With members that were considered plants before 1765, the phylum Porifera consists of simple multicellular animals commonly known as sponges. Currently, the group is estimated to have over 8,000 species; the majority of which are marine. As the name suggests, their bodies are covered with small holes/pores. This is one of their primary characteristics.
Some examples of animals in this phylum include:
· Spongilla (E.g. Spongilla alba, Spongilla chaohuensis, Spongilla aspinosa, and Spongilla friabilis, etc)
· Cliona (E.g. Cliona sulphurea, Cliona griffithsii, Cliona coccinea, and Cliona Grant, etc)
· Sycon (E.g. Sycon album, Sycon antarcticum, Sycon asperum, and Sycon barbadense, etc)
· Hyalonema (E.g. Hyalonema abyssale, Hyalonema aculeatum, Hyalonema azuerone, and Hyalonema agujanum, etc)
· Euplectella (E.g. Euplectella crassistellata, Euplectella aspera, Euplectella gibbsa, and Euplectella jovis, etc)
As mentioned, members of the phylum Porifera known as sponges are simple organisms. As such, they don't have true organs. Unlike most animals, the majority of sponges are asymmetrical. This is mostly the result of continuous morphogenesis. However, some of the species display radial symmetry. Most are also small in size (a few centimeters) with varying shapes (tube-shaped, branched, etc).
However, some can grow to be over 6 feet tall and are easily recognizable. Like some polyps, the majority of Porifera species grow upright and thus resemble a branching tree. Also, some are not highly branched and have a vase-like form.
* The bodies of phylum Porifera species are also characterized by pores and canal systems.
* Like Cnidarians, sponges are diploblasts. As such, their bodies are made up of two basic germ layers (the ectoderm and the endoderm). Here, the ectoderm makes up the outer layer while the endoderm makes up the inner layer.
* Sponges come in different colors, ranging from brightly colored (red, orange, yellow, etc) to dark brown like those found in the deep sea. Although many of the species may have a vase, tubular or branched-tree appearance, others may be flattened or shaped like a ball. For the most part, the shape of a given sponge is influenced by environmental conditions (e.g. sponges found in rough waters are usually flattened).
Based on form/morphology, those in the phylum Porifera are divided into three main groups that include:
Asconoids have the simplest or the most basic body plan of all sponges. Members of this group are generally small in size and characterized by a vase-like morphology with radial symmetry. The outer wall is very thin and encloses a large spongocoel (lined with choanocytes- Flagellated cells involved in filtering particles).
Also known as the paragaster, the spongocoel is a large cavity in which water enters through the pores. Like all the other sponges, these organisms are covered with pores that extend from the spongocoel to the external surface.
These pores, also known as ostia, are made up of large tubular cells known as porocytes. Because of their ability to contract, porocytes control the opening and closing of the pores thereby regulating the movement of water into the spongocoel.
* In asconoid sponges, the pores are tiny and numerous.
* The uppermost part of these organisms consist of a small opening known as the osculum - This is the part through which water is expelled from the spongocoel.
* Most Asconoid sponges belong to the order Homocoela (Leucosolenida). Some examples of these species include; Leucosolenia albatrossi, Leucosolenia complicata, Leucosolenia cervicornis, and Leucosolenia arachnoides etc.
Compared to Asconoid sponges in the phylum Porifera, Synconoid sponge, belonging to the order Heterocoela, have a more complex body structure. They are larger in size with a thicker wall that consists of larger pores (ostia) through which water enters the spongocoel. Rather than directly opening into the spongocoel, these pores, made of pinacocytes, open into the narrow canals known as incurrent canals.
From the incurrent canal, water enters the radial canal (consisting of flagellated cells) through the prosopyle pores. In some syconoids, the radial canal opens into another chamber known as the excurrent chamber through larger pores known as apopyles leading water into the spongocoel.
Compared to ascon types, sycon types have a narrower spongocoel cavity that opens out through the osculum.
Depending on the complexities of the canal system, syconoid sponges can be further divided into three main groups that include:
Simple sycon - Radial canal projections do not touch and spaces between them serve as the dermal ostia.
Complex sycon - Radial canals are arranged in a manner that creates tubular spaces, inhalant canals, or incurrent canals. These sponges also possess a number of pores including the prosopyles and dermal ostia.
Sycon with a cortex - Sponges in this group are characterized by fused radial canals. The incurrent canals formed from the fusion of radial canals traverse through the cortex to reach the outer surface of the radial canals.
* Unlike Asconoid sponges, synchonoid sponges do not have choanocyte cells (flagellated cells) in the lining of the spongocoel. For this reason, digestion normally takes place in the canals.
* The spongocoel of these sponges is lined by epithelial cells.
* Some examples of species in this group include Sycetta species, Sycon ciliatum, Sycon protectum, and Grantia coompressa etc.
Leuconoid is the most complex sponge body plan. Compared to the other sponges, leucons (Leuconoid sponges) do not normally have a central spongocoel. Rather, the cavity is highly branched resulting in clusters of smaller chambers (round or oval in shape) that are lined by choanoflagellates.
Like other sponges, these sponges also have numerous dermal pores. Water enters the body (choanocyte chambers) through these pores and into the incurrent canal. From the chambers, water is then passed to the excurrent canal and out of the body through the series of oscula. Leucons have more than one oscula. They are also larger in size and can grow to be over 2 meters in height.
* Leucon chambers are filled with mesenchyme.
* Sponges in this group have the most effective water filtering system.
* The majority of sponges in the phylum Porifera are leucons. Some examples of sponges in this group include Spongilla, Stellata, and Tetilla, etc.
Being some of the simplest animals on the planet, members of the phylum Porifera only have a few different types of cells. As mentioned, sponges are diploblastic and only consist of two basic germ layers. In Ascons, the outermost layer consists of flattened cells known as pinacocytes.
These cells are well fitted together like slabs to form a covering tissue known as the pinacoderm resembling the epidermis found in other animals. The edges of these cells are characterized by wavy contours which enhance cell-cell adhesion through increased surface area.
The inner part, on the other hand, consists of specialized cells known as choanocytes. Unlike pinococytes, choanocytes (collar cells) consist of a single elongated flagellum surrounded by cilia. The beating action of the flagellum help in the suction and expulsion of water.
The third type of cells are known as porocytes. Porocytes are cylindrical cells and traverse along the wall of the organism. Incurrent pores along the length of these cells allowing water to be transported into the body. Through the beating action of choanocytes flagellum (pumping water away from the cell) water is expelled through the atrium while being forced into the body through the pores. This ensures that water only moves in one direction.
Another group of cells known as amoebocytes can be found in the body of sponges located in the mesenchyme/mesoglea. These cells are motile and are involved in several important functions including secreting sponge skeleton and removal of debris etc.
There are several types of amoebocytes which include:
· Archaeocytes - Involved in regeneration and nutrition
· Sclerocytes - Secretory cells (spicules)
· Spongocytes - Secretory cells (spongin)
· Collencytes - Secretory cells (collagen)
In the phylum Porifera, sponges have a skeleton which defines/influences the general shape of the organism. As the organism grows, materials required for skeleton development are produced by specialized cells (sclerocytes and spongocytes) in the mesenchyme.
Whereas sclerocytes secrete spicules, the spongocytes are responsible for the production of spongin fiber. The skeleton of a sponge may have either of the fibers or both.
In calcareous sponges, spicules typically consist of calcium carbonate. In siliceous sponges, on the other hand, spicules are mostly made of silica. Depending on size, spicules are divided into two main groups namely, megacleres and microscleres. Megascleres, which are larger in size make up the main supporting framework on the skeleton while microscleres (smaller spicules) permeate the mesenchyme.
In siliceous sponges, the spongin (a protein) may serve to form a branching network in the skeleton structure or bind the spicules together. In some of the sponges, however (e.g. keratosan sponges), the skeleton structure is only made up of spongin with some foreign particles.
In calcareous sponges, shape, rather than size, is used to determine the type of spicules.
Some examples of spicules based on their general shape include:
Monaxon spicules - Monaxon spicules come in different shapes ranging from C-shaped to rod and twisted forms. However, they are all characterized by a single axis which may be straight or curved.
Tetraxons - As the name suggests, these spicules are characterized by four points (rays) emerging from a common point. Here, however, it's worth noting that in different sponges, the four rays vary in size or may be modified into disc-like structures.
Triaxons - Also known as hexactinal spicules, triaxons consists of three axes that cross at right angles resulting in a structure with six rays. As is the case with tetraxons, these rays vary in size. They may also be curved with knobs or spines.
Polyaxons - This group consists of spicules with a number of rays emerging from a common point. As a result, they are often star-shaped while a few that look like spiny spheres.
Some of the other types of spicules include:
Spheres - Characterized by a concentric form.
Desma - Larger spicules on which silica is deposited. Desma spicules often form from other types of spicules (e.g. tetraxon, minute monaxon, or triradiate spicules).
* In addition to influencing the shape of the sponge, the skeleton also plays an important role in ensuring that the water canals and chambers remain open. As well, it also serves to protect the organism by creating a tough texture and forming spiny projections which discourage predators.
Compared to siliceous and calcareous sponges, Hexactinellids (also known as glass sponges) have a glass-like appearance. In these sponges, the skeleton is made up of a glass-like siliceous fiber.
In a few species, projecting spicules have been shown to help transmit light from a bioluminescent crustacean living in the spongocoel of the organism. This results in a unique display of light in deepsea.
Glass sponges are also characterized by a unique general structure. For instance, like a honeycomb, they have an open body plan with large open spaces. The dermal membrane (covering) consists of a syncytium (with underlying spongin protein) which is continuous with choanablasts.
Choanoblasts are responsible for giving rise to stolons connected to the collar bearing units (connecting these units to each other in some cases). Like some of the other sponges, these sponges also contain scleroblasts involved in the production of spicules. Unlike other cells, however, these cells are not connected to the syncytium.
Members of the phylum Porifera are exclusively aquatic and therefore only found in aquatic environments. As mentioned, the majority of species are marine. Some species can be found in freshwater lakes and streams. By 2012, researchers had identified about 238 freshwater species in six major families of the phylum Porifera.
Members of the group Spongillina, for instance, are found in freshwater habitats throughout the world except in the Antarctica. While some of the species are more abundant in different parts of the world with a wide biogeographic range (E.g. S. lacustris), others like E. muelleri display a patchy distribution range; only being found in abundance in specific regions.
In both freshwater and marine habitats, sponges can be found at various depths from shallow environments/intertidal to the abyssal zone (E.g. glass sponges). All the adult forms are attached to rock surfaces among other surfaces and do not move on their own. As such, they are described as being sessile.
Despite being very simple sessile organisms, sponges have a number of important adaptations that allow them to survive in their respective habitats. For instance, during unfavorable environmental conditions (e.g. low temperatures), species like H. loodanoffi exhibit low metabolism and can remain dormant throughout winter.
When conditions improve, there is a rapid generation of all cell types as the organism becomes more active. Others, like Pharetronids, exhibit degenerative behavior in freshwater with proliferation increasing when salinity is restored.
Despite having no to very little motility, sponges can discourage competition by releasing toxic substances in the water around them. This creates unfavorable conditions that prohibit some organisms from colonizing their habitat.
As well, they can live in a symbiotic relationship with other organisms (e.g. some bacteria and algae). This allows them to benefit from each other and therefore live in harmony. For instance, by accommodating some bacteria capable of photosynthesis, sponges have easy access to the food produced.
The shape of a sponge (influenced and maintained by the skeletal structure) is also an important adaptation to their respective habitats. Along the rocky coastline where waters tend to be rough, sponges have been shown to be more flattened and encrusting. This allows them to remain tightly attached to rock surfaces and not be easily washed away.
By having a flattened shape, the surface area in contact with rock surfaces is increased while the surface area exposed to rough waters (vertically) is reduced.
* The body of sponges is also well adapted to their feeding habit. Here, flagella of the cells that line interval cavity ensure that water moves in one direction making it easy to obtain as much food particles as possible.
In the phylum Porifera, sponges are capable of sexual and asexual reproduction. They are also dioecious which means that there are male and female individuals. However, some of the species are capable of changing sexes (from male to female and vice versa e.g. H. mmorei).
During this process, a few species have been shown to even change color. While there are male and female individuals capable of changing from one sex to another, it's worth noting that they do not have permanent reproductive organs (gonads). Rather, when ready for reproduction, certain parts of the body transform into sites of gamete production.
In phylum Porifera, sponges can reproduce asexually through three main methods.
Budding - Budding has only been observed in a few sponge species. Normally, a sponge can only reproduce through budding if part of the sponge contains collencytes and archaeocytes. Whereas collencytes produce mesohyl (mesenchyme/mesoglea), archaeocytes produce the different types of cells found in sponges.
During budding, a group of cells near the base or on one side of the organism multiply to form a bulge that develops over time to become a new individual. Eventually, the new organism breaks off from the parent and the process continues. In some cases, the new individual remains attached to the parent.
Fragmentation - This type of asexual reproduction occurs when fragments of a sponge detach/break off. This may be as a result of strong water currents or the actions of other animals. The fragment may then make use of pinacocytes and choanocytes for locomotion before reshaping the mesohyl and attaching to a rock surface or any other surface.
Once attached, the organism then rebuilds itself over time to become a new functional individual.
Gemmules - Gemmules are survival pods/buds produced by most freshwater sponges and some marine sponges. These contain a number of essential components including spongil, spicules, and clusters of archaeocytes. During unfavorable environmental conditions (e.g. dry periods), freshwater sponges release thousands of these pods when dying.
Because they are more resistant to cold, the lack of oxygen, and other harsh conditions, these buds can survive for many months. When conditions improve, the gemmules germinate giving rise to new individuals.
Sexual reproduction in the phylum Porifera is the primary mode of reproduction in all sponges. Given that sponges do not have permanent gonads, some cells like archaeocytes and amoebocytes may differentiate to form parts responsible for gamete production.
Depending on the species, fertilization may be internal or external. Here, external fertilization occurs when both the eggs and sperms are ejected out through the osculum into the water. However, in some of the species, only the sperm are expelled out into the water.
From the water, the sperm cells are captured by female sponges and transported to the egg cell. Following fertilization (within the mesenchyme), the zygote develops into a larva form (ciliated larval). The larvae may be retained in the sponge or released into the water where it attaches to a surface and develops over time.
Nutrition - Members of the phylum Porifera are filter feeders. This means that they survive by sieving out microorganisms and other organic debris from the water. As water passes through the pores into the canal system and out of the body through the sculum (oscula - plural), food particles are retained and taken into the cell cytoplasm within the cavity.
Whereas the flagellum serves to direct the flow of water, microvilli of collars form a mesh that traps food particles and microorganisms to be digested. Amoebocytes also take up some of these nutrients and release waste material through the excurrents so that they can be removed.
Respiration - Unlike most animals with either gills or lungs, sponges absorb oxygen directly through diffusion from the water as it enters the organisms and leaves through the osculum.
Nervous system and circulatory system - Sponges in the phylum Porifera do not have a nervous system or a circulatory system. However, ciliated cells are suspected to play an important role in sensing any form of disturbance. In the event of any disturbance, the osculum closes in an attempt to protect the organism.
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