Phylum Porifera is placed in the kingdom Animalia , whose members are metazoans having a cellular organization. Porifera is a term, which merely refers as pore bearers that include “Poriferans” or pore bearing animals.
There are approximately 5000 species in a phylum Porifera. Initially, they were thought to be in the Plantae kingdom, because of green colour and symbiotic relationship with algae. On further research about their life cycle and feeding system, they were included in the kingdom Animalia.
Content: Phylum Porifera
Meaning of Porifera
Porifera can define as a phylum of kingdom Animalia that includes pore bearing species those are having a spongy texture and no plane of symmetry. These were the first multicellular invertebrate animals, those lack a division of labour or the presence of well developed tissues and organs.
Porifers also called “Sponges” because of their spongy appearance. The multiple minute pores in the poriferans are termed as “Ostia“, while a common large pore is termed as “Osculum” . Poriferans have no fixed shape and all possess an internal skeleton and a large cavity in between called “Spongocoel”. It includes members like Sycon, Spongilla, Euspongia, etc.
The phylum porifera possesses some unique features than the other multicellular organisms, which we will discuss below:
All the poriferans are free-living aquatic animals, where the majority of them found in marine habitats except for the members belonging to the family “Spongillidae” which lives in freshwater lakes and streams. They are found in shallow ocean environments to depths as great as five kilometers.
Sessile like plants
The mature sponges are sessile and sedentary i.e. they are found permanently affixed to the solid substratum like clamshells, rocks, etc. They cannot move here and there, and firmly attached to the sea bed, and grow like plants. The body shape is non-uniform and asymmetrical.
Outer surface is punctured by innumerable pores or Ostia that provide channels for the entry of water. Also, there are a few wide openings called oscula that spill excess water. In several freshwater sponges, the contractile vacuole is present. They can absorb and withhold fluids.
Sponges are the lower multicellular organisms, without having distinct tissue and organ system. These are cellular, diploblastic organisms, as their body wall divides into an outer pinacoderm (derived from epithelial cells) and an inner chaonoderm (derived from endothelial cells), by having a mesenchyme layer in between. Sponges deprive an excretory, respiratory, and circulatory system. They possess sensory cells, but not a well-differentiated nervous system.
Sponges have an interior space called spongocoel that may appear as a hollow cavity or infuse with several canals lined by collar cells with a thread-like flagellum.
Mode of nutrition
It is holozoic, where the specialized collar cells internalize the tiny, suspended food particles and filter them out for the digestion by the amoeboid cells. Due to this fact, sponges are also known as “Filter feeders“.
Food internalization and digestion
They lack a mouth and digestive cavity. Amoeboid cells in the mesenchyme layer mediate the movement of planktonic food and water throughout the body. The mechanism of digestion is intracellular, occurring inside the food vacuole of collar cells. Food is stored in the thesocytes.
It occurs by both asexual and sexual methods. Asexual reproduction occurs by budding, while sexual reproduction occurs via ova and sperms. In budding, the side or base of the sponges protudes out and form a new organism, and when it gets detached it develops a separate organism.
All sponges are hermaphrodite containing both male and female sexual properties. Sponges eject sperm and ova synchronously into the water. If gametes fuse from the same species, they form a larval sponge that undergoes planktonic drifting and then matures into an adult sponge. Larval sponges colonize on suitable locations and adult sponges remain sessile throughout their lifecycle.
Fertilization and development
Fertilization is generally internal, but cross-fertilization can also takes place. The process of development is also indirect by a free-swimming, ciliated-larva called amphiblastula or parenchymula. Cleavage is generally holoblastic.
The sponges can regenerate as colonies from a detached cell.
Sponges are usually categorized into ascon, sycon, and leuconoid type. The internal skeleton may comprise either fine spongin fibers, siliceous spicules, or calcareous spicules.
Examples: Sycon, Grantia, Hyalonema, Cliona, Spongilla, Euspondia, etc.
Certain species of shrimp, echinoderms live symbiotically inside the chambers of sponges and feed on the particles that are flowing through the chambers. Through this association, sponges neither get benefitted nor harmed, and their symbionts gain benefits. This type of symbiosis is called commensalism.
Classification of Phylum Porifera
Phylum Porifera is classified into three classes:
- They live in marine, shallow, and coastal water.
- Choanocytes appear as conspicuously large cells.
- It possesses a calcareous spicule skeleton build of calcium.
- The body appears cylindrical that exhibits radial symmetry.
- The body organization is of asconoid, syconoid, or leuconoid type.
- Eg., Clathrina, Scypha
- They live in the marine and the deep sea.
- Its comprises a six-rayed siliceous spicule skeleton, build of silicon.
- The body possesses cylindrical-shape with radial symmetry.
- The canal system is branched or unbranched.
- The canal system is of sycon or leucon type.
- Eg., Euplectella, Hyalonema
- They live in marine or freshwater.
- The body is asymmetrical.
- The canal system is of leucon type.
- It comprises spongin skeleton fibers and siliceous spicule skeleton of monoaxon and triaxon type.
- Eg: Spongia, Spongilla, etc.
Phylum Porifera Anatomy
Epithelial cells form an outer body, which provides protection and flexibility to all the sponges. All sponges share a characteristic feature in common, by having pores that give the phylum its name.
The pores are formed by a kind of tubular epithelial cells (porocytes) that facilitates the exchange of water and planktonic food and oxygen from the surroundings inside the cavity. The small pores are called Ostia (water enters into), while the large pores are called oscula (water exits out) of the porifers.
Collar cells or choanocytes are the tubular structures that form the inner surface of the porifers and are provided with flagella. The back and forth movement of flagella maintains a steady water flow into the ostia to bring food and oxygen, and out the osculum to remove wastes.
The fine tubular collar cells filter the suspended planktonic food and pass it to the amoeboid cells for digestion. For this reason, sponges are described as filter feeders. There is a gelatinous material found within the outer epithelial and inner collar cells.
This jellylike layer called “Mesohyl” that comprises free-moving amoebocytes, which perform diverse functions like the formation of the internal skeleton, digesting and transferring nutrients, etc. The amoebocytes form the skeletal elements like:
- Soft spongin fiber, build of flexible fibrous spongin.
- Stiff spicules provide rigidity to the porifers and categorized into monoaxon, triaxon, tetraaxon, and polyaxon type, depending upon its varying shape. Spicules possess various shapes, sizes, and compositions. Siliceous sponges have spicules made of silicon. Calcareous sponges have spicules build of calcium.
- Certain Porifers have both spicules and spongin.
Therefore, we can conclude that sponges were the first multicellular organisms whose body organization contains only cells that carry out different physiological processes of porifers. The members of Porifera deprive distinct tissues and organs system that are specialized to perform different tasks.