Lab 6: Molluscs

1. Taxonomy for Lab 6

Of the eight recognized classes of molluscs, five contain the majority of species:

Class Scaphopoda – tooth and tusk shells

Class Polyplacophora – chitons

Class Gastropoda – snails, slugs, limpets, whelks, conchs, periwinkles, etc.

Class Bivalvia – clams, oysters, mussels, scallops, cockles, shipworms, etc.

Class Cephalopoda – chambered nautilus, octopi, squids and cuttlefishes

2. Class Scaphopoda - Tooth & tusk shells

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The Class Scaphopoda contains about 400 species of molluscs called tooth or tusk shells, all of which are marine. Scaphopods range in size from 4 to 25 cm long (although most species are 2.5-5 cm in length). In comparison to other molluscs, scaphopods are a "young" group, appearing in the fossil record about 450 million years ago during the Ordovician period, and present evidence suggests that they had the same ancestor as the bivalves.

Tooth and tusk shells live sedentary lives buried in sand or mud substrates, mostly in water as deep as 6 km. Their shells grow linearly as a hollow, curved tube with an opening at each end; water enters and leaves the narrower end, which protrudes above the substrate. Unlike many molluscs, scaphopods lack ctenidia (gills), a heart and a circulatory system, and blood circulates through the various sinuses of the hemocoel as a consequence of the foot's rhythmic movements. Small food particles (such as foraminiferans) from the surrounding sediment are captured by 100-200 sticky tentacles called captacula, which are then transported to the mouth. 

3. Class Polyplacophora - Chitons

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The image above shows a dorsal view of two molluscs commonly referred to as chitons that belong to the Class Polyplacophora, a group that contains about 800 species. Chitons are common inhabitants of shallow, marine waters wherever hard substrates are present. They have a flattened foot and convex shell that is divided into eight articulating plates (valves). The plates are embedded in a part of the mantle called the girdle, which is often defended with spines or bristles. Chitons attach to rocky substrates with a powerful suction created by their broad foot muscles, which allows them to withstand strong waves and tidal currents. Most chitons feed on attached algae with their radula. 

4. Class Gastropoda - Assorted shells

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This image shows just a small sampling of the diversity found in gastropod shell designs. Malacology is the study of molluscs, and across the globe there are a number of both professional and amateur malacologists that enjoy studying and collecting mollusc shells. Unfortunately, however, the popularity of many of the more beautiful forms has lead to the demise of several species that are hunted and killed exclusively for their valuable shells, which is rather like killing an elephant for its ivory tusks! Cuttlefishes are very similar to squids in their anatomy and swimming capabilities. The principal difference between the two groups is the fact that cuttlefishes have a small, curved shell covered by the mantle (the cuttlebone often seen in bird cages) whereas squids have only a vestigial skeleton consisting of a curved plastic-like support called a pen. 

5. Class Gastropoda - An abalone

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This image shows the shell of an abalone, a marine gastropod that attaches itself to rocky substrates in the intertidal zone. The perforations (holes) in the shell provide for exhalent water flow that carries out wastes from the nephridia, anus and gills.  

6. Class Gastropoda - A nudibranch (sea slug)

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This image shows a brightly-colored nudibranch (sea slug). These forms lack any shell or true gills, and gas exchange occurs across the body surface. Carnivorous nudibranchs that feed mainly on cnidarians have dorsal projections called cerata that cover their backs; these cerata have ducts that communicate with the stomach. After nutrients are absorbed by the stomach wall, undischarged nematocysts from prey such as the Portuguese man-of-war are carried from the stomach to the tips of the cerata where they are used for their own defense! The bright (aposematic) colors of these nudibranchs may advertise the fact that they are well defended with toxic secretions or nematocysts. 

7. Class Gastropoda - A terrestrial slug

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The pulmonate ("lunged") gastropods include land and freshwater snails and slugs. All pulmonates have lost their ancestral ctenidia, and the vascularized mantle has become a lung, which fills with air by contractions of the mantle floor. The lung opens to the outside by a small opening called the pneumostome (seen on the right side of the animal). Slugs often replace shelled snails where calcium is lacking. Although they lack shells, slugs may derive some protection by secreting a sticky and noxious mucus that may be toxic to some predators. 

8. Class Bivalvia - Assorted shells

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This image shows just a small sampling of the enormous diversity and beauty found in the design of bivalve shells.  

9. Class Bivalvia - Zebra mussels

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The abundance of these tiny zebra mussels (Dreissena polymorpha) covering this native species of mussel provides a dramatic example of some of the potential hazards posed by introduced species. Released into the Great Lakes in the 1980s in ballast waters discharged by ships sailing from northern Europe, the bivalves have quickly spread as far south on the Mississippi as New Orleans and as far east as the Hudson River in New York. Using their byssal threads, zebra mussels attach to any firm surface where they filter feed on plant plankton. Large numbers build up rapidly, fouling water intake pipes of municipal water supplies and industrial plants. They also threaten native species by smothering them and exhausting the food supplies of surrounding waters. 

10. Class Bivalvia - Shipworm damage

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This image shows a piece of wood riddled with holes created by wood-boring bivalves called shipworms. These organisms can cause a tremendous amount of damage to marine timbers such as docks, pilings and ship planks. To bore through wood and other hard substrates, shipworms use modified valves that have been converted into sharp tooth-like structures, which can be seen in the image of the preserved shipworms.  

Preserved shipworms

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This image shows two preserved shipworms that have been removed from their burrows. The red arrows point to the valves of one specimen that have been modified for boring through wood. 

11. Class Cephalopoda - A chambered nautilus

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The image above shows a live chambered nautilus, a cephalopod found deep in the waters of the western Pacific Ocean. The nautilus shell is divided by transverse septa into internal chambers, and the animal lives only in the last chamber. These chambers are connected by a cord of living tissue called the siphuncle. When a chamber is abandoned, its fluid is removed and replaced with gas by the siphuncle, helping to compensate for the added mass of the shell and body, thereby maintaining buoyancy.

Chambered nautilus shell (sectioned)

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The image above shows the shell of a chambered nautilus that has been cut in half to reveal the arrangement of the internal chambers and septa. Note the small perforations (holes) in each septum that permit the passage of the siphuncle. 

12. Class Cephalopoda - A squid

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As mentioned previously, squids and cuttlefishes have two tentacles equipped with suckers only at their distal ends as well as eight arms that bear suckers along their length. In terms of skeletons, squids have only a vestigial shell called a pen and a horny beak. These cephalopods are excellent swimmers that have streamlined bodies. Lateral fins serve as stabilizers, but are held close to the body for rapid swimming. 

13. Class Cephalopoda - A cuttlefish

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 Cuttlefishes are very similar to squids in their anatomy and swimming capabilities. The principal difference between the two groups is the fact that cuttlefishes have a small, curved shell covered by the mantle (the cuttlebone often seen in bird cages) whereas squids have only a vestigial skeleton consisting of a curved plastic-like support called a pen.

14. Class Cephalopoda - An octopus

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 These cephalopods have well developed sense organs, especially eyes and are capable of complex behaviors and learning. They also can change their appearance to match their backgrounds using pigment containing chromatophores in the skin.

15. Snail radula

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This slide shows a stained section of a radula from a snail. The numerous chitinous teeth on this ribbon-like membrane are used to scrape, pierce, tear or cut off small pieces of food that are then directed in a continuous stream toward the digestive tract by conveyor belt like movements of the membrane. 

16. Veliger larvae

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This slide shows several stained veliger larvae that are found in many gastropods and bivalves. The larvae (which develop from ciliated, free-swimming trochophore larvae) have the beginnings of a foot, shell and mantle. In many molluscs the trochophore larval stage is passed in the egg, and the veliger hatches to become the only free-swimming stage. 

17. Glochidium larva

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This slide shows the glochidium larva of a bivalve. In most freshwater bivalves, eggs are fertilized internally by sperm entering with incurrent water flow. After fertilization, the eggs develop within the gill tubes (which serve as temporary brood chambers) into the tiny bivalved glochidia larvae. After being discharged, the larvae wait until they contact a passing fish and attach themselves to the gills or skin where they live as parasites for a few weeks before dropping off to begin living independent lives. This larval "hitch hiking" is seen as an adaptation for dispersal among organisms whose powers of locomotion are otherwise very limited. 

18. Squid dissection mount

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The image above shows a squid dissection mount. Numbered structures shown on the mount include the funnel, or siphon, (1); anterior vena cava (2); funnel, or siphon retractor, muscle (3); intestine (4); ink sac (5); gills (6); branchial hearts (7); posterior vena cava (8); intestinal cecum (9); mantle (10) and the anus (11). 

19. Male squid dissection

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This image (and the accompanying close-up view) show a specially prepared squid dissection encased in a Lucite block. During the dissection, a midline incision was made and both sides of the circular mantle (1) were deflected to the side to reveal the internal organs. It is these rings of mantle in the intact squid that are eaten as calamari in many restaurants! The gills (2) and major arteries on this dissected squid have been injected with red latex, while the branchial hearts (3) that supply blood to the gills and major veins have been injected with blue latex. Note the large funnel, or siphon, (4) through which water is ejected to achieve a form of locomotion by "jet propulsion". A pair of lateral fins (5) helps provide stability during swimming. Squids have eight arms (6) that bear suckers along their length and two tentacles (7) that bear suckers (8) only on their distal ends. Except for a horny beak located inside the mouth (9), the squid has only a small vestige of an internal skeleton called a pen (10).

In terms of circulation, all cephalopods have a closed circulatory system. Blood is pumped to the body by a centrally located systemic heart (11). After delivering its oxygen to the squid's tissues, poorly oxygenated blood is returned to the gills via an anterior vena cava (12) and two posterior vena cavae (13) to be pumped to the gills (2) by a pair of accessory hearts called branchial hearts (3).

Squid are predators that capture prey with their arms (6) and tentacles (7) and dispatch the prey with a powerful beak (sometimes containing venom) inside of the mouth (9). Prey are detected with large eyes (14), the sides of one of which is seen on the dissection. From there, the food passes through the esophagus into the stomach to be digested. Once partially digested, the food is diverted to a blind pouch called the intestinal cecum (15) where the process is completed, after which the remaining wastes are discharged through the anus (16). Running along side the intestine is the ink sac (17), which can discharge a load of ink through the anus (15) that may help to conceal the squid's escape from potential predators or perhaps startle them into retreating. Also seen on this dissection of a male squid is the penis (18) through which sperm are inserted into the mantle cavity of the female during a head-to-head mating. 

Male squid dissection (close-up)

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This image shows a magnified view of some of the central organs and arteries of the male squid, including the gills (1); branchial hearts (2); systemic heart (3); lateral mantle arteries (4); median mantle artery (5); anterior vena cava (6); posterior vena cava (7); penis (8); ink sac (9) and the intestinal cecum (10). 

20. Freshwater mussel dissection mount

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This image shows a number of anatomical features of a preserved, commercially prepared dissection mount of a freshwater mussel. Note the large, hatchet-shaped foot (1) that is used for burrowing into the substrate. The heart (5) has been injected with red latex, the gills (4) with blue latex and the intestine (9) with yellow latex. Digestive wastes are discharged from the intestine into the mantle cavity through the anus (10). Bivalves are distinguished from other molluscs by being having laterally compressed bodies encased in two shells (valves) that are held together by a dorsal hinge ligament (6) that causes the valves to open ventrally. The valves are drawn together by a pair of anterior (2) and posterior (8) adductor muscles, which are the parts of edible scallops that are eaten. In terms of nutrition, most bivalves are sedentary filter feeders. The posterior edges of the mantle (7) are modified to form a ventral incurrent siphon (11) that brings food and oxygen into the animal and a dorsal excurrent siphon (12) that takes carbon dioxide and wastes out. 

21. Freshwater mussel shell (outer surface)

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Bivalve shells carry out a variety of functions including support for soft tissues, protection from predators, locomotion (in scallops) and boring tunnels through hard substrates (shipworms). The shell is made of three layers: the nacreous layer, an inner iridescent layer of nacre (mother-of-pearl) composed of calcium carbonate that is continuously secreted by the mantle, the prismatic layer, a middle layer of chalky white crystals of calcium carbonate in a protein matrix and the periostracum, an outer pigmented layer composed of a protein called conchin that protects the prismatic layer from abrasion and dissolution by acids (especially important in freshwater forms where decay of leaf materials produce acids). 

22. Freshwater mussel shell (inner surface)

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The image above shows the inside of a freshwater mussel shell with all of its internal organs removed. Observe the iridescent lining of nacre (1). Near the anterior end of the valve is a raised portion called the umbo (2), which is the oldest part of the shell. The shells are held together dorsally by a spring like hinge ligament (3) that causes them to open. They are drawn together two muscles, the anterior adductor muscle (6) and the posterior adductor muscle (7). Grooves on the valves called hinge teeth allow the valves to securely interlock. In freshwater mussels there are two sets of hinge teeth, a posterior set of lateral hinge teeth (4) and an anterior set of cardinal hinge teeth (5). 

23. Freshwater mussel Dissection 1

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The image above shows a preserved, dissected freshwater mussel. Note the conspicuous fold of tissue called the mantle (1). In molluscs the mantle is a sheath of skin that hangs down in two folds around the soft body and encloses a mantle cavity, which performs many of the same functions as a coelom in other animals. The outer side of the mantle secretes the shell while the inner side is ciliated, and along with gills (2), participates in gas exchange.

Note the prominent anterior adductor muscle (3) and posterior adductor muscle (4) that draw the two valves together to enclose and protect the animal from predators. The lateral hinge teeth (5) that help the valves to securely interlock can also be seen in this image. Observe the heart (6), which is contained within the pericardial cavity (7) located in a dorsal position just below the lateral hinge teeth (5). In molluscs, this cavity represents the remains of a much-reduced coelom. Note the conspicuous, hatchet-shaped foot (8) that is used for burrowing. In the image shown, a portion of the foot has been removed to reveal the greenish digestive gland (9) and gonad (10). 

24. Freshwater mussel dissection 2

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The image above shows another view of a freshwater mussel dissection. Structures that can be seen on this image include the lateral hinge teeth (3), cardinal hinge tooth (4), anterior adductor muscle (6), posterior adductor muscle (7), gills (8), the fleshy mantle (1), a portion of exposed nacre lining the shell (2) and part of the digestive gland (5) inside what remains of the foot (most of which has been removed during the dissection). Also visible on the above image is one of the two pairs of labial palps (9), flap-like structures attached to each side of the at the anterior end of the visceral mass near the anterior adductor muscle that help guide food particles toward the mouth. 

25. Clam Model

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Click on the links below to see images of a plastic model showing the principal internal organs of a typical bivalve mollusc. 


Overview of the Model

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1. Foot; 2. Anterior adductor muscle; 3. Stomach; 4. Mantle; 5. Gills (ctenidia); 6. Posterior adductor muscle; 7. Nephridium; 8. Labial palp; 9. Pericardium (coelom)

Dorsal-Mid Section View

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1. Nephridiopore; 2. Nephrostome; 3. Pericardium (Coelom); 4. Auricle of the heart; 5. Ventricle of the heart; Gills (Ctenidium); 7. Nephridium

Anterior-Dorsal View

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1. Anterior adductor muscle; 2. Labial palp; 3. Digestive gland; 4. Stomach; 5. Gonad; 6. Intestine; 7. Pedal ganglion

Anterior-Ventral View

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1. Foot; 2. Labial palps; 3. Anterior adductor muscle; 4. Stomach; 5. Digestive gland; 6. Intestine; 7. Gonad; 8. Pedal ganglion

Posterior-Ventral View

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1. Auricle of the heart; Ventricle of the heart; 3. Pericardium (Coelom); 4. Nephridium; 5. Gills (Ctenidium); 6. Posterior adductor muscle; 7. Anus; 8. Excurrent siphon; 9. Incurrent siphon