Phylum Mollusca, Class Bivalvia, Shell, Locomotion, Digestion,Reproduction and Development and Diversity.pptx

In this presentation, Phylum Mollusca Is described. After watching this you will learn Evolutionary Perspective of Mollusca and Relationships to Other Animals, Molluscan Characteristics, Class Gastropoda, Torsion, Shell Coiling, Locomotion, Feeding and Digestion, Other Maintenance Functions, Reproduction and Development, Gastropod Diversity, Class Bivalvia, Shell and Associated Structures Gas Exchange, Filter Feeding, and Digestion, Other Maintenance Functions Reproduction and Development, Bivalve Diversity, Class Cephalopoda, Shell, Locomotion, Feeding and Digestion, Other Maintenance Functions, Learning, Reproduction and Development, Class Polyplacophora, Class Scaphopoda, Class Monoplacophora, Class Solenogastres, Class Caudofoveata, Further Phylogenetic Considerations. It is part of BS Zoology Course, Animal diversity

1. Dr. Muhammad Moosa Abro Phylum Mollusca 1 Class Bivalvia Shell and Associated Structures Gas Exchange, Filter Feeding, and Digestion Other Maintenance Functions Reproduction and Development Bivalve Diversity

2. CLASS BIVALVIA With close to 30,000 species (L . bis , twice 1 valva , leaf) is the second largest molluscan class. I ncludes the clams, oysters, mussels, and scallops. A sheetlike mantle and a shell consisting of two valves (hence, the class name) cover these laterally compressed animals. Many bivalves are edible, and some form pearls . Because most bivalves are filter feeders, they are valuable in removing bacteria from polluted water. 2 Dr. Muhammad Moosa Abro Phylum Mollusca

3. Shell and Associated Structures The two convex halves of the shell are called valves. Along the dorsal margin of the shell is a proteinaceous hinge and a series of tongue-and-groove modifications of the shell, called teeth , that prevent the valves from twisting (figure 11.8 ). The oldest part of the shell is the umbo, a swollen area near the shell’s anterior margin. Although bivalves appear to have two shells, embryologically , the shell forms as a single structure. The shell is continuous along its dorsal margin, but the mantle, in the region of the hinge, secretes relatively greater quantities of protein and relatively little calcium carbonate. 3 Dr. Muhammad Moosa Abro Phylum Mollusca

4. Shell and Associated Structures The result is an elastic hinge ligament. The elasticity of the hinge ligament opens the valves when certain muscles relax . Adductor muscles at either end of the dorsal half of the shell close the shell. Anyone who has tried to force apart the valves of a bivalve mollusc knows the effectiveness of these muscles. This is important for bivalves because their primary defense against predatory sea stars is to tenaciously refuse to open their shells. 4 Dr. Muhammad Moosa Abro Phylum Mollusca

5. Shell and Associated Structures The bivalve mantle attaches to the shell around the adductor muscles and near the shell margin. If a sand grain or a parasite lodges between the shell and the mantle, the mantle secretes nacre around the irritant, gradually forming a pearl. The Pacific oysters, Pinctada margaritifera and Pinctada mertensi , form the highest-quality pearls 5 Dr. Muhammad Moosa Abro Phylum Mollusca

6. Gas Exchange, Filter Feeding, and Digestion Bivalve adaptations to sedentary, filter-feeding lifestyles include the loss of the head and radula and, except for a few bivalves, the expansion of cilia-covered gills. Gills form folded sheets (lamellae), with one end attached to the foot and the other end attached to the mantle. The mantle cavity ventral to the gills is the inhalant region, and the cavity dorsal to the gills is the exhalant region (figure 11.9 a ). . 6 Dr. Muhammad Moosa Abro Phylum Mollusca

7. Gas Exchange, Filter Feeding, and Digestion Cilia move water into the mantle cavity through an incurrent opening of the mantle. Sometimes , this opening is at the end of a siphon, which is an extension of the mantle. A bivalve buried in the substrate can extend its siphon to the surface and still feed and exchange gases . Water moves from the mantle cavity into small pores in the surface of the gills, and from there, into vertical channels in the gills, called water tubes. 7 Dr. Muhammad Moosa Abro Phylum Mollusca

8. In moving through water tubes, blood and water are in close proximity, and gases exchange by diffusion (figure 11.9 b ). Water exits the bivalve through a part of the mantle cavity at the dorsal aspect of the gills, called the suprabranchial chamber, and through an excurrent opening in the mantle (figure 11.9 a ). The gills trap food particles brought into the mantle cavity. The food-trapping mechanism is unclear, but once food particles are trapped, cilia move them to ciliated tracts called food grooves along the dorsal and ventral margins o the gills 8 Dr. Muhammad Moosa Abro Phylum Mollusca

9. These ciliated tracts move food toward the mouth (figure 11.10 ). Cilia covering leaflike labial palps on either side of the mouth also sort filtered food particles. Cilia carry small particles into the mouth and move larger particles to the edges of the palps and gills . This rejected material, called pseudofeces , falls, or is thrown, onto the mantle, and a ciliary tract on the mantle transports the pseudofeces posteriorly. Water rushing out when valves are forcefully closed washes pseudofeces from the mantle cavity . 9 Dr. Muhammad Moosa Abro Phylum Mollusca

10. The digestive tract of bivalves is similar to that of other molluscs (figure 11.11 a ). Food entering the esophagus entangles in a mucoid food string, which extends to the stomach and is rotated by cilia lining the digestive tract. A consolidated mucoid mass , the crystalline style, projects into the stomach from a diverticulum, called the style sac (figure 11.11 b ). Enzymes for carbohydrate and fat digestion are incorporated into the crystalline style. Cilia of the style sac rotate the style against a chitinized gastric shield. 10 Dr. Muhammad Moosa Abro Phylum Mollusca

11. This abrasion and acidic conditions in the stomach dislodge enzymes. The mucoid food string winds around the crystalline style as it rotates, which pulls the food string farther into the stomach from the esophagus. This action and the acidic pH in the stomach dislodge food particles in the food string . Further sorting separates fine particles from undigestible coarse materials . The latter are sent on to the intestine . Partially digested food from the stomach enters a digestive gland for intracellular digestion. Cilia carry undigested wastes in the digestive gland back to the stomach and then to the intestine . The intestine empties through the anus near the excurrent opening, and excurrent water carries feces away. 11 Dr. Muhammad Moosa Abro Phylum Mollusca

12. 12 Other Maintenance Functions Bivalves have an open circulatory system. Blood flows from the heart to tissue sinuses, nephridia , gills, and back to the heart (figure 11.12 ). The mantle is an additional site for oxygenation . In some bivalves, a separate aorta delivers blood directly to the mantle.

13. 13 Two nephridia are below the pericardial cavity (the coelom). Their duct system connects to the coelom at one end and opens at nephridiopores in the anterior region of the suprabranchial chamber (see figure 11.11). The circulatory system of a bivalve is also used in locomotion and burrowing. Blood pumped into the foot causes the foot to extend into the substrate Other Maintenance Functions

14. 14 Muscles in the foot contract to cause the foot to swell into an anchor. Finally , retractor musscles (see figure 11.10) attached from the shell to the visceral mass and foot then pull the body and shell into the substrate. The nervous system of bivalves consists of three pairs of interconnected ganglia associated with the esophagus, the foot, and the posterior adductor muscle. The margin of the mantle is the principal sense organ . It always has sensory cells, and it may have sensory tentacles and photoreceptors. In some species (e.g., scallops), photoreceptors are in the form of complex eyes with a lens and a cornea . Other receptors include statocysts near the pedal ganglion and an osphradium in the mantle, beneath the posterior adductor muscle.

15. 15 Reproduction and Development Most bivalves are dioecious . A few are monoecious, and some of these species are protandric . Gonads are in the visceral mass, where they surround the looped intestine. Ducts of these gonads open directly to the mantle cavity or by the nephridiopore to the mantle cavity. Most bivalves exhibit external fertilization. Gametes exit through the suprabranchial chamber of the mantle cavity and the exhalant opening. Development proceeds through trochophore and veliger stages (figure 11.13a and b). When the veliger settles to the substrate, it assumes the adult form.

16. 16 Most freshwater bivalves brood their young. Fertilization occurs in the mantle cavity by sperm brought in with inhalant water. Some brood their young in maternal gills through reduced trochophore and veliger stages. Young clams are shed from the gills. Freshwater bivalves in the family Unionidae brood their young to a modified veliger stage called a glochidium , which is parasitic on fishes. These larvae possess two tiny valves, and some species have toothlike hooks. Larvae exit through the exhalant aperture and sink to the substrate. fish .

17. 17 Most die If a fish contacts a glochidium , however, the larva attaches to the gills, fins, or another body part and may begin to feed on host tissue. The fish may form a cyst around the larva. The mantles of some freshwater bivalves have elaborate modifications that present a fishlike lure to entice predatory fish. When a fish attempts to feed on the lure, the bivalve ejects glochidia onto the fish (figure 11.14 ). After several weeks of larval development, during which a glochidium begins acquiring its adult structures, the miniature clam falls from its host and takes up its filter-feeding lifestyle. The glochidium is a dispersal stage for an otherwise sedentary animal and probably has little effect on the fish.

18. 18 Bivalve Diversity Bivalves live in nearly all aquatic habitats (figure 11.15). They may completely or partially bury themselves in sand or mud; attach to solid substrates; or bore into submerged wood, coral, or limestone. The mantle margins of burrowing bivalves are frequently fused to form distinct openings in the mantle cavity (siphons ). This fusion helps direct the water washed from the mantle cavity during burrowing and helps keep sediment from accumulating in the mantle cavity. Some surface-dwelling bivalves attach to the substrate either by proteinaceous strands called byssal threads, which a gland in the foot secretes, or by cementation to the substrate.

19. 19 The common marine mussel, Mytilus , uses the former method, while oysters employ the latter. Boring bivalves live beneath the surface of limestone, clay, coral, wood, and other substrates. Boring begins when the larvae settle to the substrate, and the anterior margin of their valves mechanically abrades the substrate. Acidic secretions from the mantle margin that dissolve limestone sometimes accompany physical abrasion. As the bivalve grows, it is often imprisoned in its rocky burrow because the most recently bored portions of the burrow are larger in diameter than portions bored earlier. Bivalve Diversity

20. References 20 Miller, A.S. and Harley, J.B. ; 1999 , 2002., 2007, 2009, 2012 & 2016 Zoology, 4th , 5th, 6th, 7th, 8th , 9th& 10th Edition (International), Singapore : McGraw Hill. Hickman, C.P., Roberts, L.C/, AND Larson, A., 2018. INTEGRATED PRINCIPLES OF ZOOLOGY, 15th Edition (International), Singapore: McGRAW-Hill