C. Body is platelike, no symmetry, organs, muscular or nervous systems present. D. 2-3 mm E. marine F. glide over food and secrete digestive enzyme an

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1 Chapter 9 (multicellularity) I. Similarities A. cells are grouped B. groups of cells are specialized for various functions C. All cells in an organism is interdependent. II. Multicelled life appeared ~ 550 mya III. 2 main ways this could have happened. A. colonial hypothesis: dividing cells remained together (similar to colonial protists) B. Syncytical hypothesis: formation of plasma membranes in the cytoplasm producing small multicellular organisms. IV. Divided into 3 groups or grades A. mesazoa: believed to be the link between unicellularity and multicellularity. 1. Ciliated wormlike animals 2. Parasites in marine invertebrates classes: Rhombozoa and Orthonectida B. Parazoa: the phylums Porifera and Placozoa C. Eumetazoa: all other phylum. Phylum Placazoa I. General information A. Greek:plakos=plate zoa=animals B. only 1 species included: Trichoplax adharens. 1

2 C. Body is platelike, no symmetry, organs, muscular or nervous systems present. D. 2-3 mm E. marine F. glide over food and secrete digestive enzyme and then absorb the products. Phylum Porifera I. General Characteristics A. latin: por=pore fera=to bear B. believed to be the most primitive C sponges recognized as animals D. mostly marine other then 3 freshwater families = ~150 species E. filter feeding systems consist of pores and canals F. this groups is basically a mass of cells embedded in gelatinous material stiffened by skeleton of calcium carbonate or cilica and collogen (spicules) G. size vary from a few mm to 2m or more. H. most are asymmetrical I. Totally sessile, live attached to the bottom of water. Most commonly to rocks, shells, corals, pilings and other hard surfaces (substrate) J. approximately 9,000 species. 2

3 II. Structure and Function A. cell types and body wall 1. Simplest sponge are shaped like a vase or tube 2. Atrium or spongocoel: the interior cavity of the sponge 3. Osculum: large opening at the top going into the spongocoel. 4. The body of the sponge surround the spongocoel is perforated by pores (hence the name porifera) 5. Pinacoderm makes up the outer surface of the sponge body 6. Pinacocytes: the layer of flattened cells that make up the pinacoderm. 7. Porocytes: cells that are perforated like a ring. These make up the pores. 8. Choanocytes (collar cells): flagellated cells that have a collarlike ring of microvilli surrounding a flagellum. 8a. The collar is a netlike mesh of microfillaments connecting the villi 8b. The flagellum creates a water current through the sponge. 8c. The collar filters microscopic food particles from the water. 9. Mesohyl: the layer in between the spongocoel and pinacoderm. 3

4 9a. It has a jelly-like consistency 9b. Mesenchyme are cells that move by amoeboid movement. 1. These are specialized for reproduction, secreting skeletal elements, transporting and storing food, forming contractile rings around the openings of the sponge wall. III. Structure and function (skeleton) A. supported by a skeleton that may consist of microscopic needle-like spike called spicules 1. Formed by the mesenchyme cells 2. Made of calcium carbonate or silica and collagen. 3. Have a variety of shapes. B. Skeleton may also be made of a fibrinous protein called spongin (usually made of collagen) 1. These type of sponges are dried, beaten and washed until all cells are removed to produce a commercial sponge. IV. Water currents and ingestion A. Water currents are created by choanocytes 1. Causes water to be sucked in through pores and driven up and out of the spongocoel through the osculum. B. The water current forces bacteria and other fine suspended organic material into the choanocytes. 4

5 1. 80% of organic particles collected by some tropical sponges are invisible to even the light microscope. 2. Trapped particles are moved to the base of the collar by the microvilli where it s engulfed by the cell 3. Food is transferred to a mesenchyme cell (amoebocyte) in the mesohyl. 4. After the food is ingested, the products are passed by diffusion, to other parts of the sponge. C. Water currents also provide gas exchange, waste removal, and transfer of gametes. D. Many species have symbiotic relationships with smaller cyanobacteria and algae. 1. Symbiosis: live as a parasite but both host and parasite benefit. 2. Algae has protection and sponge gets some food given off by the algae. V. Body Forms A. Ascon 1. Vase-like 2. Simplest and least common 3. Not more than a few centimeters tall. 5

6 3a. Size is limited because as the volume of spongocoel increases, the flagellated surface doesn t increase proportionately. Therefore if the asconoid was large it would contain more water than it s choanocytes could move. 4. Outer opening of the porocytes are called ostia and lead directly to the spongocoel. 5. Choanocytes line the spongocoel and the flagellar movements draw water into the spongocoel via the ostia and out by way of the osculum. B. Sycon 1. This sponge has finger-like projection folds 2. The inner layer of the projection are called the flagellated canal or the radial canal. 2a. In this area are the only place that the choanocytes are found. 3. The pinacoderm side is called the incurrent canals. 4. The path of water flow is the same as in ascons. C. Leucon 1. Extensively branched system 2. Choanocytes are found only in chambers 2a. This allows for more surface area 2b. There are approximately 10,000 to 18,000 choanocytes per cubed mm 6

7 3. Dermal pores are made by porocytes 4. The spongocoel is absent 4a. Water leaves the sponge body through excurrent canals 4b. The exiting pore is larger and the choanocytes push water that way. 5. One leuconoid sponge that was 10cm high and 1 cm in diameter was found to have 2,250,000 flagellated chambers. 5a. A sponge this size could pass 22.5 liters in 24 hours. 6. Irregular in shape with many oscula (plural for osculum) located over body. 7. They can grow to large sizes 7a. Most fit in a bushel basket. 7b. Some species reach over a meter in height. 8. Most species found in shallow water are leuconoid. 7

8 VI. Defense Mechanisms A. most defenses appear to be chemical. B. spicules are not a deterent. C. 50% Antarctic and 64-75% tropical sponges are toxic 1. Predators will usually feed on nontoxic or weakly toxic sponges. D. Predators include sea stars and sea slugs. VII. Reproduction A. most are monoecious 1. Both sexes occur in the same individual 2. Sperm and eggs are produced at different times, therefore not allowing for selffertilization. B. Sperm production and disbursement. 1. Certain choanocytes lose their collar cells and flagella 2. Through meiosis flagellated sperm are formed. 3. Sperm cells exit one sponge through the osculum and enter another sponge with incurrent water. 8

9 4. Sperm are trapped by the choanocytes of another sponge and put into a vacoule. 5. The sperm are stored in the choanocytes. These choanocytes lose their collar and flagellum, become amoeboid and transport the sperm to the egg cells in the mesohyl. C. egg production 1. Choanocytes go through meiosis to form eggs (again losing collar cells and flagella) 2. The egg moves into the mesohyl and is stored there until it is fertilized by the sperm. D. Post fertilization 1. Early development occurs in the mesohyl 2. The zygote, as a result of cleavage, develops into a flagellated larval stage (an immature stage that undergoes changes before attaining adult body form) 3. The larva breaks free of mesohyl and is carried by water currents out of the osculum and into regular water current. 4. After 2 days, at most, of free swimming the larva settles down into the substrate of water bottom and begins formation into the adult body form. 9

10 E. Asexual reproduction 1. Some marine and most freshwater sponges go through asexual reproduction. 2. Gemmules (latin for little bud.), which are resistant capsules containing amoeboid cells are stored in the adult sponge. 3. When the parent sponge dies in the winter gemmules are released. These are able to survive both freezing and drying environments. 4. Once favorable conditions are present the cells are releases out of an opening called the micropyle and organize into an adult sponge. *** NOTE*** Some sponges have the power of regeneration. This is where the sponge has the ability to grow back parts of its body that has been removed. 10