Еffect Оf Еnvironmental Оn Нeart Rate In Daphnia Essay Examples & Outline

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In this lab experiment, the functioning of the effect of salinity and temperature on open circulatory system of a fresh water Daphnia species was examined by measuring the heart rate in the two environmental variables. A null hypothesis stated at the beginning of the experiment was that there is no difference in heartbeat rate when the Daphnia species was exposed to a saline environment or temperatures of 10oC and 20oC.

The results of the experiment showed that the heartbeat rate of Daphnia species is affected by both changes in temperature and salinity. The heart rate decreased with time when the organism was exposed to saline water, and it increased as temperature rose from 10oC to 20oC. These results rejected the null hypothesis.


Daphnia species have an open blood circulatory system with the heart located dorsally and anteriorly from the circulatory fluid. These planktonic crustaceans have a relatively small size body that require a sufficiently rapid transportation of nutrients, waste products and respiratory gases to and from its body cells. The body fluid is easily visible as it rapidly flows through the body cavity. Like many fresh water organisms, Daphnia usually have osmoregulatory problems; too much water with few solutes.

They use chloride-absorbing glands to absorb ions. At a temperature of 20oC, the heart rate is about 200 times per minute. However, environmental changes such as concentration of oxygen, temperature and salinity affect metabolism activities and physiological functions in the organism. Changes in temperature affect physiological processes, with increase resulting in increased biological rates and vice versa. Daphnia are osmoconforming crustaceans with an internal salt content that matches that of their external fresh water environment (Vernberg and Winona). As a result of these variables, the organism has to respond in order to cope up with the environmental changes.

This experiment investigates the effect of changes in salinity and temperature on Daphnia by measuring the heart rate at different levels of these variables.


a)Effect of temperature:

A fresh and live specimen of Daphnia was grease mounted in a small petri dish, and the dish filled with water at room temperature. The dish was then positioned under the dissection microscope at a magnification of 2X to observe the heart. After about 10 minutes to allow the animal to acclimatize, the small dish containing the animal was placed inside a large petri dish with a mixture of ice and water.

A thermometer to measure temperature was placed inside the large petri dish and the dish placed at 10 cm away from a heating lamp. When the temperature inside the large dish attained 10 oC, the heart rate of the animal was measured for ten seconds and recorded. This was later converted to beats per minute. Two measurements were taken at every point to obtain the average. These steps were repeated at every 2oC temperature rise until a temperature of 20oC was reached.

b)Effect of salinity:

In the first step an individual Daphnia specimen was grease mounted in a petri dish, and the dish filled with water at room temperature. The petri dish was then positioned under the dissection microscope at a magnification of 2X and left for about 10 minutes for the animal to acclimatize to the new environment.

The heartbeat rate was then measured for 10 seconds. This was repeated again to obtain the average value and recorded under fresh water. In the second step, the water in the petri dish was replaced with a 1% saline solution. The heart rate of the animal was then measured at 1, 6, 8 & 10 minutes respectively. This was also repeated to obtain average values.


The table below shows the results of the average heartbeat rate per minute obtained when it was measured in fresh water and at 1% salinity versus time.

Table P5.1 Heart rate and salinity in Daphnia species

Saline level Heart beats in 10 seconds Heart beat per minute
1ST Reading 2nd Reading 1ST Reading 2nd Reading Average
Freshwater 17 23 102 138 120
1% saline/1 minute 21 15 126 90 108
6 minutes 7 9 42 54 48
8 minutes 6 5 36 30 33
10 minutes 3 1 18 6 12

When the mean heartbeat rate per minute was plotted against salinity and time, the graph shown in figure 1 below was obtained.

The table below shows the results obtained when the heart beat rate of the Daphnia species was measured at varying temperatures.

Table P5.2. Heart rate per minute as temperature increases for Daphnia species

Temperature (oC) Heart beats in 10 seconds Heart beat per minute
( Average per minute
1st reading 2nd reading
10 8 7 48 42 45
12 10 9 60 54 57
14 11 11 66 66 66
16 19 20 114 120 117
18 24 23 144 138 141
20 26 28 156 168 162

When the values of mean heartbeat rate per minute were plotted against temperature, the graph shown in figure 2 below was obtained.

For the purpose of statistical testing of the hypothesis that the heart rate of Daphnia species changes with varying temperatures, the data recorded by other student pairs was required. The table below shows the results obtained by student pairs.

Student pair Heart rate (beats per minute) at 10oC Heart rate (beats per minute) at 20oC
1. 156 264
2. 60 113
3. 81 177
4. 45 162
5. 165 309
6. 108 177
7. 132 399
Mean 106.7143 228.7143

A histogram for the data above is shown in figure 3 below, indicating how heartbeat rate was varying at 10oC and 20oC for student pair results.


When the Daphnia species is exposed to salinity, the rate of heartbeat decreases as time increases. This indicates that a change to saline environment affects the physiological and metabolic processes in the organism. The results reject the null hypothesis which stated that salinity does not affect the Daphnia. This fresh water organism is intolerant to of saline conditions. Exposure to saline environment destabilizes ion balance. This reduces the rate of physiological processes in the cells of the organism, and thus, the rate of heartbeat decreases as more salts are absorbed in the body.

The results also that the heart rate of Daphnia animals is affected by temperature. As the temperature increased, the heartbeat rate also increased. The calculated t value (2.93) critical value (2.179) so p 0.05. This indicates that the null hypothesis which stated that there would be no difference in heart rate was rejected. The rise in temperature from 10oC to 20oC increases the biological rate by 2.14 times. The animals are cold blooded with lack of mechanism for thermoregulation.

This implies that their body temperature is the same with the external environment. The chemical reactions occurring in the body of the organism depend on certain proteins or enzymes to proceed. As the external temperature is increased, these metabolic reactions also increase. This is because the rate of chemical reactions increase as temperature increases. In order to provide sufficient oxygen to the cells that carry out increased metabolic activities, the heart pump rate increases. Daphnia normally experience variations in temperature in their environment, and their rate of metabolism changes as temperature increases or decreases.


I would like to acknowledge my laboratory partner who assisted me in setting up and running this lab experiment to obtain the required results. I also appreciate our lab demonstrator who guided us on the procedure of this experiment and offered technical assistance in drafting this report.


Vernberg, F. John and B Winona. Functional Adaptations of Marine Organisms. New York: Elsevier, 2013.


Appendix1. Calculation of t-test to compare heartbeat rate at 10oC and 20oC
Calculation of SS

Heartbeat rate per minute at 10oC (X1-mean) (X1-mean
156 49.286 2429.080
60 -46.714 2182.226
81 -25.714 661.225
45 -61.714 3808.655
165 58.286 3397.223
108 1.286 1.653
132 25.286 639.367
n =7, d.f =6 Total = 13119.429 = SS1

Heartbeat rate per minute at 20oC (X1-mean) (X1-mean
264 35.286 1245.081
113 -115.714 13389.799
177 -51.714 2674.369
162 -66.714 4450.798
309 80.286 6445.794
177 -51.714 2674.369
399 170.286 28997.220
n =7, d.f =6 Total = 59877.429=SS2

Calculation of pooled variance

= 6083.0715

Calculation of t
t = = =

Critical t value: = t0.05 ( + ) = = 2.179
The calculated t value (2.93) critical value (2.179) so p 0.05
Therefore, the null hypothesis that there would be no difference in heart rate when the Daphnia species was exposed to temperatures of 10oC and 20oC was rejected.
Appendix 2. Calculation

= (
T2 - higher temperature
T1 - lower temperature
R2 – Rate at T2
R1 – Rate at T1

= 2.14