MARINE BIOLOGY: CLAM DISSECTION
Revised by Anne Maben, Marine Biology Instructor,
Long Beach Polytechnic High,
from a UCSC Invertebrate Zoology Lab
INTRODUCTION:
The class of mollusks includes clams, oysters, mussels, and scallops. They get their name from the wood-like
valves or shells that make up their exoskeleton. Clams have a big hatchet-shaped foot for moving about and for
burrowing in mud or sand. Water enters and leaves a bivalve by way of two tubes called siphons. One siphon
takes in water while the other expels water and wastes. Water taken in contains oxygen and food particles
consisting of detritus and plankton. As the water flows across the gills, oxygen and carbon dioxide are exchanged.
Mucus on the gills traps microscopic food particles, and tiny hairlike cilia on the gills move the food laden mucus
toward the mouth. Lip-like structures called palps help sort the food and direct it into the mouth. Bivalves do not
have a radula. The food suspended in mucus moves through the digestive organs, which break it down and
absorb it.
Before plastic came into use, the shells of bivalves were commonly used to make buttons. Bivalves such as
clams are valuable as food; in fact they make up a major share of the marine invertebrate cash crop. Except for
the shell, bivalves can be eaten whole. But when water in which they grow becomes polluted with chemicals or
disease organisms, bivalves should not be eaten. At certain times of year, for example, microscopic organisms
called dinoflagellates multiply rapidly in the enclosed waters of bays and estuaries. When they grow so thick that
the pigment in their bodies makes the water look red, the phenomenon is called a red tide. Toxic substances
produced by dinoflagellates (Gonyaulax) can concentrate in the bivalves that use them as food. Although the
bivalves are not harmed (their cellular physiology walls away the toxins within their fatty tissues,) the toxin attacks
the nervous system of humans who eat them. This "paralytic shellfish poisoning" can be fatal to humans.
In this activity, we will observe the external and internal features of a
bivalve and attempt to trace its water flow.
PROCEDURE
1. Count the growth lines can you see counting from the umbo out to
the edge of the valve.
2. Lay the clam on a dissecting pan with the umbo to the left. Insert a
razor blade between the valves and move the blade dorsally along
the valve margin to cut the adductor muscles.
3. Carefully slit the hinge ligament at the umbo.
4. Hold the clam so its anterior end faces you and
its umbo faces up .
5. Find the mantle and the gills on both sides of the
soft body cavity.
6. Observe, sketch, and identify the structures
shown in Fig. 1.
Figure 1.
7. Expose the clam's interior anatomy by gently
separating the mantle from the upper valve with
a blunt probe and then lifting the upper valve
(see Fig. 2).
8. Draw and label the
internal parts of your
clam as
seen from this view.
9. Reposition the clam
with its umbo to the left
as shown in Figure 3.
Lift and fold the top
mantle to expose the
gills and body cavity
below. Draw and label
the internal parts of
your clam with the
structures found in
Table 1 of the
discussion.
10. Using different-colored
pencils or crayons,
sketch arrows on your
drawing to show the
pathway of food and
water into and out of
the clam's body.
11. Describe the functions
of each major part in
Table 1 of the
Discussion.
Figure 2.
Figure 3.
ANATOMICAL FEATURE
adductor muscles
digestive gland
excurrent siphon
foot
gills
hinge ligament
hinge teeth
incurrent siphon
intestine
lines of growth
mantle
muscle scars
nacre
pallial line
palps
stomach
umbo
FUNCTION
QUESTIONS FOR DISCUSSION:
1. How many growth lines can you see counting from the umbo out to the edge of the valve?
2. What part of the clam secretes the shell?
3. What chemical does it secrete?
4. What effect does the cutting of the muscles have on the valves?
5. How many gills does the clam have?
6. How do the gills play a part in food gathering?
7. What is the specific function of the muscular foot in clams?
8. Why is a continuous circulation of water essential to the clam?
9. Why do you think the outlet of the digestive system is near the excurrent siphon?