Poriferans

Phylum Porifera

         The phylum Porifera (the sponges) includes about 5000 species almost all of which are marine (there are more about 15o freshwaters species, member of the family Spongilidae. Sponge occur worldwide at all latitudes from intertidal zone to deep sea. Their range in size from a few millimeters to 2 meters accros. Porifera means "pore-bearing" and refers to the numerous pores and channels that permeate a sponge's body.

           Sponges are the simplest multi-cellular organisms, but they lack the germ layers of more complex metazoans, have a cellular level of organization lacking true tissues and organs. The Body is a mass of cells embedded in a gelatinous matrix (mesohyl) which is supported by a framework of spicules, as well as collagen and spongin fibers.Adult sponges can be asymmetrical or radially symmetrical and come in a variety of shapes: arboresecent (tree-like), flabellate (fan-shaped), caliculate (cup shaped), tubular (tube shaped), globular (ball shaped), and amorphous (shapeless) among others. 

How they feeding

         Sponges are sessile (they don’t move) and depend on water movement to bring in food and oxygen and remove wastes.Sponges generate their own flow of water having a unique water current system. Openings are connected by a series of canals, which are lined by choanocytes (the flagellated collar cells) that maintain the current and filter out food particles. The choanocyte’s collar consists of microvilli joined together by delicate microfibrils,which filter out tiny food particles. The beating of the flagellum draws water through the collar and out the top. Big particles to pass through the collar get trapped in mucus and slide down the collar to the base where they are phagocytized.

          Sponges can filter enormous volumes of water as much as 20,000 times the volume of the sponge in 24 hours. Sponges mostly consume bacteria and may filter as much as 90% of those passing through. Some sponges also supplement their filter feeding by hosting symbionts such as green algae, dinoflagellates or cyanobacteria that provide nutrients to the sponge. Corals (which are Cnidarians not sponges) similarly have symbiotic algae that live with them.

Sponge Cell Types

1. Archaeocytes

          Archaeocytes move around within the mesohyl.  They are ameboid in appearance and carry out several tasks. Archaeocytes are responsible for digesting food particles captured  by the choanocytes & store digested food material. Archaeocytes can differentiate into other specialized cell types. Secrete structural components, specialized on archaeocytes called sclerocytes, spongocytes and collenocytes secrete respectively spicules, spongin and collagen, great importance to:

- systematists, as an indispensable factor in species identification

- sponges, which generally depend upon the support elements for maintaining shape,       and possibly, for discouraging predation

2. Choanocytes

          Choanocytes (collar cells) engage in filter feeding. One end is embedded in mesohyl and the other end protrudes. The protruding end of the choanocyte has a flagellum that moves water through a mesh-like collar where small particles are trapped. Functions of collar cells:

- They generate currents that help maintain circulation of seawater within & through the spoge.

- They capture small food particles

- They capture incoming sperm for fertilization

3. Porocytes

       Porocytes: These are tubular cells that in the simplest type of sponge (asconoid sponges) form tubes through the wall of the sponge and allow water to flow into the central chamber.

4. Pinacocytes

            Pinacocytes: Layers of pinacocytes form a flat, thin epithelium-like layer (unlike true epithelium [a tissue] the individual cells are not joined by bands of extracellular proteins). Pinacocytes cover exterior and some interior surfaces. Pinacocytes have some ability to contract and some are arranged in bands around pores and use to regulate the flow of water in and out of the sponge.

Canal System on Porifera

Most sponges have one of three types of canal system:

  1. Asconoid

     Asconoid is the simplest system.  Water enters through pores into a single large central cavity (the spongocoel) which is lined with choanocytes. There is a single large osculum.

  2. Syconoid

     In syconoid canal systems there is still a single spongocoel and osculum, but the lining of the spongocoel is folded back to make radial canals lined with choanocytes.

  3. Leuconoid

      Leuconoid organization is most complex and permits an increase in sponge size. Most leuconoids form large masses with numerous oscula. Clusters of choanocyte-lined chambers receive water from narrow incurrent canals and drive water into excurrent canals that eventually reach the osculum.  There is no spongocoel.

Group of Sponge

There are three classes of sponges:

   1. Class Calcarea

       - Calcareous sponges whose spicules are made of calcium carbonate.

       - Tend to be small (<10cm) and tubular or vase shaped.  

       - May be asconoid, syconoid or leuconoid in structure.

          

   2. Class Hexactinellida
- Skeleton is made of six-rayed siliceous spicules bound in a glasslike lattice. Nearly all are deep sea forms.
- Body of hexactinellids consists of a single, syncytial tissue (mass of protoplasm containing many nuclei, but not divided into cells).
- This bilayered syncytium encloses a collagenous mesohyl and various cells including choanoblasts which extend into the flagellated chamber through openings in the reticulum.

   3. Class Demospongiae
- The Demospongiae includes about 80% of all species and includes the freshwater Spongillidae. Spicules siliceous, spongin or both.
- All members are leuconoid and most of the large sponges are members of Demospongiae. Loggerhead sponges may be a couple of meters in diameter.
- Includes the bath sponges, which have only spongin skeletons.

Sponge Reproduction

         Sponges reproduce both sexually and asexually. Most sexually reproducing species are hermaphrodites (individuals produce both male and female gametes at different times). Sperm are shed into the water and taken up by other sponges. Individuals with eggs use special cells called archaeocytes  to transport sperm to the eggs. Zygotes develop into ciliated larvae that are released into the water and eventually settle and develop into a sponge.

       Asexual reproduction is either by budding or more commonly the production of gemmules which are clusters of cells surrounded by a protective coat.

Conclusion