initially by Carrie Resnick

From the latin molluscus, meaning soft.
Over 150,000 known species

Examples: snails, slugs, oysters, clams, octopuses, squids
Environment: mostly marine, some freshwater, some terrestrial. Some molluscs have adapted to live in deserts. (SM)(5)
Major classes: Polyplacophora, Gastropoda, Bivalvia and Cephalopoda

Major types of mollusks. (AK) (25)

Classes of Mollusks (MT)
Gastropoda: This is the largest and most abundant class of mollusks, which include snails, periwinkles, conches, whelks, limpets, and sea slugs. Gastropods are classified by the difference in shell form, which most commonly is external. They posses a single shell, in which they are able to withdraw inside to protect themselves from predators. They inhabit nearly every habitat on earth and are the only class of mollusks that have ventured permanently on land.(KL)(16)
Bivalvia: Members of this class include clams, mussels, oysters, scallops, and cockles. These organisms are an important food source for fish, gastropods, and shore birds in addition to humans. They posses a two-valved shell, which functions to protect the animal inside. All bivalves are aquatic. A few types of bivalves, such as scallops and file shells, can swim through water by clapping the valves together.
Cephalopoda: All cephalopods are carnivorous, and they feed off of fish, worms, Crustacea, and other mollusks. They are large mollusks who have a highly developed nervous system. They have abundant giant nerve fibers that give them rapid reflexes that are useful when catching prey. In addition, their heads contain a crown of tentacles, which are specialized organs used for subduing prey. Members of this class include squids, octopuses, cuttlefish, and nautiluses.

Amphineura: has different subclass mollusks including Polyplacophora and Aplacophoras (which are wormlike, deepwater mollusks) (AC)(23)
Polyplacophora: Also known as Chitons, polyplacophora are a group of organisms that have eight valves on their shells. All chitons are marine and are dioecious (have separate males and females). They are abundant on rocky coasts around the world. They have a simple nervous stem, numerous special sensory organs, a pair of kidneys, a heart, and an open blood system.
Most Polyplacophora live in the rocky intertidal zone or shallow sublittoral zones. Some live in deep water, can be more than 7000 m. They usually look like flattened, elongated ovals. (MLK) (22

Monoplacophores: These live on seabeds. They tend to be only up to an inch in length. They are segmented like worms, in that vital organs are duplicated in each segment of the mollusk. Monoplacophores tend to be more primitive and live in deep parts of the ocean, away from more advanced potential predators.

Scaphopods: These came on shore of the oceans about 550 million years ago. They tend to be small, curved, white tubes. There are up to 350 known species, that live in sandy seabeds ranging from the intertidal area to the abyssal zone. These tend to be 5 inches in length, maximum. (SI)(11)
A mollusk is another name for a shellfish.
Shellfish are not all fish and many of them live on land. Not all of them have shells. They are not fish, although many of them live in the water. (SR) (15)
The word Molusk comes from the latin term meaning "soft", in reference to the soft bodies of these organisms though most of them are covered in shells. (MC)

Aplacophora: aplacophora have no shell and are appear to be similar to worms. They are generally less than 5 millimeters long. In replace of their shell, they have small needlelike spicules, or spikes, protruding from their skin. They live in very deep waters, more than 6000m below sea level. (JS) (24)

Mollusks Diagram (SR) (14)

Diagnostic Characteristics:
All mollusks have generally the same body plan. This includes the mantel, a tissue the folds over the visceral mass. The farther extending part of the mantel where the gills, anus and excretory pores are housed is called the mantle cavity. Mollusks also have a visceral mass, where organs, gonads and a coiled digestive track can be found. Mollusks have a head-foot, which contains both the sensory and motor organs. LW 1) The mantle, visceral mass and foot do not look the same in all mollusks; for example, the foot of cephalopods (literally “head foot”) has become the siphon and parts of the head and tentacles. Gastropods go through torsion as embryos, which causes their anus and mantle cavity to be places above their head.
Mollusks have a coelem, a fluid filled cavity that develops within the mesoderm. This functions as a hydrostatic skeleton and also provides space within which the internal organs can be suspended by the mesentaries. All mollusks have a soft body, which is generally protected by a hard, calcium containing shell. In some forms, however, the shell has been lost in the course of evolution, as in slugs and octopuses.
Mollusks also have a toothed toungue called a radula. It is composed primarily of chitin. The radula serves to scrape off algae and other food materials. In some species, it is used in combat. (LW) (1)
Anatomy of a Mollusk:external image t761279a.gif(MT)
Acquiring and digesting food:
Many mollusks have a radula, a straplike organ that is used to scrape up food. Some mollusks in the Gastropoda class are predators and have adapted radulas that can cut holes into the shells of other mollusks and tear tissue. More often the radula is used by gastropods and chitons to graze on algae. Mollusks in the class Bivalia do not have radulas and are mostly suspension-feeders, meaning they trap food particles in mucus on their gills and bring the particles to their mouths using cilia. Cephalopods have mouths at the end of their tentacles, from which they bite and inject poison to immobilize prey.
Mollusks have complete digestive tracks, also known as alimentary canals. These are tubes that run from the mouth to the anus through which food moves in one direction. The food moves through specialized regions when digestion and nutrient absorption can happen in steps. Food does not have to be completely digested for new food to be ingested.

In bivalves such as clams, mussels and oysters, have gills, larger than necessary for respiration, that function to strain suspended material out of the water. Hair-like filaments called cilia produce a water current over the gills, and other cilia move the trapped food particles along the gill face and into food grooves. Bivalves have shells consisting of two laterally poised symmetrical valves. Normally the valves are held open by an elastic ligament near the hinge, but when threatened or felt threatened, the bivalve can clam up using its adductor muscles, muscle that pulls in some ligament (foot) towards the central line of the body. The soft body of a bivalve consists of a visceral mass, a hatchet shaped foot, sided by a pair of gills and a mantle lobe next to each shell that secrete the shell. The foot is a distinctive feature of a bivalve, located toward the front of the body. Usually they are fused below to enclose a mantle cavity, leaving an opening for the foot. The foot is located toward the front of the body. The foot is well developed in clams and other strong burrowers but almost nonexistent in scallops and oysters. The rear part of the bivalve is defined by a pair of tubes or siphons. One of these siphons carries water in while the other pumps water out; in the process, the bivalve receives oxygen and food items and eliminates wastes. Bivalves are thus filter feeders. (12,13)(ZS)
Mollusks usually have an open circulatory system, which includes a dorsal heart that pumps hemolymph, or circulatory fluid. The fluid moves from arteries to sinuses, bathing the organs as it goes. Cephalopods have closed circulatory systems.
An open circulatory system means that mollusks have neither true hearts, nor true capillaries. Instead they have blood vessels that act as pumps to move blood through the body. To get around not having capillaries, blood vessels are directly joined with open sinuses. As blood is forced out of larger vessels into these sinuses, it bathes the internal organs. At the same time, other vessels receive blood that is forced back into the sinuses. (ZXU)(2)Because the mollusk has a discrete circulatory and respiratory system, they have a greater capacity for oxygen intake and distribution which allows them to be greater in body mass. For this reason, mollusca includes the largest and most advanced of living invertebrates. (KS)
Metabolic waste removal:
The excretory organs of the mollusks are called nephridia.
After the nutrients have been digested by the glands lining the stomach and sent into the blood, the undigested materials are packages and tightly compressed to form a solid. The mantle cavity, which is at the tail end of the mollusk, is what is essentially responsible for the physical excretion of waste. The nephridia, which are tubules that collect fluid from the coelom, empties into the mantle after exchanging needed salts and other nutrients with body tissues. The packaging of wastes also ends up in the mantle cavity. From here waste is released into the water to be carried away by the current. (NG)

Sensing the Environment
Some mollusks have eyes as complex as vertebrates. Some eyes are clusters of photoreceptors and others include lenses. Gastropods have often have eyes at the ends of the tentacles. “Mollusks, specifically cephalopods, have larger, more complex eyes. In a recent test, squids were able to capture transparent prey in water by using polarized vision.” (SD)(21)
Cephalopods also have a unique ability in changing colors. They have the ability to change into yellow, black, orange, red, blue, and brown, and can do this in less than a second. they do this through cells called Chromatophores. (GR)(8)
Mollusks move using a muscular foot. The foot is also used as an anchor by many mollusks attaching themselves to rocks, boats and the like. Bivalves such as scallops can move across the seafloor by flapping their shells. Cephalopods are the fastest mollusks, since they are predators. They move backwards by drawing water into their mantle cavities and then squirting it out through an anterior siphon.
Some common ways of movement for mollusks include jet propulsion, crawling, fin undulations, and pulsation of arms. Also, suction cups are common in some mollusks to grab onto food or anchor themselves onto rocks. Suction cups are equipped with chemical receptors. (DB) (26)

Aquatic mollusks have gills for respiration. For example, clam gills project down from a localized area on the main body mass, and are flattened and long.
Different types of mollusks have different type of respiration. Aquatic mollusks are able to breathe through their gills. Most aquatic mollusks have one or two gills. Some mollusks are able to breathe using cutaneous respiration thought their mantle. Cutaneous respiration is breathing through the skin. Finally, some mollusks such as terrestrial snails are able to breathe with lungs. (7)(MF)
Aquatic mollusks have siphons, tubules which pump water in both directions for gas exchange.(ORS 9) Some mollusks can also exchange gases through their skin(ORS 10)
Many mollusks have calcerous shells, or shells made of calciumcarbonate. The shells are secreted from the mantle. However, some mollusks, such as slugs and squids, have either reduced of internal shells, due to evolution. Mollusks in the Gastropoda family have single, spiraled shells that they retreat into when threatened.
Besides shells, some mollusks may have a special adaptation on their skin- complicated pigment cells referred to as chromatophores. Seen especially in celaphopods, these cells can be controlled in a fashion to help protect the mollusk through a multitude of ways. These pigment cells can be changed to fit a certain color or pattern for camouflage, so the mollusk will not be seen by predators searching for prey. Chromatophores are also used by mollusks in a different technique called "startle displays"- bright or sudden changes of color and pattern in order to frighten and drive away predators. One last method of using chromatophores is communication- many consider some cephalopods intelligent enough to use their advanced coloration displays for a means of interaction and perhaps even the spreading of information, such as the presence of a nearby predator. This is perhaps the most advanced color-changing system for all animals, as it is nigh-instantaneous and incredibly detailed in its performance. (TM)(3,4)
Osmotic Balance
Many mollusks are marine invertebrates. Most marine invertebrates are osmoconformers, meaning their osmolarity is the same as the sea water. However, some are osmoregulators, meaning they must maintain the amount of salt and water in their bodies in relation to the environment to maintain homeostasis. In hyperosmotic environments, osmoregulation mollusks must take in water and discharge salts to maintain homeostasis. Conversely, in hypoosmotic environments, these mollusks have to take in salts and lose water to maintain homeostasis. Freshwater mollusks must take in salts and lose water. However, not all mollusks live in marine environments, so these mechanisms for osmoregulation can differ.

The Temperature balance
Aquatic vertebrates, which many mollusks are, are mainly thermoconformers, meaning they can't control their body temperatures.Reproduction
A Mollusk is either male or female and reproduces sexually with a large organ called a gonad.

With freshwater mussels the male releases sperm into the water which is carried to the female by the water current. The female collects the sperm and fertilizes her eggs. The eggs develop into the larva, or glochidia, within the female. The glochidia will then be released in the early spring or fall to attach to a host fish where it will develop into a juvenile mussel. Once it has reached this stage it will leave the fish and find a place to attach itself so it can begin to filter feed. (17) (RL)When mollusk larva hatch, they are known as trochophora, microscopic organisms that swim and acquire food using cilia. Trochophora also have a group of flagella on top of them. (NI) 19[Once the egg of a mollusk is fertilized it develops into a trochophore which extends into the larve. Mollusks can can gender within one season. The process of reporduction is both internal and external depedning of the type of mollusk. In tentacle mollusks, and certain snails, fertilization occurs inside the body of the female. In other mollusks the eggs are released into the water and then fertilized while still swimming. (MP) 20]

Review Questions1) What are the similarities and differences noticed between the circulation systems among the mollusks, Insects and Crustacea?(YA)2) How are unique charachteristics of the Mollusks, such as the shell and muscular foot, beneficial to the organism? LW
3) What is the difference between respiration through gills and cutaneous respiration? When is each system used? (RG)
4) Compare and contrast two of the major classes of mollusks. What differences between them effect their choice of environment, possible adaptations, and hierarchy within their food webs?(CW)
5) How is the structure of the mollusk beneficial to the transport of nutrients? (NG)

Other Information:
Some mollusks have separate sexes and others are hermaphrodites. Many mollusks spend a phase of their lives as ciliated larva called trochophore. Mollusks have ganglia arranged in segments that control their behaviors along with brains and ventral nerve cords.
Mollusks in the News! (LW) (NG)2. (TM)4. (TM)5. 6. (MT)7. (MF)8. (GR)**Mollusc**a.html (ORS)10. (ZS)13. (ZS)14. (SR)15. (SR)16. (RL)18. (MC)19 (NI)20 (MP)21. (SD)22. (MLK)23. (AC)24. (JS)25. (AK)26. initial information (CSR): Campbell and Reece, "Biology (Sixth Edition)" Benjamin Cummings, (c) 2002