external image spongebob_2004-12-29.jpgKINGDOM: ANIMALIA


What are they?

Sponges, of the phylum porifera, are porous (have holes), sessile as adults (stay in one place), multicellular organisms. As larvae, however, they are free-floating. They, of all organisms, are closest to the choanoflagellates that gave rise to the animal kingdom. Sponges are often mistaken for plants, which they are not. The cell layers that make up a sponge are loose organizations that are not quite tissues because the cells are not specified. There are 9,000 or so various species of sponges. Only about 100 occupy fresh water environments, while the rest are marine. Porifera have no definite symmetry, have a multicellular body with few tissues and no organs or body cavity, reproduce sexually or asexually, and have no nervous system. There are approximately 15,000 species of sponges, all of which belong to one of three groupings. These groups are the glass sponges, the calcareous sponges and the demosponges.
Sponges have medicinal potential due to the presence in sponges themselves or their microbial symbionts of chemicals that may be used to control viruses, bacteria, tumors and fungi. http://en.wikipedia.org/wiki/Sponge#Antibiotic_compounds

What do they look like and how are they structured?

external image spongeanatomy.jpg

Sponges can be asymmetrical or radially symmetrical, and they come in a variety of sizes, colors, and shapes. Sponges occupy both freshwater and marine environments, at a variety of depths. Sponges are often found places including coral reefs, mangroves, and seagrass ecosystems. Sponges vary in size from 1 centimeter to 2 meters tall. Their basic body structure is like a sac punctured with holes. The word porifera literally means "pore bearer". Sponges draw water in through their pores into the central cavity or spongecoel, after which it flows out a larger opening called the osculum. The pores from which the sponges take in water are called the Osita. The choanocyte inside the spongecoel contains flagellum, string apendages that create a current of water through the sponge. More complex sponges have several oscula as well as branched water canals and folded body walls. The cells around the pores and oscula are capable of contracting enough to close the openings when needed. Sponges have no muscle or nerves, but the individual cells of the organism can react to the environmental changes they sense. Sponges have no organs or specialized tissue. Flagellated cells move water into the cell's central cavity through the pores, and the sponge's individual cells excrete waste, take in oxygen, and digest food found in the water.

Two layers of cells, separated by gelatinous mesophyl make up the sponge. Amoebocytes, called so for their pseudopodia, travel through the mesophyl. They have many functions: taking up food, digesting it, and carrying it around the sponge to other cells. In some types of sponges, there are sharp spicules made of calcium carbonate (silica). Other types produce flexible fibers that are composed of spongin, a collagen protein.
Sponges are made of four different cells including collar cells (line the interior), porocyte cells (make up the pores), epidermal cells (form the outer layer), and amoebocytes (in the mesohyl, help tranport nutrients, and produce organs for sexual reproduction).
Sponges have three different types of body plans, but these do not determine taxonomic groups. Asconoids are tube-shaped with several pores. They have a water canal that runs straight through the body. Many asconoids live in groups and attach themselves to an object. Synconoid sponges are larger with thicker body walls that form the water canals into the osculum and out of the spongocoel. Leuconoids are the largest sponges made of masses of tissue and have a complex canal system).

General structure of sponges including the basic body parts
General structure of sponges including the basic body parts

Porifera anatomy. (AS)
Porifera anatomy. (AS)


Sponges are suspension feeders. They trap food from the water that circulates throughout their porous bodies. In order to grow just by 100 grams a sponge must filter 1,000 kg of water! The flow of water through the sponge is unidirectional, driven by the flagella which line the surface of the chambers connected by a series of canals. Some sponges create a low pressure system above their excurrent opening, which pushes water out of the sponge. They are able to control the flow of water through them by closing certain openings and opening others. They usually eat the bacteria the flows them, which is caught by filtering. Others eat small bits of organic matter that passes through them and others species have algal symbionts, which give them nutrients. Sponges in the cladorhizidae family are actually carnivorous, they eat small crustaceans whole. They trap them their spicules and digest them extracellularly. Sponges regularly change their body shape to improve their filter feeding system. This constant change occurs through amoeboid (resembling an ameba in changeability of form) movements of cells inside the sponge and their change from one form to another.


Water flow in Porifera


Sponges are hermaphrodites (from the Greek Hermes and Aphrodite meaning both male and female). They produce both sperm and eggs. Sponges produce either sexually or asexually. Asexual reproduction is by means of external budding, and occasional internal budding. Budding is when a new organism grows directly from ( a specialized or sometimes random) the parent organism. This bud can stay attached to the sponge, and become part of a sort of colony, or it can detach and anchor in a new place. Internal Budding (found in freshwater sponges) is when gemmules (the internal growths) begin growing in the sponge and beocme independent organisms when their parent sponge dies.
Sponge species which sexually reproduce release the "male" gametes into the water where it is absorbed by its neighbors the same way they obtain food. The spermatozoa (the male gemetes) then change into amoeba like cells and make their way to the eggs in the mesophyl. Gametes come from choanocytes or amoebocytes, while eggs reside in the mesophyl. Sperm cells are carried out of the sponge by the water current. Cross fertilization occurs in the mesophyl where zygotes then develop into flagellated larvae. The swimming larvae disperse from the parent sponge, and upon finding a suitable surface and substratum, the larvae develop into sessile adults.

Sponges are also capable of regeneration - or the replacement of missing parts. Porifera use regeneration for both repair and to reproduce asexually from fragments broken off the parent sponge. When conditions deteriorate, for example as temperatures drop, many freshwater species and a few marine ones produce gemmules, "survival pods" of unspecialized cells that remain dormant until conditions improve and then either form compeltely new sponges or re-colonize the skeletons of their parents. http://en.wikipedia.org/wiki/Sponge


Reveiw Questions:
1. What two ways do sponges reproduce and explain each briefly.
2. What properties of sponges deems them animals? Why might we mistake sponges for plants?
3. In order to improve their feeding system, what do sponges do to themselves?

Campbell, Neil A., and Jane B. Reece. Biology. Sixth Edition. Boston: Benjamin-Cummings Company, 2002.
"Choanocyte - What is a(n) choanocyte | Encyclopedia.com: Dictionary of Zoology." Encyclopedia - Online
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