What is a Mollusk?

Mollusks are members of the phylum mollusca. Mollusks have been evolving in the sea for over 600 million years and there are more than 100 000 mollusks species. The mollusks phylum includes tiny snails, squid, octopus, clams, oysters, abalone, and cuttlefish. They are considered to be the most intelligent of the invertabrate species. The study of mollusks is called malacology. There are four main classes in the phylum mollusca, gastropods, bivalve, cephalopod and Polyplacophora. Gastropods include nudibranches, slugs and snails. They compose over one third of the mollusk group and most are single spiral shells but some have no shells. Bivalvia includes clams oysters and scallops. Bivalvia have two identical shells that are hinged together. Cephalopods include octopus, squid and cuttlefish and unlike most mollusks they have a closed circulatory system. They are further divided into grouops depending on the amount of tentacles and they all have the abillity to change colour making it easy for them to hide from danger. Polyplacophora include chitons and commonly clamp themselves to rocks and other surfaces. They have no eyes or tentacles. Mollusks have soft, thick and fleshy bodies and range from very small to as big as six feet accross. Mollusks have bilateral symmetry meaning that if cut down the middle from anterior to posterior end they would be the same on either side and have cephalization. Mollusks have a muscular skeleton consisting of an outer shell(s)and a soft body with a muscular foot, a mantle and a visceral mass.

Movement

Land mollusks like slugs and snails move slowly by means of a flat sole called a foot. Mollusks living in the ocean move by jet propulsion. The mollusk ejects water from a cavity within their bodies shooting them forward. Some molllusks are mainly sessile and can stay glued to rocks or other surfaces for all their lives. Animals like chitton do this. If they do choose to move, it will be very minimal and will involve the foot to contract producing tiny waves of motion. The class that uses this type of motion is the class Polyplacophora.

Respiration

Mollusks use gills as a form of resperation as well as filters for foods, though it is known that most mollusk species only use their gills for breathing. The gills are also refered to as "ctenidia". Aquatic mollusks (i.e snails, clams and squid) breathe through the gills inside their mantel cavities. But land mollusks use an adapted mantle cavity lined with blood vessels to breath "skin breathing".

Reproduction

Mollusc's reproductive organs are situated in the visceral mass (refer to female squid picture). Both simple and highly complex forms of sexual reproduction are inhibited. Egg fertilization occurs externally (except most belonging to the class Cephalopoda), sometimes in broadcast spawning (large egg and sperm quantities released in water at the same time). Molluscs are protostomes, they have spiral cleavage and require separate sexes for reproduction. Some can be hermaphrodites, for example snails because of their slow movement they have the ability to change sexes. After the egg is fertilized, it becomes a larva, which is motile. This is called a trochophore larva. When this later elongates in the next developmental stage it is called a veliger larva.

Circulation/Internal Transport

The oxygen and nutrients that are taken in by molluscs are carried through it’s body by blood. The blood is not always contained within vessels, making it an open circulatory system. This means that, when the blood is not being contained in the blood vessels, it works its way through the body tissues in sinuses (open spaces). This type of circulatory system works well for molluscs who are slow moving, or sessile, for example clams, however it is not sufficient for more motile molluscs like squid and octopi, which function by means of a closed circulatory system.

Feeding + Digestion

Most mollusks are herbivores, carnivores, or filter feeders though there are some species that are detritus feeders or are parasitic. Many mollusks (ie. snails and slugs) feed through a tongue shaped structure called a radula. A radula is a layer of flexible skin with hundreds of tiny teeth making it appear like sandpaper. Inside the radula there is stiff rod of cartilage. It places the tip of the radula on the food and pulls the flexible area of skin back and forth over the cartilage. Herbivores use their radula to scrap algae off of surfaces in the water or to eat the buds and roots of flowers on land plants. Carnivores extend their radula into the shell of the prey and tear up and swallow the preys tissue. Mollusks have a very complex digestive tract. After being taken by the radula, the food goes to the digestive glands and then to its intestine. Wastes exit via the anus and food enters the mouth. In the class gastopods, the mouth and anus are at the same end but are different openings. In some mollusks like oysters, clams and scallops, they use there gils to sift through food in the water. There is a layer of sticky mucus on the gills that the food sticks to and then is pushed to the mouth by cilia on the gills as well.

Excretion

The mollusks undigested food becomes a solid waste that is eliminated through the anus as feces. Another form of waste removal for mollusks is Cellular Metabolism which produces nitrogen-containing waste in the form of ammonia. The poisonous ammonia is removed through the body fluids. The mollusks use a simple tubed shaped organ called the nephridia to eliminate the ammonia from the blood stream and to the outside.

Molluscs and their environments

Molluscs have a large impact on the environment, as they play many differnet roles in their ecosytem. For example, Molluscs are detritus feeders, (meaning they feed on tiny bits of decaying plants and organisms)and therefore, in a way clean up their surroundings. Molluscs are able to adapt and live in a variety of habitats ranging from mountain to deserts to the deep ocean, including intertidal zones, freshwater ponds and streams, and even in our own back yeards.


Molluscs play many different roles in living systems, including the intertidal zone shown above.

Squid Dissection Lab

Vocabulary

Mollusks - soft-bodied invertebrate animal that is characterized by an internal or external shell, a foot, a mantle, and visceral mass; member of the phylum Mollusca.

Foot - muscular structure in mollusks that usually contains the mouth and other feeding structures.

Mantle- thin, delicate layer of tissue that covers most of a mollusk’s body and secretes the shell when one is present.

Shell - structure in mollusks made by glands in the mantle that secrete calcium carbonate.

Visceral mass - structure in mollusks that contains internal organs.

Radula - in some mollusks, layer of flexible skin with hundreds of tiny teeth used for feeding

Gill - filamentous respiratory structure in an aquatic animal

Open circulatory system- system in which blood does not always travel inside blood vessels

Closed circulatory system- system in which blood always moves inside blood vessels

Nephridium - simple tube shaped excretory organ used to remove ammonia from the blood and release it from the body

Gastropod - mollusk that moves by means of a broad muscular foot located on its ventral side; usually has a one piece shell for protection.

Bivalve - mollusk that lives within a shell made of two sections that are hinged together.

Cephalopod - marine mollusk whose head is attached to its foot, which is divided into tentacles.

Mollusk Facts

~Mollusks are a very diverse group of invertabrates that live in a wide variety of environments. They can be found living in gardens, trees, freshwater streams and ponds, estuaries, tidal pools, beaches, and in the ocean.
~Mullusks play an important role in their ecosystems. For example, they are detritus feeders, and therefore clean up their surroundings. Alot of mollusks are parasites, and also host parasites. Mollusks are also a food source for humans.
~The mollusk phylum is the second largest inverdabrate phylum next to anthropods.
~There are seven classes of mollusks;Polyplacophora, Aplacophora,Gastropoda, Scaphapoda,Blvalvia and Cephpoda.