-understand how an oyster carries out basic life functions
-Identify key features of a bivalve mollusk
How do oysters feed? How do they breathe? How do oysters sense the world around them?
The survival of an organism depends on its ability to sense and respond to its external environment. To understand how oysters are able to perform basic life functions, scientists perform dissections to familiarize themselves with the structural make-up or the anatomy of an oyster. By learning how an oyster functions scientists can gain a better understanding of how oysters contribute to maintaining ecological balance within their environment, and how threats like water quality concerns can impact oyster functioning.
Oysters are taxonomically classified as mollusks which is one of the most diverse groups of animals on the planet. Mollusks are soft bodied animals and include familiar organisms like snails, octopuses, squids, and clams. There are thought to be at least 50,000 species in the world that are classified as mollusks. Oysters are further classified as bivalves, which describes the two external shells that form the outer, protective layer of an oyster. The scientific name for the eastern oyster is Crassostrea virginica.
Discuss why it is useful to know the anatomy of an organism. Introduce the oyster in terms of important taxonomic concepts (i.e. oysters are invertebrates which are soft bodied animals lacking an endoskeleton, that they fall under the phylum Mollusca and are bivalves. Oyster anatomy is very different to human anatomy. Let’s see how oysters are structurally organized.
-Divide the class into 2-3 students. Provide each group with a live oyster on a tray.
-Have the students examine and describe the oyster
-Identify the two shells or valves and compare them. One is more cupped and rough, the other smooth and flat; note – in nature the deeper valve is the one that is cemented down, the flatter valve acts as a lid. Are the two shells the same size? Is one thicker than the other?
-What is the shape of the oyster? Identify the hinge or umbo area, the narrow point where the two shells come together. This is the oldest part of the shell, as oysters grow shell is laid down at the opposite end. It is also a point at which the shells are attached to one another. The other end is referred to as the bill (the ventral margin) is free to open.
-Look for other organisms on the outside of the shell, or ‘scars’ of organisms that were once there (sponges leave holes on the shell surface; barnacles leave an oval round mark; oyster drills leave a single hole, limey tube worms leave a network of calciferous tubes).
Have students draw the exoskeleton, or shell of the oyster and label the umbo and bill. Discuss with students the function of the shell.
Anatomy of an Oyster:
Referring to the Oyster Anatomy Diagram point out the following organs to the students
Adductor muscle – this is a notably different type of tissue, generally shaped like an oval. The muscle controls the opening and closing of the shells. The adductor muscle leaves a scar on the shell at the point where it is attached. Have students find the muscle scar.
Mantle – this is the loose outer tissue that covers the entire body of the oyster. The mantle is always in contact with the shell but is not attached to it. The primary function of the mantle is to produce the oyster shell.
Gills – are the oyster’s largest organ and consist of 4 folds of tissue that are located under the mantle edge. Gills are the main respiratory organ but also play a role in feeding. By beating tiny hairs called cilia, the gills are able to create a current pumping water over the gills. By this method the gills are able to collect food particles (phytoplankton) and move them onto the labial palps for further sorting.
Labial palps and mouth – follow the gills toward the umbo area. There will be a slit followed by two thicker layers of tissue these are the palps. The mouth is the U-shaped slit that is located between the palps. The labial palps are specialized organs that control the total amount of food that is passed along to the mouth.
Stomach and digestive glands – locate the area where the stomach can be found. The stomach lies under the mantle layer and will be brown in color. The stomach connects to the intestines and digestive glands, and is where food is broken down into usable nutrients. Try dissecting the stomach to assess its contents.
Rectum – can be found along the edge of the adductor muscle. It is a tube through which wastes are eliminated.
Heart – the heart lies above the adductor muscle. Sometimes you can see it beating. It is located in a clear sac and looks like a tiny sponge connected to a tube. Oysters have blood but it is not pigmented red like human blood. The heart pumps the blood through the oyster’s body. Oysters have an open circulatory system, there are no definite veins and arteries through which the blood can flow instead blood drains through open sinuses within the body.
Tentacles on mantle edge – Oysters sense their external environmental through tentacles that are attached to the mantle edge. These tentacles can sense light changes, temperature changes, and can sense the presence of sediment and chemicals in the water. Oysters do not have a brain, but they do have a simple nervous system containing nerves and organs called ganglia. This simple nervous system controls the mouth and mantle, as well as the internal organs. These will not be visible in the dissection.
Inner Shell Surface –have students describe the inner surface of the shell
Gonads: these are only visible when oysters are in spawning mode during summer and fall. This is the only time that the sex of an oyster can be discerned. During the rest of the year the gonads are inconspicuous.
Without referring to the diagram have students point out the main features of the oyster to one another and discuss the functions of the various structures. Discuss with students how the oyster’s anatomy allows them to live in the environment that they inhabit.
Have students draw or label their own oyster anatomy diagrams. Have students compare and contrast oyster anatomy with that of a human.
Instead of doing a dissection an image of an oyster can be discussed in class. Students can draw the anatomy of the oyster an annotate body parts and functions.