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03. SPONGES & CORALS

3(a) Structure of Sponges

Sponges are members of the Phylum Porifera (meaning ‘pore-bearing’) and are the oldest of all multi-cellular animals. They have been around for approximately 750 million years. They lack true tissues but have many cell types that take on these functions. Sponges are exclusively aquatic, filter feeders that actively pump water through their bodies to eat, breathe and excrete.

Specialised Cells: Sponges have special collar cells (or choanocytes) that are unique in the animal kingdom. They have flagella, whip-like structures that work to set up water currents so the sponge can sieve food particles from the water. The water comes in through an inhalant pore and leaves the sponge via an exhalant pore. The special cells of the sponge include those that filter sea water; cells that are phagocytic (that engulf and digest food particles); those that form the external 'skin', breathing pores and tubes through which water enters and leaves the body; and those that secrete the mineral and organic skeletons, called spicules and fibres, respectively. Sponges are also unique because nearly all their cells can change function as required (totipotency).

Skeletons: Many species produce either silica (siliceous) or calcium carbonate (calcitic) skeletons, providing some structure to otherwise basically shapeless growth forms.

Adult sponges are sessile and firmly attached to the seabed, while the larvae are motile, short- lived, and crawl across the seabed before attaching to it and developing into adults.

Obvious pores in the colourful sponge, Clathria mima
Obvious pores in the colourful sponge, Clathria mima. Image: Dr. John Hooper, QM.
Sponge Cliona
Sponge Cliona sp., showing the canals through which water flows. Image: Dr. John Hooper, QM.

Diagram of a Sponge

Diagram of a Sponge
Arc – totipotent phagocytic cells (archaeocytes)
Bas – basipinacocytes (cells lining internal aquiferous system)
Cho – choanocytes (collar cells)
ChoCh – choanocyte chamber
Exo – exopinacocytes (cells lining exterior surface)
Fla – flagellum on choanocytes
Osc – exhalant pores (oscula)
Ost – inhalant pores (ostic)
Spi – spicules (siliceous or calcitic depending on the class)
Red Arrows - inhalant water current with food particles etc
Blue Arrows - exhalant water current with waste products.

Diagram by Dr. John Hooper in The Great Barrier Reef: Biology, Environment and Management
Editors P. Hutchings, M. Kingsford & O. Hoegh-Guldberg.  CSIRO Publishing, Copyright Australian Coral Reef Society, 2008. http://www.publish.csiro.au/pid/5921.htm


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