The Molluscan World: Life in a Shell (or not) Note: These links do not work. Use the links within the outline to access the images in the popup windows. This text is the same as the scrolling text in the popup windows.

I. What is a mollusc? (Page 1)

Mollusca: http://courses.ncsu.edu/zo495x/common/zo155_site/wrap/molluscs/molluscs_popups/mollusca.html

Mollusca is another Phylum in the Protostomia line. It is most closely related to Phylum Annelida which contains the segmented worms, such as earthworms. While it may seem that molluscs, such as a snail or a clam, have little in common with segmented worms, animals in both Phyla have larvae that are almost identical. Recent DNA evidence also indicates a close relationship between the two groups.

Body Plan: http://courses.ncsu.edu/zo495x/common/zo155_site/wrap/molluscs/molluscs_popups/body_plan.html

The major external features of molluscs are illustrated in this generalized body plan. The is a tough sheath that covers the dorsal side of the body. The shell lies above the mantle. There is a thick, muscular structure called the foot on the ventral side of the body, and a specialized feeding structure, the , within the mouth. The mollusc shown here represents the hypothetical ancestor from which modern molluscs are derived. We shall see that these four features are modified to various extents in different molluscan groups.

Mantle: http://courses.ncsu.edu/zo495x/common/zo155_site/wrap/molluscs/molluscs_popups/mantle.html

Molluscs have a tough dorsal covering called the mantle. It may be seen projecting from beneath the shell of the chiton shown here. It is thought that the mantle first appeared in a flatworm-like ancestor to provide a protective shield over the animal’s soft body. The mantle began to secrete spicules of calcium carbonate which increased its strength, and these eventually fused to form plates and finally the solid shell of modern molluscs.

Foot: http://courses.ncsu.edu/zo495x/common/zo155_site/wrap/molluscs/molluscs_popups/foot.html

This photograph is the ventral surface of the chiton seen is the previous image. The thick, muscular foot occupies most of the ventral surface. Undulations of the foot coupled with mucus secretion provides a slow, creeping locomotion similar to that of planarians. Indeed, some of the tiny molluscs also bear flatworm-like cilia on the foot to facilitate locomotion. This is one indication that molluscs evolved from a flatworm ancestor.

Radula: http://courses.ncsu.edu/zo495x/common/zo155_site/wrap/molluscs/molluscs_popups/radula.html

The radula is a fascinating feeding device found only in molluscs. In its generalized form, as shown here, the radula is a belt-like structure bearing teeth which is located inside the mouth. It can be extended out through the mouth when feeding. Extension and retraction of the radula are controlled by muscles within the head.

Scrape: http://courses.ncsu.edu/zo495x/common/zo155_site/wrap/molluscs/molluscs_popups/scrape.html

The radula of primitive molluscs was originally designed to scrape food, mainly vegetation, from surfaces such as rock. This function has been retained in several groups of modern molluscs and is illustrated by this diagram which shows teeth scraping a rock as the belt-like radula moves back and forth. The photograph on the left is the radula of a slug. On the right we see the more complex radula of a whelk.

Shell: http://courses.ncsu.edu/zo495x/common/zo155_site/wrap/molluscs/molluscs_popups/shell.html

Perhaps the most important innovation achieved by molluscs is the development of a protective shell. The shell is secreted by the mantle and consists of at least two hard layers containing crystals or sheets of calcium carbonate. These layers are similar to bone (which also contains calcium salts), but have an arrangement of molecules that make shell harder. On the other hand, there is less protein in shell causing it to be more brittle than bone. The inner shell layer is harder than the middle layer and has a smooth, glossy sheen. It is called the nacreous layer and is the source of “mother o’ pearl”. The outer shell covering is a tough, fibrous layer rich in protein.

Outer Surface: http://courses.ncsu.edu/zo495x/common/zo155_site/wrap/molluscs/molluscs_popups/outer_surface.html

Here we see a highly magnified view of a shell surface. Many shells are distinguished by a characteristic pattern of lines and ridges on the outer surface.

Increase in Size: http://courses.ncsu.edu/zo495x/common/zo155_site/wrap/molluscs/molluscs_popups/increase_in_size.html

The molluscan shell fits loosely around the body as compared to other animals with external skeletons. It offers some support for the body, but has primarily a protective function. Since the shell is not shed as the animal grows, it must grow as well. The rings in this shell indicate seasons of shell growth, somewhat like the rings in a tree trunk which reveal the tree’s age.

Advantages: http://courses.ncsu.edu/zo495x/common/zo155_site/wrap/molluscs/molluscs_popups/advantages.html

The advantages of a shell are obvious. Molluscs like the snail have a moveable home into which they can retract when danger threatens. The sedentary clam can tightly close its pair of shells and remain safe within. The shell protects the soft-bodied molluscs from predators, mechanical damage and also from desiccation, when they are out of the water. In some specialized molluscs, the shell serves additional purposes such as burrowing, boring or containing gas for buoyancy.

Evolutionary Trend: http://courses.ncsu.edu/zo495x/common/zo155_site/wrap/molluscs/molluscs_popups/evolutionary_trend.html

Although the shell is an effective protective device, it is heavy and limits locomotion to speeds as slow as, well, a snail. Thus, it should not be surprising that some molluscs have lost their shell, trading protection for speed and mobility. In the images shown here, we see two shelled molluscs in the bottom panels and two of their shell-less relatives (sea slugs) at the top. The squids and octopi are prime examples. They belong to a molluscan group in which almost all members have lost the protective shell.

Advanced: http://courses.ncsu.edu/zo495x/common/zo155_site/wrap/molluscs/molluscs_popups/advanced.html

Unlike the cnidarians and flatworms, molluscs have a digestive tract with both a mouth and an anal opening. This allows one-way movement of food through the tract and leads to development of specialized digestive regions and the attachment of enzyme secreting organs. Digestion is thus more efficient in molluscs and is entirely extracellular. In parallel with formation of a complete digestive tract, a separate circulatory system appears in the molluscan line. A heart is present and housed within a small cavity. Both the digestive organs and the cavity are embedded within a mass of mesodermal tissue, called the visceral mass, that comprises most of the mollusc’s body. Thus the cavity is surrounded by mesodermal tissue and meets the definition of “coelom”—a body cavity lined by mesoderm. In molluscs, the coelom is very small, but in animals of most other Phyla, such the segmented worms, it occupies most of the body’s interior. Molluscs are also advanced in acquiring oxygen; they have developed a gill for this purpose. More details on molluscan respiration will follow shortly.

II. What are the kinds of molluscs? (Page 2-5)

Primitive Features: http://courses.ncsu.edu/zo495x/common/zo155_site/wrap/molluscs/molluscs_popups/primitive_features.html

The body plan of chitons has several primitive features, especially regarding internal anatomy. In other ways though, it is specialized for a life adhering to rocks and shells. The dorsal surface of the chiton’s body is covered by 8 overlapping plates. This differs from the solid shells of other molluscs and is an adaptation that permits flexibility while conforming to the shape of an uneven surface. The mantle extends out from beneath the shell plates and bears scales, bristles or calcareous spicules in some species. The broad ventral foot is surrounded by gills, which are protected by the over-hanging mantle. Chitons do not have a well defined head, but the mouth opens ventrally at the anterior end.

Low Tide: http://courses.ncsu.edu/zo495x/common/zo155_site/wrap/molluscs/molluscs_popups/low_tide.html

Chitons are usually inactive when exposed at low tide, but begin to feed when covered by water. They tend to avoid light and are often found in crevices or under ledges during the day. It is not unusual to see one chiton clinging to the shell of another.

Long Radula: http://courses.ncsu.edu/zo495x/common/zo155_site/wrap/molluscs/molluscs_popups/long_radula.html

The radula of chitons is unusually long compared to that of other molluscs and bears teeth that are mineralized to increase hardness and durability. These adaptations are necessary for a life spent scraping hard, abrasive surfaces. Interestingly, some of the teeth are capped by magnetite (an iron-containing mineral) and are actually attracted to a magnet.

Clamp: http://courses.ncsu.edu/zo495x/common/zo155_site/wrap/molluscs/molluscs_popups/clamp.html

Chitons can cling tenaciously to rocky surfaces. For chitons living in the intertidal zone, this important capability prevents them from being carried away in the surging tide. When necessary, the chiton uses the mantle as well as the foot for attachment. It presses the outer edge of the mantle to the rock, then raises the inner portion to create a vacuum that acts like a suction cup. With this combined action of foot and mantle it is almost impossible to pry a chiton from its rock without injuring the animal.

Many Species: http://courses.ncsu.edu/zo495x/common/zo155_site/wrap/molluscs/molluscs_popups/many_species.html

Chitons vary little in appearance. Most are within the 3-12 cm size range and are colored in drab shades of red, brown, yellow or green. They all have 8 shell plates with the anterior ends of the plates buried in the underlying mantle. This is more extreme in a few species such as the chiton in the upper middle of this image. Only a small region of each plate is visible above the mantle in this species.

Gumboot Chiton: http://courses.ncsu.edu/zo495x/common/zo155_site/wrap/molluscs/molluscs_popups/gumboot_chiton.html

This spectacular Gumboot chiton is abundant along the Northwestern coast, especially California. It is the giant of the chiton world. Shell Plates: http://courses.ncsu.edu/zo495x/common/zo155_site/wrap/molluscs/molluscs_popups/shell_plates.html

The mantle of the Gumboot chiton has completely covered the shell plates. In this living gumboot, only bulges beneath the mantle reveal their location.

Ventral Foot: http://courses.ncsu.edu/zo495x/common/zo155_site/wrap/molluscs/molluscs_popups/ventral_foot.html

The broad foot of this snail can be seen through the glass of an aquarium. The snail moves as muscular waves running along the sole of its foot act against mucus secreted by the foot. These rhythmic waves can be seen if you look closely. Each wave extends across the width of the foot and moves along its length. Several waves are visible at the same time at different positions along the sole.

Torsion: http://courses.ncsu.edu/zo495x/common/zo155_site/wrap/molluscs/molluscs_popups/torsion.html

Torsion occurs during the larval stage of all snails and slugs. In this diagram the original position of the mantle cavity and anus are shown in the larva on the left. After rotation by 180 degrees, the mantle cavity and anus lie above the head. This change of position allows water to flow toward the gills as the animal moves forward.

Anterior Position: http://courses.ncsu.edu/zo495x/common/zo155_site/wrap/molluscs/molluscs_popups/anterior_position.html

As can be seen in this diagram, the gills are located anteriorly in an adult snail. There is also a disadvantage to this arrangement, however, since the anus now opens in front of the gills and near the mouth.

Abalone: http://courses.ncsu.edu/zo495x/common/zo155_site/wrap/molluscs/molluscs_popups/abalone.html

Abalones are one the most primitive gastropods. They have solved the problem of sanitation by creating a one- way water flow: into the mantle cavity through holes along the edge of the shell and out through holes located more posteriorly. Thus uncontaminated water flows over the gills, then past the anus and out of the shell. As the abalone grows, larger holes are formed anteriorly and the most posterior holes are closed, becoming bumps on the shell.

Keyhole Limpets: http://courses.ncsu.edu/zo495x/common/zo155_site/wrap/molluscs/molluscs_popups/keyhole_limpets.html

Keyhole limpets have a single opening at the top of their conical shells. Water enters beneath the shell on both sides, passes over the gills, then out the apical hole, carrying waste water with it. The hole is most obvious in an empty shell, such as the one shown here on the lower right. Living limpets are shown elsewhere in this image. If you look closely, you can see the shell openings in some limpets. The arrow points to the anus protruding through the hole in the shell of one animal.

Anus: http://courses.ncsu.edu/zo495x/common/zo155_site/wrap/molluscs/molluscs_popups/anus.html

The most common sanitation solution is seen in this diagram. As shells became highly coiled, space was restricted on one side, usually the right, and the right-hand gill was lost. In the typical snail shown here, water enters the mantle cavity from the left side of the animal and exits on the right. The anal opening has also moved to the right assuring that waste water will not contaminate the gill or be taken in through the mouth. : http://courses.ncsu.edu/zo495x/common/zo155_site/wrap/molluscs/molluscs_popups/siphon.html

Many gastropod species have an extended groove at the edge of the shell opening as indicated in the diagram. On the right, the siphon of a living animal can be seen lying within this groove. The siphon can be extended to take in water in front of the gastropod. This may allow the animal to sample the water ahead before moving into it.

Conical: http://courses.ncsu.edu/zo495x/common/zo155_site/wrap/molluscs/molluscs_popups/conical.html

Some gastropods have a simple, conical shell as illustrated by the diagram. Limpets live on rocky substrates, often in the intertidal zone. The shell can be pressed down, much like that of a chiton’s, to clamp the limpet to the rock. This type of shell would be unwieldy in a large gastropod, since the enlarged visceral mass would require a very long cone.

Coiled: http://courses.ncsu.edu/zo495x/common/zo155_site/wrap/molluscs/molluscs_popups/coiled.html

As gastropods grew larger, the shell began to coil in order to accommodate the long visceral mass. Initially, the coiling was in one plane and oriented vertically. While this shell shape is popular with snail cartoonists, it is less frequently found in nature. The relatively tall shell is top heavy and difficult to maneuver in tight spaces.

Visceral Mass: http://courses.ncsu.edu/zo495x/common/zo155_site/wrap/molluscs/molluscs_popups/visceral_mass.html

The majority of gastropod species feature a shell in which the coiling forms a 3-dimensional spiral. This shape allows the visceral mass to coil into a more compact shape as seen in this snail which has been removed from its shell.

Compact: http://courses.ncsu.edu/zo495x/common/zo155_site/wrap/molluscs/molluscs_popups/compact.html

Snails have developed a variety of spirally coiled shell forms. Clockwise from the upper left we see a garden snail with shallow spiral protruding to the side, then two shells with progressively deeper spiraling that causes the shell to lean to one side. The fourth snail has a spiral that is more compact in the dorsal-ventral plane and rests more evenly on the back. In all cases the shell is more compact and less top heavy than the shell in the previous image. All snails are able to withdraw first the head and then the foot completely into their shell.

Seal: http://courses.ncsu.edu/zo495x/common/zo155_site/wrap/molluscs/molluscs_popups/seal.html

Most gastropods can seal the opening to their shell. This is accomplished by a horny plate, called the operculum, that is attached to the back of the foot. When the snail retracts into its shell, the foot is pulled in last and the operculum forms a seal which can be further enhanced by mucus secretion. The bottom panels show a periwinkle feeding and the same animal sealed within its shell. The seal not only protects the snail from predators, but can prevent its body from desiccation when exposed to dry conditions. Also note the extended siphon of the snail shown in the diagram.

Coiled Shells: http://courses.ncsu.edu/zo495x/common/zo155_site/wrap/molluscs/molluscs_popups/coiled_shells.html

As anyone who collects sea shells knows, the spiral shells of marine gastropods are quite diverse. Here a just a few examples, using animals photographed their natural habitats. Lost the Shell: http://courses.ncsu.edu/zo495x/common/zo155_site/wrap/molluscs/molluscs_popups/lost_the_shell.html

We are looking at a preserved sea slug lying on its back. This gastropod is one of many to have given up the protection of a shell in return for mobility. Although shell-less, it has the typical ventral foot and a mouth at the head end. A large number of gills are present in this specimen.

Sea Hares: http://courses.ncsu.edu/zo495x/common/zo155_site/wrap/molluscs/molluscs_popups/sea_hares.html

The sea hare lacks a shell, but has retained the mantle which is folded up over the back of the animal in the top panel. In some species, such as the sea hare at the bottom, the edges of the mantle are extended into wing-like structures and can be used for swimming. There are also sea hares that secrete a slimy purple ink if handled roughly. This substance is obtained from the red algae on which the animals feed and appears to be a defense against predation. One species of West coast sea hare reaches an amazing length of 75 cm and can weigh up to 35 pounds! It is probably the largest gastropod in the world.

Sea Slugs: http://courses.ncsu.edu/zo495x/common/zo155_site/wrap/molluscs/molluscs_popups/sea_slugs.html

Sea slugs are among the most beautiful animals in the sea. Since they lack both a shell and a mantle, the gills are often exposed on the dorsal side of the body. Besides gills, many sea slugs have projections called cerata which can have a variety of exotic shapes. While the coloration of some sea slugs in protective, allowing them to escape detection by blending into the background, in most cases the vivid colors are clearly for another purpose. Many sea slugs avoid predation by storing noxious chemicals within their body. Young fish have been observed to swallow sea slugs, but immediately spit them out. The distinctive color patterns may remind predators to avoid that species of sea slug after one bad experience. Other sea slugs feed on cnidarians, managing somehow to disarm the nematocysts. They then store the nematocysts at the body surface and discharge them if threatened. This orange sea slug stores nematocysts from the coral polyps that it consumes.

Slugs: http://courses.ncsu.edu/zo495x/common/zo155_site/wrap/molluscs/molluscs_popups/slugs.html

Some species of land slugs have lost the shell entirely, whereas others have a thin shell plate beneath the mantle. It is thought that a lack of calcium in their diet may have driven shell loss in these gastropods. Slugs originally lived in calcium poor regions, although in modern times they have been introduced to most parts of the world. Slugs secrete copious amounts of slime which protect them from drying out and allow them to avoid injury when crawling over sharp objects. These animals are notorious for the sticky slime trails they leave behind.

Limpet: http://courses.ncsu.edu/zo495x/common/zo155_site/wrap/molluscs/molluscs_popups/limpet.html

Limpets scrape algae and other micro-organisms from the rocky surfaces where they live. Those that inhabit the intertidal zone scrape small depressions that fit the size of their shell and then return to the same spot each day after feeding. This is the Giant Keyhole limpet that grows to at least 4 inches in diameter.

Abalones: http://courses.ncsu.edu/zo495x/common/zo155_site/wrap/molluscs/molluscs_popups/abalones.html

This Red Abalone is grazing on the microscopic algae covering an underwater rock.

Marine Snails: http://courses.ncsu.edu/zo495x/common/zo155_site/wrap/molluscs/molluscs_popups/marine_snails.html A variety of marine snails live within the intertidal zone. Each species stays within a specific zone of the rocky coast. Some species live within the mid-tide region indicated by the biologist, while others live above it or below it. They all feed by scraping the rocky surface.

Herbivores: http://courses.ncsu.edu/zo495x/common/zo155_site/wrap/molluscs/molluscs_popups/herbivores.html

This terrestrial snail is consuming vegetation on a wet walkway. Aquatic snails will do the same cleaning job on the inner glass of an aquarium.

The Radula: http://courses.ncsu.edu/zo495x/common/zo155_site/wrap/molluscs/molluscs_popups/their_radula.html

The radula of this snail can be seen as it is repeatedly extended, then retracted into the mouth.

Triton: http://courses.ncsu.edu/zo495x/common/zo155_site/wrap/molluscs/molluscs_popups/triton.html

Tritons are large gastropods that live on the sea floor. They incapacitate their prey with salivary secretions containing sulfuric acid or other toxins.

Bonnet Shell: http://courses.ncsu.edu/zo495x/common/zo155_site/wrap/molluscs/molluscs_popups/bonnet_shell.html

The bonnet shells and tuns have a feeding style similar to tritons. Note the large foot beneath each of these living gastropods.

Volutes and Olives: http://courses.ncsu.edu/zo495x/common/zo155_site/wrap/molluscs/molluscs_popups/volutes_and_olives.html

Most volutes and olives live on the sandy floor in shallow water. They capture prey by smothering it in their large, flexible foot. Note the spotted foot and the small siphon extending from the shell in each of these animals.

Whelk: http://courses.ncsu.edu/zo495x/common/zo155_site/wrap/molluscs/molluscs_popups/whelk.html

This whelk uses its siphon to locate prey by sampling the water ahead. It is able to perceive a food source up to 30 meters away, but may require more than a day to get there. The whelk feeds on bivalves which it pries open by gripping with the foot and using edge of its shell as a wedge.

Soft Flesh: http://courses.ncsu.edu/zo495x/common/zo155_site/wrap/molluscs/molluscs_popups/soft_flesh.html

The whelk in this photograph has opened a cockle shell and is consuming the flesh. Like most carnivorous gastropods, its mouth is located at the end of a proboscis that can be extended to facilitate feeding or to subdue prey.

Cone Snails: http://courses.ncsu.edu/zo495x/common/zo155_site/wrap/molluscs/molluscs_popups/cone_snails.html

The radula of cone snails bears a sharp, needle-like tooth on the end of a long proboscis, as can be seen at the arrow in this photograph. The proboscis can be extended rapidly, with enough force to harpoon the cone snail’s prey. The needle is usually left behind in the prey, but not before it has injected a neurotoxin which causes immediate paralysis and eventual death. Cone snails can kill marine worms, other gastropods, and even fish using this device. The tooth is replaced and filled with poison after each use.

Oyster Drill: http://courses.ncsu.edu/zo495x/common/zo155_site/wrap/molluscs/molluscs_popups/oyster_drill.html

Both of these gastropods feed on bivalve molluscs by boring a hole through the shell, inserting the radula and tearing out pieces of flesh. The boring process is accomplished mainly by acid secretion. The oyster drill, which preys on oysters, has an acid secreting organ on the sole of its foot. It secretes acid for about 30 minutes, then removes bits of weakened shell with the radula. Repetitions of this process can create a hole 2 mm deep within about 8 hours. Oyster drills have been known to penetrate shells up to 5 mm thick.

Cowries: http://courses.ncsu.edu/zo495x/common/zo155_site/wrap/molluscs/molluscs_popups/cowries.html

Some shelled gastropods, like these cowries, feed on the polyps of hydroid colonies and corals. The cowrie in the upper panel is feeding on a sea whip. The cowrie at the bottom is preparing to remove an orange coral polyp from its protective cup.

Man-of-War: http://courses.ncsu.edu/zo495x/common/zo155_site/wrap/molluscs/molluscs_popups/man_of_war.html

As we saw earlier, many species of sea slugs feed on cnidarians and store the undigested nematocysts for their own use. The man-of-war sea slug is the champion at this game. It feeds on siphonophores and can actually consume the tentacles of a Portuguese Man-of-war. A sting from this sea slug is understandably powerful and can do as much damage as the Portuguese Man-of-war itself.

Conch: http://courses.ncsu.edu/zo495x/common/zo155_site/wrap/molluscs/molluscs_popups/conch.html

This conch feeds on detritus within the sand or mud. It uses its long, mobile siphon like a vacuum to suck potential food items into the mantle cavity where they are sorted and carried to the mouth. Slipper shells utilize mucus secretions within the mantle cavity to strain tiny food organisms from the water.

Modified: http://courses.ncsu.edu/zo495x/common/zo155_site/wrap/molluscs/molluscs_popups/modified.html

These diagrams show the general outlines of the ancestral mollusc and a bivalve, both seen from the rear. Both animals have gills hanging within the mantle cavity on either side of the visceral mass. The bivalve body has been laterally flattened, and the shell has developed a dorsal hinge. The bivalve foot still extends ventrally, but has become long and flattened.

Comparison: http://courses.ncsu.edu/zo495x/common/zo155_site/wrap/molluscs/molluscs_popups/comparison.html

The bivalve molluscs have some features in common with gastropods, but there are also some major differences. In bivalves, torsion does not occur, so the anus is located at the posterior end of the body. Unlike most molluscs, the bivalves do not have a radula. In fact, they have nothing at the anterior end that resembles a head other than the mouth. Bivalves have a pair of siphons to move water in and out of the mantle cavity.

Mantle Cavity: http://courses.ncsu.edu/zo495x/common/zo155_site/wrap/molluscs/molluscs_popups/mantle_cavity.html

This diagram is a gives a good view of the gills hanging beneath the mantle. Water enters the mantle cavity through an incurrent siphon, flows forward over the gills, then dorsal and posterior to exit through the excurrent siphon. The continuous flow of water allows efficient gas exchange across the gill surface, the primary function of gills.

Hinge: http://courses.ncsu.edu/zo495x/common/zo155_site/wrap/molluscs/molluscs_popups/hinge.html

The two halves of the bivalve shell are hinged on the dorsal side. The hinge is held together by internal ligaments and strengthened by “hinge teeth” in many bivalves.

Muscles: http://courses.ncsu.edu/zo495x/common/zo155_site/wrap/molluscs/molluscs_popups/muscles.html

The shell-closing muscles can be seen at the arrows in this clam. When the clam is opened, the muscles tear free on one side leaving scars on the inside of the shell.

Slightly Open: http://courses.ncsu.edu/zo495x/common/zo155_site/wrap/molluscs/molluscs_popups/slightly_open.html

When the shell-muscles relax, the shell opens due to elastic recoil of the shell ligaments. If the bivalve is undisturbed, the shell remains open to allow water circulation and feeding. When the bivalve is threatened or exposed to the air, the shell closes tightly.

Shell Opens: http://courses.ncsu.edu/zo495x/common/zo155_site/wrap/molluscs/molluscs_popups/shell_opens.html

Watch the clam on the left as its shell opens and the edges of the mantle extend outward.

Bivalve Foot: http://courses.ncsu.edu/zo495x/common/zo155_site/wrap/molluscs/molluscs_popups/bivalve_foot.html

The wedge-shaped foot of a clam is visible in the partially dissected specimen shown here. When digging, a bivalve extends its foot into the substratum and expands the end of the foot to create an anchor. Retraction of the foot into the shell, then pulls the animal downward. After several repetitions of this process, the entire shell is beneath the surface.

Digging: http://courses.ncsu.edu/zo495x/common/zo155_site/wrap/molluscs/molluscs_popups/digging.html

One reason for the success of bivalves is their ability to live beneath the surface of the beach or sea bed. In this sequence of images, a razor clam has been dropped on to the sandy sea bottom. Within seconds, the animal has inserted its foot into the sand and pulled its body vertically downward. Soon, only the incurrent siphon is visible.

Extended: http://courses.ncsu.edu/zo495x/common/zo155_site/wrap/molluscs/molluscs_popups/extended.html

Each of these two fingernail clams has extended its foot. Notice that the foot becomes thinner as it lengthens.

Locomotion: http://courses.ncsu.edu/zo495x/common/zo155_site/wrap/molluscs/molluscs_popups/locomotion.html

By anchoring the extended foot, then retracting it, bivalves can move across the sand. Here we see several small clams dragging their shells through wet sand just above the tide line. One clam seems to be going in a circle.

Buried: http://courses.ncsu.edu/zo495x/common/zo155_site/wrap/molluscs/molluscs_popups/buried.html Most bivalves live their lives partially or completely buried. If the entire shell is buried, siphons extend up to the surface so that water can circulate through the body. Some clams burrow deeply into the sand and have siphons much longer that those shown here.

Gill Structure and Function: http://courses.ncsu.edu/zo495x/common/zo155_site/wrap/molluscs/molluscs_bigimages/gills.html

The gills of bivalve molluscs are utilized for feeding as well as for respiration. Since the respiration in all shelled molluscs requires water to be circulated over the gills, it was probably inevitable that some species would modify the gills for filter feeding. In the typical bivalve shown here, water enters the mantle cavity and flows over the gills before exiting through the excurrent opening. The gills are made up of thin filaments spaced closely together. As water passes between the filaments, tiny food organisms and debris are filtered out, remaining on the surface of the filaments. Note that the filaments are covered with cilia that beat to create water flow in the direction indicated by arrows.

Next, the filtered particles are moved along the filaments by cilia until they arrive at a "food groove". There are several of these running at right angles to the filaments along the surface of the gills. Mucus secreted by the gills covers the particles, and they are moved along the grooves as thin mucus threads, finally arriving at the palps near the mouth.

Sorting: http://courses.ncsu.edu/zo495x/common/zo155_site/wrap/molluscs/molluscs_popups/sorting.html

When particles collected on the gills reach the palps, a sorting process occurs. Larger particles, usually edible, are carried along the ridged surface of the palp to the mouth. Tiny particles, usually fine sediment, fall into grooves and are moved ventrally until they fall into the mantle cavity. At regular intervals, the mantle cavity contracts vigorously and the rejected particles are expelled through the incurrent siphon. This is a reversal of the usual direction of water flow through the siphon.

Sessile: http://courses.ncsu.edu/zo495x/common/zo155_site/wrap/molluscs/molluscs_popups/sessile.html

These mussels attached to the rocky surface as larvae. They will live their entire lives in this rocky crevice.

Threads: http://courses.ncsu.edu/zo495x/common/zo155_site/wrap/molluscs/molluscs_popups/threads.html

Some bivalves attach permanently to the substratum by means of tough, fibrous threads. These threads may be seen hanging from the shells of a pen clam and zebra mussel recently removed from their attachment site on rocks.

In the bottom panel, the byssal gland is demonstrated in a dissected clam. This gland produces the threads and one can be seen growing out of the gland.

Oyster Shells: http://courses.ncsu.edu/zo495x/common/zo155_site/wrap/molluscs/molluscs_popups/oyster_shells.html

Oyster larvae use secretions from the foot to attach to a solid underwater surface, usually rock. The shell-less larva presses the left side of its mantle against the surface and as the left half of the shell is secreted, it becomes fused to the substratum. Larvae sometimes settle on shells and it is common to find the shells of several oysters fused together.

Scallop: http://courses.ncsu.edu/zo495x/common/zo155_site/wrap/molluscs/molluscs_popups/scallop.html The scallop can generate a type of “jet propulsion” by clapping its shells together. The rapidly expelled water drives the scallop backward in a jerky swimming motion. Scallops spend most of their time lying on the sea bed, but can move rapidly if disturbed, as shown in the upper panel. The large muscle that closes the shell is the scallop part eaten by humans.

Wood: http://courses.ncsu.edu/zo495x/common/zo155_site/wrap/molluscs/molluscs_popups/wood.html

Wood that has been underwater for any length of time will become infested by boring bivalves. In these photographs, the tunnels created by burrowing activity are clearly visible.

Shipworm: http://courses.ncsu.edu/zo495x/common/zo155_site/wrap/molluscs/molluscs_popups/shipworm.html

The notorious shipworm is actually a bivalve mollusc. Although it has an elongated worm-like shape, this body of this animal is covered by a mantle. A shell is present, but has been reduced to two small valves at the anterior end of the body. These shells have sharp edges and are used to bore a tunnel through wood as they open and the animal rotates its “head”. The posterior siphons always protrude from the tunnel opening, so as the animal grows, the body becomes longer. Shipworms actually eat the wood they have destroyed. They store sawdust from the boring process in a special stomach sac where the cellulose is broken down by symbiotic bacteria, similar to those of the termite. The natural habitat of shipworms is near coastal regions with floating timber or underwater tree roots. Of course, they can also live in manmade objects such as piers and wooden boat hulls.

Piddick: http://courses.ncsu.edu/zo495x/common/zo155_site/wrap/molluscs/molluscs_popups/piddick.html

This mollusc is a piddick, one of many bivalves that bore into rocky surfaces. The foot of the animal is to the right and the siphons to the left. Piddicks bore by attaching the sucker-like foot and scraping a hole with the toothed edge of the shell. The piddick is credited with helping to form the English Channel through generations of boring into the chalky substratum.

Clams: http://courses.ncsu.edu/zo495x/common/zo155_site/wrap/molluscs/molluscs_popups/clams.html

Here are three edible clams. Hard-shelled clams are the most popular in the world of commerce. One species actually has the scientific name Mercenaria mercenaria.

Mussels: http://courses.ncsu.edu/zo495x/common/zo155_site/wrap/molluscs/molluscs_popups/mussels.html

Several species of mussels are edible.

Scallops: http://courses.ncsu.edu/zo495x/common/zo155_site/wrap/molluscs/molluscs_popups/scallops.html

Scallops are considered a delicacy, although only the shell-closing muscle is eaten.

Oysters: http://courses.ncsu.edu/zo495x/common/zo155_site/wrap/molluscs/molluscs_popups/oysters.html

Oysters are often served whole.

Pearls: http://courses.ncsu.edu/zo495x/common/zo155_site/wrap/molluscs/molluscs_popups/pearls.html

Oysters are the source of natural pearls. If a small, foreign object becomes lodged between the shell and the mantle, the mantle surrounds it and secretes concentric layers of nacreous shell. If the object is more or less round and does not stick to the shell, a spherical pearl is formed. The foreign object is usually a parasite, so the reaction of the mantle protects the oyster from injury. In the production of cultured pearls, a small bead is cut from the shell of a freshwater mussel and placed into the oyster. It can take up to 3 years to form a marketable pearl. Most bivalves can form pearls, but the nacreous secretions are of lower quality than those of the Pearl Oyster.

Giant Clam: http://courses.ncsu.edu/zo495x/common/zo155_site/wrap/molluscs/molluscs_popups/giant_clam.html

These bivalves are sessile. They attach to the substratum by threads as larvae and become partially buried as they grow. The adult clams are true giants, reaching a size of 1.3 meters. They have colorful mantles with expanded edges that contain photosynthetic symbionts which are crucial in supplementing the clam’s nutrition. The wavy edges of the shell allows increased mantle surface area, thus providing more room for symbionts with maximum exposure to sunlight.

Mantles: http://courses.ncsu.edu/zo495x/common/zo155_site/wrap/molluscs/molluscs_popups/mantles.html

The colorful mantles of these marine bivalves can be seen protruding from their gaping shells. The three images at the end of this sequence are giant clams. The last image is of a giant clam species that bores deeply into coral beds until its shell opening is flush with the coral surface.

Zebra Mussels: http://courses.ncsu.edu/zo495x/common/zo155_site/wrap/molluscs/molluscs_popups/zebra_mussels.html

Zebra mussels were introduced into the great lakes in the 1980’s when a ship from the Baltic region dumped water ballast into the St. Lawrence sea way. The mussels have undergone a massive population explosion and can be found in large, dense mats containing as many as 700,000 individuals. They attach to every available surface and threaten to clog the intake pipes of water plants and industrial facilities, including the cooling pipes of nuclear reactors.

Mississippi River: http://courses.ncsu.edu/zo495x/common/zo155_site/wrap/molluscs/molluscs_popups/mississippi_river.html

Zebra mussels have also had an adverse effect on the ecosystem by removing massive amounts of plankton from the water and by overgrowing and smothering native clams. These freshwater clams from the Mississippi have small Zebra mussels attached to their shells as indicated by arrows.

Members: http://courses.ncsu.edu/zo495x/common/zo155_site/wrap/molluscs/molluscs_popups/members.html

The group is comprised of squids, cuttlefish, octopi and the Chambered . Only the Nautilus has an external shell. Squids and cuttlefish have 10 tentacles, the octopi 8 and the Nautilus has at least 70. Whereas squids and cuttlefish are active swimmers living in the water column, the octopus and nautilus spend most of their time on the sea bed.

Some Resemble Fish: http://courses.ncsu.edu/zo495x/common/zo155_site/wrap/molluscs/molluscs_popups/fish.html

Squids and cuttlefish live within the water column and resemble their main competitors, fish, in many ways. Like fish, they have a streamlined body, a well-defined head with large eyes, fins for stabilization, and the ability to control buoyancy. Some cephalopods even swim together in schools. Since fish are vertebrate animals and share no recent common ancestor with cephalopds, the similarities are an example of convergent evolution, with both animal groups adapting to the same ecological niche. Body Axes: http://courses.ncsu.edu/zo495x/common/zo155_site/wrap/molluscs/molluscs_popups/body_axes.html

As cephalopods evolved from the ancestral mollusc, the body elongated along the dorsal-ventral axis. Thus the tentacles, which represent the foot, remained in a ventral location. However, the head and eyes are located at the base of the tentacles, and since the animal moves forward with the tentacles leading, it is customary to refer to this end as “anterior”. Note also that the mantle cavity containing gills was retained, but that the shell moved to an interior location.

Head-Footed: http://courses.ncsu.edu/zo495x/common/zo155_site/wrap/molluscs/molluscs_popups/head- footed.html

This diagram of a squid illustrates the location of the tentacles around the head. As in all squids and cuttlefish, eight of the tentacles are relatively short and two are long.

Suckers: http://courses.ncsu.edu/zo495x/common/zo155_site/wrap/molluscs/molluscs_popups/suckers.html

The tentacles of all cephalopods, except the Nautilus, are equipped with suckers. The suckers of a preserved squid are shown close-up in the right panel. In the squids and cuttlefish, the suckers are on short stalks and may bear teeth around the rims. These suckers act as strong suction cups for attaching to solid objects or capturing prey. They, do not open into the tentacles and definitely do not suck blood, a common misconception.

Cephalopod Mantle: http://courses.ncsu.edu/zo495x/common/zo155_site/wrap/molluscs/molluscs_popups/cephalopod_mantle.html

The cephalopod body is surrounded by a tough mantle that is open at the anterior end. It expands and contracts continuously to circulate water through the mantle cavity, thus providing oxygen to the gills. A part of the ancestral foot has been modified to form a siphon that protrudes through the mantle opening. Strong contraction of the mantle expels water through the siphon, creating a jet propulsion system.

A Siphon: http://courses.ncsu.edu/zo495x/common/zo155_site/wrap/molluscs/molluscs_popups/a_siphon.html

The siphon can be clearly seen in these photographs of a preserved squid. The siphon is movable and can be pointed backward or forward. This allows a rapid jet propulsion type of locomotion in either direction.

Greatly Reduced: http://courses.ncsu.edu/zo495x/common/zo155_site/wrap/molluscs/molluscs_popups/greatly_reduced.html

Cephalopods were once very abundant. During the age of dinosaurs, at least 8,000 species lived in great numbers in the sea. This is more than ten times the number of cephalopod species alive today. Most of these ancient cephalopods were ammonites, as known by the large number of fossilized shells found throughout the world. Other ancient cephalopods were related to the Nautilus. All of the ammonites went extinct at the same time as the dinosaurs; the head and tentacles shown in the diagram represent a guess at how the animal may have appeared in life. Of all the shelled cephalopods once so abundant, only four species of Nautilus survive. In the main cephalopod line, the shell underwent reduction as revealed in fossils of the now extinct Belemnites. As cuttlefish and squids evolved, the shell was further reduced to a flat, rod-like structure beneath the skin on the dorsal side of the body. In octopi, the shell has been lost completely.

Jaws: http://courses.ncsu.edu/zo495x/common/zo155_site/wrap/molluscs/molluscs_popups/jaws.html The cephalopod mouth still contains a radula that is used by some species to rasp tissue from prey or to drill though the shells of other molluscs. The primary feeding tool, however, is the pair of sharp jaws that surround the mouth.

Nautilus: http://courses.ncsu.edu/zo495x/common/zo155_site/wrap/molluscs/molluscs_popups/nautilus.html

Although protected by a heavy shell, the Nautilus can move easily through the water by jet propulsion. This is possible because the shell is partially filled with gas, making it buoyant.

Buoyancy: http://courses.ncsu.edu/zo495x/common/zo155_site/wrap/molluscs/molluscs_popups/buoyancy.html

Buoyancy can be regulated by altering the proportion of water vs. gas within the shell. Internally, Nautilus shells consist of a spiral containing closed chambers. The animal’s body occupies the large, open chamber in front. As it grows, the animal moves forward into a larger space and eventually secretes a partition behind it, creating a new chamber. A unique structure, the , is a tubular extension of the body that runs through all of the chambers. To increase gas within the shell, the siphuncle pumps ions out of the water-filled chambers, causing water to enter the siphuncle by osmosis where it is carried away in blood vessels. At the same time, gas bubbles out of the blood and into the chambers. A reversal of this process can restore water to the shell. This unique system allows the Nautilus to rise or sink through the water column or to hang motionless at neutral buoyancy. Modern fish are the only other animals with this ability, but they achieve neutral buoyancy by a different method as we shall see later in the course.

Reduced Shells: http://courses.ncsu.edu/zo495x/common/zo155_site/wrap/molluscs/molluscs_popups/reduced_shells.html

Cuttlefish control buoyancy in the same way as the Nautilus. Although the cuttlefish shell is internal, is does contain small chambers for gas and water accumulation. In fact a few species of cuttlefish have a coiled internal shell resembling that of the Nautilus. The shell of cuttlefish is the source of “” often hung in bird cages to supply minerals.

Some Squids: http://courses.ncsu.edu/zo495x/common/zo155_site/wrap/molluscs/molluscs_popups/some_squids.html

Although the internal shell of squids is too flat to house gas, some species have developed another method for buoyancy control. By secreting ammonium ions into their body cavity, they create a solution of ammonium chloride which is lighter than the sodium chloride of sea water. The animal shown here is a small, deep water species, but many larger squid (including the giant squid) use a similar mechanism.

Swim: http://courses.ncsu.edu/zo495x/common/zo155_site/wrap/molluscs/molluscs_popups/swim.html

When using their jet propulsion system, cephalopds hold their tentacles tightly together to streamline the body.

Capture Prey: http://courses.ncsu.edu/zo495x/common/zo155_site/wrap/molluscs/molluscs_popups/capture_prey.html

The squid diagrammed here has shot forward and used its two long tentacles to catch a fish.

Backward: http://courses.ncsu.edu/zo495x/common/zo155_site/wrap/molluscs/molluscs_popups/backward.html This Rainbow Squid has its siphon pointed forward, so it is moving backward. Squid can shoot backward very quickly, but this squid is in no hurry and has its tentacles spread.

Ink: http://courses.ncsu.edu/zo495x/common/zo155_site/wrap/molluscs/molluscs_popups/ink.html

Cephalopods can eject a cloud of ink when threatened, as seen in the octopus shown here. Dissection of the cephalopod digestive tract reveals that an is attached to the intestine, near the anus. The anus is positioned at the edge of the mantle, so ink can be released with the “jet stream” when needed. The ink of cuttlefish is a rich brown color and is the source of “sepia”, a popular artist’s pigment.

Distract: http://courses.ncsu.edu/zo495x/common/zo155_site/wrap/molluscs/molluscs_popups/distract.html

Watch the replays of this video as a squid ejects ink and jets away. The ink mass is similar in size and shape to the fleeing squid and may serve to confuse a predator.

Escapes: http://courses.ncsu.edu/zo495x/common/zo155_site/wrap/molluscs/molluscs_popups/escapes.html

This octopus has decided to escape from the photographer. The ink emitted from the siphon is not only a visual distraction, but also contains chemicals to mask the cephalopod’s scent.

Broad Fins: http://courses.ncsu.edu/zo495x/common/zo155_site/wrap/molluscs/molluscs_popups/broad_fins.html

Observe the rippling motion of the fins as this cuttlefish moves through the water.

Cuttlefish Changing Color: http://courses.ncsu.edu/zo495x/common/zo155_site/wrap/molluscs/molluscs_popups/change_color.html

Watch closely and you will see some subtle color changes on the head and tentacles of this cuttlefish. The animal seems to be performing for the cameraman.

Chromatophores: http://courses.ncsu.edu/zo495x/common/zo155_site/wrap/molluscs/molluscs_popups/.html

Chromatophores are small sacs of pigment scattered over the surface of the body. They are colored orange and brown in the squid shown here. Each pigment cell is surrounded by strands of muscle that connect to a capsule around the cell. When the muscles contract, the pigment sac within expands making the color visible on the body. Most cephalopods have several different pigments in separate sacs, so can exhibit a wide range color patterns.

Camouflage: http://courses.ncsu.edu/zo495x/common/zo155_site/wrap/molluscs/molluscs_popups/camoflage.html

The ability to change color allows cephalopods to match their background, similar to a chameleon. The octopus on the left has done a great job in matching the surrounding colors of the sea bed. The cuttlefish on the right has its tentacles hidden beneath the body and is pretending to be a rock or perhaps it is masquerading as a flatfish. Camouflage can be used both to thwart predators and to hide from potential prey.

Very Rapid: http://courses.ncsu.edu/zo495x/common/zo155_site/wrap/molluscs/molluscs_popups/very_rapid.html

Color change can be very dramatic. In these sequential photographs, a squid changes from pale to brown in seconds as it swims from a light to dark background.

Fool Predators: http://courses.ncsu.edu/zo495x/common/zo155_site/wrap/molluscs/molluscs_popups/fool_predators.html

There is more than one way to fool a predator. This small squid is attempting to confuse the photographer standing over it by spreading the fins and creating large, dark eye spots above them. It now looks more like a fish than a squid.

Attract a Mate: http://courses.ncsu.edu/zo495x/common/zo155_site/wrap/molluscs/molluscs_popups/attract_a_mate.html

Color patterns are also used for social signaling. Amazingly, the male in the center of this photograph is sending two signals at once. The side of its body visible to the female on the right is pale, a positive sign, while the other side is dark, a signal of aggression to the male competitor on the left.

Bioluminescence: http://courses.ncsu.edu/zo495x/common/zo155_site/wrap/molluscs/molluscs_popups/bioluminescence.html

This deep sea squid is flashing lights over its entire body.

Bacteria: http://courses.ncsu.edu/zo495x/common/zo155_site/wrap/molluscs/molluscs_popups/bacteria.html

This Bobtail Squid uses luminescent bacteria to produce light. The partially dissected specimen on the right reveals the location of the so-called light organs, where the symbiotic bacteria live.

Giant Squid: http://courses.ncsu.edu/zo495x/common/zo155_site/wrap/molluscs/molluscs_popups/giant_squid.html

The giant squid is the largest invertebrate animal. It can reach a length of 60 feet, including the tentacles. Specimens occasionally wash up on the shore or become entangled in nets. These two giant squid were found in New Zealand and Tasmania.

Attack Whales: http://courses.ncsu.edu/zo495x/common/zo155_site/wrap/molluscs/molluscs_popups/attack_whales.html

The giant squid is the only animal large enough to threaten the Sperm whale. Although the drawing on the left is an artist’s creation, evidence of squid attacks is provided by the large sucker scars found on the skin of some whales. The squid does not always win, however, since pieces of Giant Squid have been found in the stomach of Sperm whales.

Bottom Dwellers: http://courses.ncsu.edu/zo495x/common/zo155_site/wrap/molluscs/molluscs_popups/bottom_dwellers.html

This octopus is resting on the sea bed, probably waiting for prey to come within reach.

Can Swim: http://courses.ncsu.edu/zo495x/common/zo155_site/wrap/molluscs/molluscs_popups/can_swim.html

When swimming by jet propulsion, octopi hold the tentacles together as seen in this Giant Pacific Octopus.

Move: http://courses.ncsu.edu/zo495x/common/zo155_site/wrap/molluscs/molluscs_popups/move.html The octopus spends most of its time on the sea floor. It uses the tentacles to pull itself along the sea bed.

Strong Tentacles: http://courses.ncsu.edu/zo495x/common/zo155_site/wrap/molluscs/molluscs_popups/strong_tentacles.html

This octopus is using its tentacles to climb over an obstacle.

Hide: http://courses.ncsu.edu/zo495x/common/zo155_site/wrap/molluscs/molluscs_popups/hide.html

The octopus has a soft, flexible body and can squeeze itself into surprisingly small spaces. In the bottom photographs, an octopus can be seen entering and then hiding within a bivalve shell.

Crevices: http://courses.ncsu.edu/zo495x/common/zo155_site/wrap/molluscs/molluscs_popups/crevices.html

Octopi spend most of their time hidden in a den, and often snare prey from their hiding place. The green octopus shown here is probably looking for a meal.

Many Tentacles: http://courses.ncsu.edu/zo495x/common/zo155_site/wrap/molluscs/molluscs_popups/many_tentacles.html

The many tentacles of the Nautilus lack suckers, but some of them are prehensile and are used to convey food to the mouth.

Consume Fish: http://courses.ncsu.edu/zo495x/common/zo155_site/wrap/molluscs/molluscs_popups/consume_fish.html

The cuttlefish shown here has captured a small fish. Some cuttlefish feed high in the water column, while others patrol the sea bed looking for fish or small invertebrates.

Other Molluscs: http://courses.ncsu.edu/zo495x/common/zo155_site/wrap/molluscs/molluscs_popups/other_molluscs.html

This squid appears to have captured a gastropod.

Crabs: http://courses.ncsu.edu/zo495x/common/zo155_site/wrap/molluscs/molluscs_popups/crabs.html

The octopus seen here has brought a crab to its mouth. Note the attachment of several suckers to the crab’s carapace.

Venom: http://courses.ncsu.edu/zo495x/common/zo155_site/wrap/molluscs/molluscs_popups/venom.html

All of the octopi have a poisonous bite, but the strength of the poison varies. The blue-ringed octopus has one of the most toxic poisons known. Although this octopus is small, only 10-12 cm across, its bite can be fatal to humans. The beak of a typical octopus is shown protruding from its mouth in the inset.

Divers: http://courses.ncsu.edu/zo495x/common/zo155_site/wrap/molluscs/molluscs_popups/divers.html

The octopus is a shy animal and even the larger ones pose little threat to humans. This octopus was surprised by a diver while finishing its meal of a smaller octopus.

III. How do molluscs perform the basic life processes? (Page 6) Circulatory System: http://courses.ncsu.edu/zo495x/common/zo155_site/wrap/molluscs/molluscs_popups/circulatory_system.html

The gill and heart of a typical mollusc are shown here in a chiton. As blood flows over the gills it absorbs oxygen. Blood from the gills is carried to the heart, where it is pumped out through a large blood vessel, the aorta. Blood is transported to several blood cavities within the body, where it bathes the tissues before returning to the gill region. This system represents a major advance over the flatworms which absorb oxygen through the body surface and lack any kind of pumping mechanism to circulate fluids.

Cavities: http://courses.ncsu.edu/zo495x/common/zo155_site/wrap/molluscs/molluscs_popups/cavities.html

One of the largest blood cavities is found in the foot, as shown here in a clam. In bivalves, the foot can be extended and stiffened by pumping more blood into it. Because the blood is not completely contained within the heart and vessels, this type of circulation is called an “open” system. Note also that deoxygenated blood (colored blue) flows over the gills before returning to the heart.

Hearts: http://courses.ncsu.edu/zo495x/common/zo155_site/wrap/molluscs/molluscs_popups/hearts.html

The cephalopods are exceptional molluscs in many ways, including a more advanced circulatory system. Blood is retained within vessels at all times, passing through the tissues in tiny capillaries similar to those of vertebrate animals. Furthermore, cephalopods have a separate pair of pumps, called gill hearts, to circulate blood to the gills under high pressure. The blood is then returned to the main heart and pumped to the body. This is similar to the mammalian system, although in mammals the two pumps are represented by the right and left ventricles of a single heart.

Aquatic Habitat: http://courses.ncsu.edu/zo495x/common/zo155_site/wrap/molluscs/molluscs_popups/aquatic_habitat.html

The ancestors of this Ramshorn snail evolved lungs to live on land and lost their gills in the process. When the Ramshorn moved to a freshwater habitat, it was unable to develop gills. (A structure lost in the course of evolution rarely reappears.) The Ramshorn lives on underwater plants near the edge of ponds and must come to the water surface periodically to breath air.

Digestive Organs: http://courses.ncsu.edu/zo495x/common/zo155_site/wrap/molluscs/molluscs_popups/digestive_organs.html

As we have seen, molluscs have a one-way digestive tract. This makes digestion more efficient by allowing it to occur in successive stages. Molluscs have a stomach compartment as well as salivary glands and digestive glands that secrete enzymes at various points along the tract. Cephalopods have the most complex digestive system. The anus of all molluscs is located within the mantle cavity, near the excurrent water stream. In cephalopods it empties into the siphon.

Excretory Organ: http://courses.ncsu.edu/zo495x/common/zo155_site/wrap/molluscs/molluscs_popups/excretory_organ.html

The excretory system of molluscs is associated with the cavity around the heart. Some filtering of the blood occurs here, and metabolic wastes accumulate within the cavity before entering the excretory organ. Wastes and excess water pass out of the body through an excretory pore near the anus.

IV. How do molluscs reproduce? (Page 7) Swimming Larva: http://courses.ncsu.edu/zo495x/common/zo155_site/wrap/molluscs/molluscs_popups/swimming_larva.html

A trochophore larval stage occurs in the more primitive molluscs as well as in marine annelids. This is one line of evidence that the two Phyla are closely related.

Veliger Larvae: http://courses.ncsu.edu/zo495x/common/zo155_site/wrap/molluscs/molluscs_popups/veliger.html

The swimming larval stage of most bivalves and gastropods is called a veliger. It has lobes fringed with cilia with which it swims. A foot and a shell gradually develop, after which the veliger sheds the lobes of cilia and assumes the adult life style. Note the partially formed shell in the bivalve veliger on the right.

Freshwater Bivalves: http://courses.ncsu.edu/zo495x/common/zo155_site/wrap/molluscs/molluscs_popups/freshwater_bivalves.html

This diagram depicts the life cycle of most freshwater clams. A female takes in sperm released by a male and fertilization in internal. The eggs hatch on the gills of the mother who soon expels them into the water. The larvae, called glochidia, have hooks with which they attach to the gills of a fish. Most larvae fail to find a fish and die. Successful larvae live as ectoparasites until they have developed into tiny clams.

Copulation: http://courses.ncsu.edu/zo495x/common/zo155_site/wrap/molluscs/molluscs_popups/copulation.html

During copulation, each snail transfers sperm bundles to the other through a gonopore located near the head.

Fertile Eggs: http://courses.ncsu.edu/zo495x/common/zo155_site/wrap/molluscs/molluscs_popups/fertile_eggs.html

The pond snail on the left is depositing her eggs on an underwater stem. Eggs of a marsh snail are shown on the right.

Court: http://courses.ncsu.edu/zo495x/common/zo155_site/wrap/molluscs/molluscs_popups/court.html

Color plays an important role in cephalopod courting. The small male squid shown here is trying to bond with a larger female by flashing a striped color pattern.

During Copulation: http://courses.ncsu.edu/zo495x/common/zo155_site/wrap/molluscs/molluscs_popups/during_copulation.html

During copulation, a male cephalopod uses the tip of a specialized tentacle to transfer a bundle of sperm into the mantle cavity of its mate.

Guard: http://courses.ncsu.edu/zo495x/common/zo155_site/wrap/molluscs/molluscs_popups/guard.html

This female octopus is closely guarding her eggs.

Develop: http://courses.ncsu.edu/zo495x/common/zo155_site/wrap/molluscs/molluscs_popups/develop.html

Developing embryos are visible within these cuttlefish eggs.

V. What type of nervous system and sense organs do molluscs have? (Page 8) Nervous System: http://courses.ncsu.edu/zo495x/common/zo155_site/wrap/molluscs/molluscs_popups/nervous_system.html

The primitive mollusc nervous system consists of a pair of nerve cords connected to a anterior ring around the digestive tract. There are cross connections between the cords as seen in free-living flatworms.

Ganglia: http://courses.ncsu.edu/zo495x/common/zo155_site/wrap/molluscs/molluscs_popups/ganglia.html

In most gastropods, the head region contains well-defined ganglia. Note the nerves connecting the ganglia to sensory regions of the head.

Brain: http://courses.ncsu.edu/zo495x/common/zo155_site/wrap/molluscs/molluscs_popups/brain.html

The cephalopod brain is large and complex. The cerebral ganglion is in close proximity to the peripheral eyes and the large . The entire brain region is surrounded by cartilage, much like that of a shark, and the statocysts closely resemble the inner ears of fish.

Complex Behavior: http://courses.ncsu.edu/zo495x/common/zo155_site/wrap/molluscs/molluscs_popups/complex_behavior.html

The octopus is by far the most intelligent invertebrate animal. Those in captivity have been closely observed and provide a good model for the study of refined discrimination ability. The octopus has proved to be quite an escape artist. There are numerous tales of octopi leaving supposedly escape-proof tanks and wandering through the room. In one famous story, a biologist couldn’t understand why fish kept disappearing from their aquarium, so one night he set up a video camera to catch the thief. To his great surprise, he saw an octopus climb out of its tank, cross the room, enter the aquarium, and catch a fish. The octopus ate the fish, then climbed back into its own tank before morning.

Mimicking: http://courses.ncsu.edu/zo495x/common/zo155_site/wrap/molluscs/molluscs_popups/mimicking.html

Many cephalopods can masquerade as other forms of sea life, but the newly discovered Mimic Octopus is the master. It is shown here pretending to be a long-armed type of sea star and a crab. The crab disguise in facilitated by the long eye stalks found only in this octopus. It has also been seen to mimic the flounder and the sea snake.

Sensory Tentacles: http://courses.ncsu.edu/zo495x/common/zo155_site/wrap/molluscs/molluscs_popups/sensory_tentacles.html

Snails and slugs have tentacles near the mouth that are sensitive to touch and chemical stimuli.

Eyes: http://courses.ncsu.edu/zo495x/common/zo155_site/wrap/molluscs/molluscs_popups/eyes.html

Many bivalves have sensory tentacles extending from the mantle, as seen in this scallop. Most bivalves do not have eyes, but scallop has a row of simple, light-sensing eyes along both edges of the mantle.

Eye Stalks: http://courses.ncsu.edu/zo495x/common/zo155_site/wrap/molluscs/molluscs_popups/stalks.html

Gastropod eyes are sensitive to light, but cannot form an image. Watch the eye stalks of this snail fully extend and then contract.

Camera-type Eye: http://courses.ncsu.edu/zo495x/common/zo155_site/wrap/molluscs/molluscs_popups/camera-type_eye.html

The cephalopod eyes can form an image and are designed like a camera. In another example of convergent evolution, cephalopods have developed eyes with a lens, cornea, iris and retina that are remarkably similar to the eyes of vertebrate animals.

VI. Are molluscs important to humans? (Page 9)

Consumed: http://courses.ncsu.edu/zo495x/common/zo155_site/wrap/molluscs/molluscs_popups/consumed.html

Bivalves such as the clam, oyster and scallop are popular seafood in the United States. In other parts of the world gastropods and cephalopods are well-liked. Conchs are eaten in great quantity throughout the Caribbean, as indicated by masses of discarded shells. Squid are prevalent in coastal markets of the orient.

Dredged: http://courses.ncsu.edu/zo495x/common/zo155_site/wrap/molluscs/molluscs_popups/dredged.html

Both commercial harvesting of clams and their collection by local residents is a common site along American coasts.

The Abalone: http://courses.ncsu.edu/zo495x/common/zo155_site/wrap/molluscs/molluscs_popups/the_abalone.html

The large foot of the abalone is considered a delicacy in some parts of the world, including Northern California. Over-harvesting of California abalone and their collection by divers has created a scarcity. The demand for abalone is exacerbated by sea otters which are abundant along the Western coast and relish this gastropod. Otters are commonly seen floating on their backs and breaking an abalone shell with a rock held between the paws. The competition between humans and otters for this mollusc has created some political firestorms and has led to a recent attempt to culture the abalone.

Greatly Declined: http://courses.ncsu.edu/zo495x/common/zo155_site/wrap/molluscs/molluscs_popups/greatly_declined.html

Native bivalves have declined dramatically during the last 100 years. Chesapeake Bay serves as a good example. In 1701 a traveler was so impressed with the massive oyster reefs in the bay that he wrote:

“The abundance of oysters is incredible. There are whole banks of them so that the ships must avoid them…. They surpass those in England by far in size…they are four times as large. I often cut them in two, before I could put them in my mouth.”

Today these oyster reefs are gone due in part to over-harvesting. In the 1800’s oyster harvests were counted by millions of bushels, with 15 million bushels collected in 1880 by Maryland oystermen alone. Today the harvest is in the thousands, rather than millions of bushels and the oyster population is only 1% of former levels. This is unfortunate for the ecology bay as well as for the fisherman. Bivalves perform an important function in clearing the water of excess algae and other microorganisms as they filter-feed. It is estimated that at the height of the oyster population, the entire volume of water in the Chesapeake bay was filtered by oysters within 3-4 days. Now the same amount of filtering requires one year. Efforts are underway to increase oyster numbers in the bay area.

Massive Kills: http://courses.ncsu.edu/zo495x/common/zo155_site/wrap/molluscs/molluscs_popups/massive_kills.html Over-growth of algae or release of toxic chemicals into the water can lead to large kills of bivalves. Young animals are the most sensitive. In this photograph, a resident is demonstrating a massive death of juvenile clams in Narragansett Bay.

Water Pollution: http://courses.ncsu.edu/zo495x/common/zo155_site/wrap/molluscs/molluscs_bigimages/water_pollution.html

Many freshwater habitats in the United States, such as Chesapeake Bay, are contaminated by high levels of phosphate and nitrogen compounds. These result from sewage containing detergents or human waste and from drainage of agricultural land containing fertilizers. When excessive phosphorus and nitrogen is present in the water, algal blooms occur. This reduces the amount of sunlight reaching underwater plants, resulting in reduced plant life. Additionally, as the algae die and settle to the bottom, their decomposition removes oxygen from the water. Animals sensitive to oxygen content, such as oysters, crabs and some fish then die. In areas where bivalves have previously been over-harvested, the problem is worse since there is less filtering of algae from the water. In some locales, heavy metals such as mercury contaminate the water, creating further problems.

Mariculture: http://courses.ncsu.edu/zo495x/common/zo155_site/wrap/molluscs/molluscs_popups/mariculture.html

Many species of mollusc are now being produced by culturing. Here we see an oyster farm that uses the suspended bag method. For examples of mariculture around the world, study the large images on this page.

Mussel Farm: http://courses.ncsu.edu/zo495x/common/zo155_site/wrap/molluscs/molluscs_bigimages/mussel_farm.html

The culture of bivalves is widespread in the orient, where mussels, clams and scallops are popular seafoods.

Scallop Farming: http://courses.ncsu.edu/zo495x/common/zo155_site/wrap/molluscs/molluscs_bigimages/scallop_farming.html

This scallop farming operation is in Japan.

Abalone Culture: http://courses.ncsu.edu/zo495x/common/zo155_site/wrap/molluscs/molluscs_bigimages/abalone_culture.html

These abalone farmers are feeding kelp to abalone in submerged cages. The culture of abalone is well established in Korea and parts of China, but has only recently been attempted in California.

Jewelry: http://courses.ncsu.edu/zo495x/common/zo155_site/wrap/molluscs/molluscs_popups/jewelry.html

Shells have been utilized as jewelry throughout human history. The necklace and bracelet above were made by early native Americans. Pearls were highly esteemed by Romans and Egyptians as in most modern cultures.

The Pearls: http://courses.ncsu.edu/zo495x/common/zo155_site/wrap/molluscs/molluscs_popups/the_pearls.html

The mother-of-pearl secreted by the mantle to line the shell is the source of all pearls. The oyster is the source of most commercial pearls, but pearls from freshwater clams and mussels also make lovely jewelry and are much less expensive. Pearl Oysters: http://courses.ncsu.edu/zo495x/common/zo155_site/wrap/molluscs/molluscs_popups/pearl_oysters.html

Wild pearl oysters are shown in these photographs. The species at the bottom is the source of black pearls.

Garden Slugs: http://courses.ncsu.edu/zo495x/common/zo155_site/wrap/molluscs/molluscs_popups/garden_slugs.html

Slugs feed on plant tissues and cause damage as they rasp on leaves, stems, flowers and roots. They may produce holes in leaves or just scar the leaf surface. Small seedlings are especially susceptible to injury. Silvery slime trails are evidence of slug infestations.

Giant African Snail: http://courses.ncsu.edu/zo495x/common/zo155_site/wrap/molluscs/molluscs_popups/giant_african_snail.html

The Giant African Snail is native to East Africa but has been introduced throughout the indo-pacific region, including Hawaii. Several Caribbean islands are also infested. At a length of eight inches, this animal is by far the largest land snail. It is a voracious eater, feeding on crops and other vegetation. Even worse, it carries a parasite that can cause meningitis in humans. Agricultural officials are watching closely for signs that it has reached the USA.

Damage: http://courses.ncsu.edu/zo495x/common/zo155_site/wrap/molluscs/molluscs_popups/damage.html

The oyster drill is the greatest predator of the common Eastern Oyster. Oyster beds rarely escape damage from this little gastropod. Cultivated oysters are usually grown in regions of low salinity which the oyster can tolerate, but the oyster drill cannot.

Attacking Boats: http://courses.ncsu.edu/zo495x/common/zo155_site/wrap/molluscs/molluscs_popups/attacking_boats.html

This drawing is based on the encounter of a French steamship, the Alectron, with a Giant Squid in 1861. Rather than the squid attacking the ship, the ship’s crew enthusiastically attacked the squid, harpooning it and trying to haul in on board by a noose around the tail. The squid escaped minus the end of its tail, but the captain spun such an exciting tale that an engraving was created based on a drawing made from an officer of the Alectron. While it is highly unlikely that a human swimmer or diver would encounter a Giant Squid in the sea, the size of the beak would certainly be a cause for concern!

Sting: http://courses.ncsu.edu/zo495x/common/zo155_site/wrap/molluscs/molluscs_popups/sting.html

Three venomous cone snails are shown here. The bottom snail is called “The Glory of India”. Like most cone snails, it has spread throughout the indo-pacific region. The two cone snails at the top are also found in Hawaiian waters. These gastropods are among the fastest predators known; they can inflict a sting in milliseconds. Humans are known to have died from cone shell stings, one in less than 4 hours.

Algae That Contaminate Bivalves: http://courses.ncsu.edu/zo495x/common/zo155_site/wrap/molluscs/molluscs_bigimages/contaminated.html

"Bad" oysters, clams and mussels occur when the animals have been feeding on toxic microorganisms, usually algae. Since the algae are most abundant in the summer, this is not a safe time to collect oysters. Algal blooms can occur at other times as well, causing warning signs to be posted in shellfish areas. While algal toxins do not seem to harm the bivalves, they can cause intestinal distress when consumed by humans. The microorganisms responsible for red tides are especially toxic and can cause human death.