals the presence of many tiny Ostia or inhalant pores. The free end of every cylindrical branch has an opening at the summit known as osculum.
They are fringed with calcareous monaxon spicules or oscular fringes which check the entry of any foreign substance. The body below the osculum is narrow, forming the collar region. The body surface has elevations emerging as spicules (oxeotes) rendering the bristle appearance.
Canal System
Sycons exhibit a canal system which is an anatomical peculiarity, unique to them. It penetrates the body with watcher channels. The canal type precisely seen in Sycon is Syncoid Stage I type, more advanced than asconoid canal system. The Ostia opens into a central cavity through a system of canals, the central cavity is known as spongocoel.
The body is organized into a complex network of canals and pores, which goes on to form the aquiferous or the canal system. The body wall is composed of Choanoderm, Mesenchyme and Pinacoderm.
Composition of the canal system:
Ostia – these dermal pores are lined by thin membranes. These possess two openings, one for entry and one for the exit. The closing and opening of Ostia for water flow is regulated by myocytes.
Sponogocoel/Paragastric or gastral cavity – osculum leads to a large central canal known as spongocoel, named variously. Its wall is lined by pinacocytes, which are ectodermal flattened cells. Osculum is engirdled by a layer of cells known as myocytes acting as sphincters, contractile in nature. Spongocoel appears to open to the outside through the osculum.
Radial canals – evaginations of the body wall. The body wall lining of the spongocoel is shoved periodically as finger-like projections known as radial canals whose walls are lined with choanocytes. The outer end of the radial canal is closed while the inner end is openly mediating with the spongocoel via the excurrent canal.
Incurrent canal – this tubular structure is found between two successive radial canals, hence are alternatively arranged. Invaginated folds of the body wall. The inner end of the incurrent canal is blind whose walls are lined by pinacocytes (flat and ectodermal). Gastral cortex, a thickened mesoglea is found between the radial and the incurrent canal. Ostia is located on the pore membrane engirdled by contractile myocytes acting as sphincters, checking the openings.
Prosopyles – these minute pores are found between the radial and incurrent canals. Each of these is an intercellular channel in Sycons, through which the incurrent canals open into the radial canals. Porocytes, to which prosopyles open into, are cylindrical and thick-walled, possessing a nucleus in the cytoplasm towards one end. It regulates the inflow of water and is highly contractile.
Excurrent canals: these canals are used by radial canals to mediate with the spongocoel and are wide and short canals whose walls are lined by pinacocytes.
Apopyles – these are engirdled by contractile myocytes and are the openings of the radial canals in spongocoel.
Current Of Water
Water circulation in Sycon follows this route. Water races into the body through innumerable Ostia along the external surface, each of which leads to the incurrent canal. From here, water flows into the radial canal through prosopyle. From the radial canal, water gushes into the spongocoel through the apopyles. Hence, spongocoel is the shared chamber in which all the radial canals of the body open into.
Finally, the spongocoel opens to the outside through the osculum, an aperture. Hence, water enters through numerous Ostia but exits through the osculum, which is a single opening. In the radial canals, the planar beating of the flagella of choanocytes generates a current that simultaneously draws in water and gushes it out too.
Microscopic Organization
A microscopic view shows the presence of a single layer of cells covering the outer surface, which is designated as the dermal layer. It is from here that the spicules seem to emerge. The layer possesses large cells known as pinacocytes.
Spongocoel possesses a lining of flattened endodermal cells while the radial canal is lined by unique collar cells, each possessing flagellum (a long whip-like structure). These are collar cells or choanocytes. Each choanocyte is oval or round shaped exhibiting vacuoles in their cytoplasm. The open end of the cell body has a relatively lengthier flagellum whose base is engirdled by contractile transparent collar-like projections of the cytoplasm.
Flagellum emerges from the basal granule associated with the rhizoplast. The collar-like projections consist of cytoplasmic tentacles, whose number ranges from 20-30. The skeleton of Sycon is made up of spicules that arise from the scleroblasts and are organized to safeguard the delicate parts. A common observance is the presence of tetraradiate or triradiate spicules. The mesenchyme is composed of a gelatinous proteinaceous matrix containing spicules and many amoeboid cells.
There are many types of amoeboids such as:
Collenocytes
Myocytes
Thesocytes
Archaeocytes
Gland cells
Chromocytes
Respiration
Simple diffusion causes exchange of gases between sponge cells and the flowing water.
Dissolved oxygen in the water, diffuses into the cells causing the oxidation of protoplasmic molecules, along with the release of energy captured in ATP.
ATP is critical in supplying energy to metabolizing cells.
Contractile vacuoles in the amoeboid cells of freshwater sponges are said to take part in osmoregulation and excretion.
Nutrition
Water current brings along microbes on which the sponges feed. They enter through the numerous Ostia.
They are filter feeders, subsisting on planktons and other organic substances.
The choanocytes engulf these and transfer them to amoeboid cells present under the choanocytes wherein digestion occurs. Here the products of assimilation are passed to different body parts.
Digestion is intracellular and nutrition here is holozoic. Some of the amoeboid cells have chlorophyll performing autotrophic nutrition.
In the flagellated chambers beating of flagella or collar cells, leads water to pass through collar causing food substances to cling to them.
Food particles are trapped with the help of microvilli of collars, serving as filters.
As a result, the engulfed food particles are taken to food vacuoles after the pseudopodial action of choanocytes at the bed of their collars.
Initially, the phase is acidic in food vacuoles and then alkaline.
Food here undergoes digestion partially, which is then transferred to amoebocytes present in the mesenchyme, which are in constant motion.
Amoebocytes cause complete food digestion, the residue is pushed through outgoing water current. In addition to this, amoebocytes also dispense digested food to all cells while some store it, to be used later.
Many enzymes are isolated from them including fat-digesting, starch and protein enzymes.
Reproduction
Both sexual and asexual modes of reproduction is observed. Asexually, they produce a bud, in some other instances they produce special structures that resemble gemmules of freshwater sponges. Both ova and sperms in sexual reproduction are produced from archaeocytes found in mesoglea. Sperm cells possess long tails, swimming freely in the water current while the ova are amoeboid, moving through the mesoglea.
Sperm cells do not enter directly into the ovum. When the sperm cells enter the radial canal, the nearest choanocytes to the egg captures it. Choanocyte that absorbs the sperm is the carrier cell, it ejects its flagellum and collar approaching the egg. With time, the sperm loses its tail and enters the egg. The earliest development stages are inside the mother sponge. Once the developmental stage is complete, the larva gushes into the radial canal and then to the outside.
