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Lab Title:
Scuds on Drugs
Purpose:
To observe the internal and external anatomy of a scud.
To observe the effect of common over-the-counter drugs on the heart rate of scuds.
Materials:
Scud sp., compound microscope, depression slide and coverslip, eyedropper, beaker
with drug solution,* beaker of aquarium water
*5 mg of either aspirin, ibuprofen (Advil), acetaminophen (Extra Strength Tylenol),
acetaminophen and diphendydramine (Extra Strength Tylenol PM), or acetaminophen
and caffeine (Aspirin-Free Excedrin) dissolved in 500 ml of spring water (or aquarium
water)
Directions: While the Technician goes to the supply station and prepares a wet mount of a scud, the
Recorder should copy the lab title, purpose and team members' names and jobs on the lab report. Place
the group number in the upper left-hand corner of the paper. Be certain to answer all questions using
complete sentences; do not use the words "it" or "they" in your answers. Instead, restate the question in
your answer.
Background Information: Scuds of the order Amphipoda are arthropods that are approximately 5-20 mm
in length. They are of immense importance in the ecology of ponds because they often occur in vast
numbers and feed on tiny algae and bacteria, and in turn, are preyed upon by a host of organisms
including hydra and small fishes. They are good subjects for laboratory experiments because, even
though their bodies are encased in shells, their carapaces are transparent, allowing you to see their
internal body functions. In this experiment, we will first observe the anatomy of the scud. Then we will
add a dilute solution of an over-the-counter drug to the scud to observe the effect of the drug on the heart
rate.
Procedure:
A.
Focus the scud slide under low power. Since you are not yet using a coverslip, do NOT attempt to
focus on high power as this could get the lens wet. Have each group member observe the head,
which sticks out of the carapace, and count its antennae.
1.
2.
3.
4.
B.
Have each group member look at the scud and count its legs.
5.
6.
7.
C.
How many pairs of legs does the scud have?
What are two possible functions of the legs?
To which class of arthropods do scuds belong? List three physical traits that support your
answer.
Have each group member observe the digestive tract. It is a long tube that curves forward from the
mouth and then bends back in a straight line through the body, turning downward again at the end
of the body to the anus. If you are patient, you can see the waves of contraction of the gut which
help mix up the food and push it along the digestive tract.
8.
D.
What is the dark spot on the head and what is its function?
How many pairs of antennae are found on the scud's head?
What are two ways that a scud might use its antennae?
Observe and accurately describe the movement of the scud.
What is the function of the anus?
Above the gut, just behind the head, you will be able to see the oval-shaped heart and, just behind
the heart, the brood chamber. At almost any time you will see developing eggs or embryos in this
part. You may even be able to see the hearts of the more advanced embryos beating. Scud have
an interesting and somewhat complex life cycle. Usually, all the eggs they lay are unfertilized and
result in offspring that are exclusively female. When they hatch, they remain in the brood pouch for
a day or two feeding on a secretion produced by the mother. Then the space between the two sides
of the shell widens and the offspring squeeze through the gap to begin life on their own. When the
environment becomes unfavorable, however, such as during a severe drought or at the onset of
winter, scuds lay eggs and some of these hatch into males. Males are smaller than females and
have longer antennae. After mating, the female produces thick-shelled eggs that are contained in a
compartment until the female molts. The compartment then sinks to the bottom where the eggs
remain dormant until favorable conditions return; then they hatch into females that lay many
summer eggs.
9.
10.
E.
Look again at the heart. You will be able to see the blood flowing up the back toward the heart.
When scud are in highly oxygenated water, their blood is clear. But in badly oxygenated water, their
bodies make hemoglobin to assist in the uptake of oxygen. Hemoglobin causes their blood to have
a pink or reddish color. Observe the flow of blood through the body.
11.
12.
F.
What is the average heart rate per minute of the scud?
average rate.]
[Show how you calculated the
Read these instructions completely before beginning: Place the scud in the solution given you by
your Instructor. Note the time! You will test the heart rate of this scud at three different times: the
first test will be 5 minutes after placing it in the solution. When you have determined the average
heart rate of the "drugged" scud, return it to the solution and wait another five minutes. Test the
heart rate again, and again return it to the drugged solution. Wait another five minutes, test its heart
rate, and this time return the scud to a drug-free solution. While you are waiting, you should answer
the following questions.
14.
15.
16.
H.
Has the scud been living in a low-oxygen or high-oxygen environment? Explain your answer.
Is the scud circulatory system an open or a closed one? Explain how you determined your
answer.
Add the coverslip to the slide. You may need to add another drop of water if the slide has dried out
a little. Count the number of times the scud's heart beats in 10 seconds (the Technician should
count and the Maintenance Director should use the stopwatch). Convert this number to the number
of beats per minute. Repeat this procedure two more times. Calculate the average heart rate of the
scud.
13.
G.
How many eggs or embryos are in the brood chamber?
What is the advantage of producing an almost entirely female population?
At what time did you place the scud in the solution?
What solution is your group testing on the scud?
What effect do you think this solution will have on the heart rate of the scud?
After 5 minutes, remove the scud and place it on the slide, covering it with the coverslip. Each
group member should take turns determining the heart rate of the scud by counting the number of
beats in 10 seconds and then multiplying by six to find the heart beats per minute. When you are
finished counting the heart rate of the scud at 5 minutes, 10 minutes and 15 minutes in the solution,
IMMEDIATELY return it to the drug-free solution indicated by your Instructor.
17.
18.
19.
20.
At what time did you observe the heart rate of the "drugged" scud?
What is the average heart rate per minute of the scud that has been exposed to the drug for 5
minutes? [Show how you calculated the average rate.]
What is the average heart rate per minute of the scud that has been exposed to the drug for
10 minutes? [Show how you calculated the average rate.]
What is the average heart rate per minute of the scud that has been exposed to the drug for
15 minutes? [Show how you calculated the average rate.]
Conclusions
21.
22.
23.
24.
25.
26.
In the experiment involving the over-the-counter drug, what is the problem question?
What is the independent variable in this experiment?
What is the dependent variable in this experiment?
What is the control?
Was your hypothesis proved correct? What is the effect of the drug on the heart rate of the scud?
Describe one procedural change that you could make in this experiment that might improve the
accuracy of your experimental results.
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