The Effects of Temperature on Daphnia Magna

Ectothermic animals, better known as cold-blooded animals, regulate their body temperature by relying on external sources of heat. Temperature change, therefore, arguably has a greater effect on ectotherms than other animals. The flux in body temperatures ectotherms experience causes some of them to have a stunted growth rate, due to temperature being partly responsible for the release of kinetic energy in enzymes (Van Der Have 1996). Daphnia magna serve as a prime example of the impact of temperature on biological growth. These translucent aquatic fleas undergo changes in their biological processes when exposed to dramatic variations in temperature. Although daphnia are invertebrates by classification, they possess hearts that are more similar to vertebrates’ than to their crustacean relatives (Baylor 1942). In conditions where oxygen is poor, they have been observed to experience less development. The current experiment provides a broad overview of the effects of environmental change in planktonic freshwater crustaceans. Particularly, it shows how daphnia magna adapt physiologically to drastic temperature changes until it reaches its limits, from which point its biological system is changed and eventually gets impaired.

Method

            A minor amount of Vaseline was applied on a microscope slide, at the deep point of the concave. Next, a daphnia specimen was extracted from the sample and placed in the slide using a pipette dropper with its end altered. The transfer was done with a slight amount of water on the depression with the Vaseline containing the specimen. This was carried out without filling the cavity to the brim. Next, the daphnia was held in place using a cover glass that was pressed down onto the slide. Air bubbles were promptly removed at this time, after which quick confirmation if the specimen’s heartrate was observable was carried out. Warm and cold water was then combined in a Styrofoam cup to get 15°C exactly. The daphnia slide was then set on a petri plate and lowered onto the water on the cup. Regular checks with a thermometer were done to ensure that the water remained at 15°. Next, the cup containing the daphnia

slide was set on a dissecting microscope’s stage plate, with incident illumination on. Adjustments to the microscope’s focus were made until the specimen was clearly visible. The daphnia was then left in place for 5 minutes to allow its heartbeat to stabilize. This was done to make proper identification of the heart lying in anterior and dorsal view easier. After that, 5 separate measurements of the specimen’s heartrate were conducted, and the frequency of beats per 20 seconds were logged. The temperature was measured after each successive attempt. The entire sampling and measurement process was then repeated at different temperatures, i.e. at 5°C, 10°C, 20°C, 25°C, 30°C. As with the first attempt, the daphnia sample was given 5 minutes to help its heartrate stabilize across each temperature. For comparison, a series of measurements were again conducted at 15°C at the end of the experiment. Results for all measurements have been tabulated in the lab book; reading was modified to beats per minute.

Results

Daphnia Heartrate in Beats Per Minute (bpm)

Temperature

5°C

10°C

15°C

20°C

25°C

30°C

35°C

15°C

Number of times

1

96

132

174

222

270

324

 -

 -

2

90

150

162

210

258

300

 -

 -

3

108

150

180

228

240

306

 -

 -

4

90

144

210

198

246

294

 -

 -

5

78

162

174

228

270

312

 -

 -

Standard Deviation

10.8995

10.8995

18.0000

13.0077

13.6821

11.5412

 -

 -

Q10 Values

 -

1.9481

5-15°C

1.4715

10-20°C

1.4267

15-25°C

1.4144

20-30°C

  -

25-35°C

 -

 -

            Table 1: The data above shows the effect of alterations in temperature on the daphnia’s heartrate, as measured in the following settings: 5°C, 10°C, 15°C, 20°C, 25°C, and 30°C. No results were given for 35°C and the comparative 15°C measurements due to the ensuing irregularity of the daphnia’s heartrate, indicating that it had reached its upper thermal thresholds, which promptly ended the experiment. To add more context, the Q10, SD and mean values were also added.

            Figure 1: The Daphnia specimen’s heartrate was taken 5 separate times at the following temperatures: 5°C, 10°C, 15°C, 20°C, 25°C, 30°C. Means for all measurements at each temperature were taken and drawn against these in the scatterplot. Curve fitting was employed to show the apparent trend line above.

            Figure 2: The Q10 values plotted against the centre points computed from the temperature ranges. For instance, the Q10 value for the 5°C-15°C range is drawn at 10°C on the principal axis. Here, Q10 is an indication of the sensitivity of the daphnia’s biological processes to a temperature change of +10°C.

           Normality Tests

Temp (degrees C)

Kolmogorov-Smirnova

Shapiro-Wilk

Statistic

df

Sig.

Statistic

df

Sig.

Heartrate (bpm)

5

.058

87

.200*

.983

87

.290

10

.090

87

.078

.972

87

.059

15

.058

82

.200*

.989

82

.686

20

.089

76

.200*

.975

76

.144

25

.050

72

.200*

.984

72

.520

Table 2: This table exhibits the results of the primary normality tests

*. A lower limit of the true significance.

a. Lilliefors Significance Correction

            Above are the outcomes of the Kolmogorov-Smirnov test of normality. These results indicate whether or not the data is normally distributed, i.e. it fulfils the initial parametric assumption. Granted that no value in the ‘Sig.’ section are below 0.05, it appears that there no substantial difference between the data sample and the normal distribution exists. Further, as the data is more or less distributed normally, the initial assumption is met, and succeeding tests can be performed.

Measuring Homogeneity of Variances

Heartrate (bpm)

Levene Statistic

df1

df2

Sig.

2.228

4

399

.065

Table 3: Results of the Levene’s F-test.

            ‘Sig.’ values below 0.05 mean substantial differences in variance between samples. Since this test has a ‘Sig.’ value of 0.065, it can be safely assumed that the test’s variance is fairly homogenous and fulfils both parametric assumptions. As intended, conducting the ANOVA test could be done under correct settings.

ANOVA

Heartrate (bpm)

Sum of Squares

df

Mean Square

F

Sig.

Between Groups

714860.779

4

178715.195

64.368

.000

Within Groups

1107803.486

399

2776.450

Total

1822664.264

403

Table 4: Results of the ANOVA test.

            The ANOVA test was carried out to locate statistically significant variations across the means of the various samples. The ‘Sig.’ value being less than 0.05, it appears that such a statistically significant difference exists between the samples’ means.

Various Comparisons

Dependent Variable:  Heartrate (bpm)

Tukey HSD

(I) Temp (degrees C)

(J) Temp (degrees C)

Mean Difference (I-J)

Std. Error

Sig.

95% Confidence Interval

Lower Bound

Upper Bound

5

10

-33.22837*

7.98914

.000

-55.1210

-11.3358

15

-70.98071*

8.11002

.000

-93.2045

-48.7569

20

-100.31740*

8.27318

.000

-122.9883

-77.6465

25

-113.49690*

8.39494

.000

-136.5015

-90.4923

10

5

33.22837*

7.98914

.000

11.3358

55.1210

15

-37.75234*

8.11002

.000

-59.9762

-15.5285

20

-67.08903*

8.27318

.000

-89.7600

-44.4181

25

-80.26854*

8.39494

.000

-103.2731

-57.2639

15

5

70.98071*

8.11002

.000

48.7569

93.2045

10

37.75234*

8.11002

.000

15.5285

59.9762

20

-29.33669*

8.38996

.005

-52.3276

-6.3457

25

-42.51620*

8.51005

.000

-65.8362

-19.1962

20

5

100.31740*

8.27318

.000

77.6465

122.9883

10

67.08903*

8.27318

.000

44.4181

89.7600

15

29.33669*

8.38996

.005

6.3457

52.3276

25

-13.17951

8.66568

.549

-36.9260

10.5670

25

5

113.49690*

8.39494

.000

90.4923

136.5015

10

80.26854*

8.39494

.000

57.2639

103.2731

15

42.51620*

8.51005

.000

19.1962

65.8362

20

13.17951

8.66568

.549

-10.5670

36.9260

Table 5: Results of the post hoc tests conducted to determine which samples vary.

            ‘Sig.’ values below 0.05 mean substantial differences in variance between samples. These outcomes demonstrate that there a statistically significant difference between the means of every sample exists, save for between the samples at 20°C and 25°C. The ‘Sig.’ value here is 0.549 which shows that there is no statistically significant difference in this range.

            The primary trend that the results display is that increases in circumjacent temperature leads to increases in the daphnia’s heartrate. Figure 1’s curve of best fit and the means shown in Table 1, which rose from 92.4 bpm at 5°C to 307.2 at 30°C. The data further demonstrates how the daphnia specimen had reached its upper thermal threshold at c.35°C and showed signs of irregular heartbeat, which immediately halted the experiment. Regarding standard deviations at each temperature, there is no evident pattern to speak of.

As the Q10 ­­values indicate in Figure 2, temperature changes of +10°C in lower temperatures produce more effects on daphnia’s heartrate as changes of the same degree in higher temperatures. Likewise, the rate of decline in Q10 values becomes less significant as temperatures rise, as shown by the trend in Figure 2. Finally, the class data indicates that statistically significant differences exist between the means of the data gathered at each temperature, save for the means collected between 20°C and 25°C.

Discussion

            As ectotherms, the internal body temperature of Daphnia magna are extremely susceptible to temperature changes in their environment. As freshwater animals, they are affected by changes in the temperature of the surrounding water. When the temperature increases, oxygen levels decrease, leading the Daphnia to pump blood more quickly so it can take in more oxygen efficiently, leading to a higher heartrate and body temperature (Khan 2008). Further, as the heartrate increases, so do the animal’s metabolic levels, as chemical reactions happen at an increased rate as more kinetic energy is generated. Heartrate consequently rises to meet the heightened demand for oxygen in the cells (Dennis 1999). The dramatic increase in heart rate by a factor of 2-3 during the initial +10°C increase in temperature is likely caused by the rapid decrease of oxygen in the water as a result of heating up. In response, the daphnia tries to offset the dearth in oxygen by increasing its heartrate to pump its blood faster. It generates more haemoglobin so it could take up oxygen more efficiently and distribute it to its cells. The entire process could repeat and do so at a greater pace as the temperature rises past 35°C, which is the daphnia’s thermal threshold, and an unmaintainable heartrate is sustained. Pressure intensifies in the heart causing arteries and blood vessels to become more and more strained, leading to irregular heartbeat, which eventually slows down and ultimately stops (Kivivuori 1996).

References

Khan, Q. and Khan M., 2008. Effects of temperature on waterflea Daphnia magna (Crustacea:Cladocera). Nature Precedings,

Green, J., 1956. Growth, size and reproduction in (Crustacea: cladocera). Journal of Zoology, vol. 126, no. 2, pp. 173-204.

Baylor, E. R., 1942. Cardiac pharmacology of the cladoceran, daphnia. The Biological Bulletin, vol. 83, no. 2, pp. 165-172.

Van Der Have, T.M., and de Jong, G., 1996. Adult size in ectotherms: Temperature effects on growth and differentiation. Journal of Theoretical Biology, vol. 183, no. 3, pp. 329-340.

Kivivuori, E., and Lahdes, O., 1996. How to measure the thermal death of Daphnia? A comparison of different heat tests and effects of heat injury. Journal of Thermal Biology, vol. 21, no. 5-6, pp. 305-311.