Method:
The first test was carried out at 18oC – room temperature. A small amount of petroleum jelly was smeared on a cavity slide; as this keeps the daphnia immobile enough to perform the experiment. By slowing the movements of the daphnia down the heart rate can be determined without unduly stressing the daphnia. Using the pipette a daphnia was removed from the beaker and placed on a cavity slide; the daphnia still had some water around it, so the temperature was maintained, a cover slip was placed on top. The slide was placed under the microscope and positioned so that the heart can be seen. The stop clock was set and using the pencil & paper the number of heartbeats was recorded [by dotting the paper to each equivalent/responding heart beat] over a period of 10 seconds. After recording the result, the test was repeated two more times with the same daphnia. In between the 10 second intervals as we were not actually observing, the light source was turned off in order to keep from heating up the slide and to keep evaporation to minimum. This test was carried out with three separate daphnia and each set of results was multiplied by three, so that the calculated number of beats per minute [BPM] could be estimated for each daphnia.
A quantity of daphnia were transferred to petri dish and floated in the water bath [35oC], and the same for the water bath 40oC, both were left for a period of time. The investigation was repeated as before, and the results recorded.
Also quantities of daphnia were transferred to a test tube, and this was placed in the pan of ice, and allowed to cool to 0oC. As before the investigation was repeated with the daphnia and the results recorded.
Table 1. Results for 0oC.
Table 2. Results for 18oC.
Table 3. Results for 35oC.
There are no results for 40oC as all the daphnia died.
Evaluation:
Conducting this experiment demonstrates that by using a fluctuation in temperatures to which the daphnia are in either slows the heart rate down or increases it. By comparing the heart rates of three daphnia in each temperature scale, you can tell whether temperature is a stimulant, depressant, or has no effect on the heart rate.
The physiologic mechanisms, by which the change in heart rate is brought about is by chemical reactions that occur in the cells of daphnia, which are dependant on certain enzymes, or proteins to help reactions proceed.
As we increased the temperature of the water, the metabolism increased as well, because chemical reactions occur faster at higher temperatures. This means that the heart rate will speed up in order to provide oxygen to the cell as the metabolism increases. This increase of metabolism and subsequently heart rates can be seen in the tables of results. By using different temperatures we have been able to link increased temperature with increased heart rate; and also we have a good range of results.
The slowest heart rate was at 0oC, the results were 189, 195 and 204 BPM. The results show that 0oC is a depressant factor in metabolism this is due to the fact that the temperature was low, and so slowing down the chemical reactions in the cells of the daphnia.
Table 3 the results for 35oC show the fastest heart rate; 396, 339 and 324 BPM were recorded. This relates to the prediction: higher temperature – faster the chemical reaction – faster the metabolism – faster the heart rate. The hypothesis was correct and a higher temperature is a stimulant in metabolism for daphnia.
What is evident in all of the tables of results in each 10 seconds there is an increase in heart rate. The cause of this could be the light from the microscope, it was turned off but a small amount of heat could have permeated the daphnia, and thus causing a slight increase in metabolism. Also another explanation could be due to the daphnia itself. In the life cycle of daphnia they have both sexual and asexual phases; some of the daphnia used in this investigation did have a brood of eggs in their brood pouches; these could be seen under the microscope. There was also a variation in sizes of the daphnia; typical daphnia live for 40 – 56 days, some of the daphnia used could have been at different stages in their life, plus males are typically smaller than females. Both of these factors from their sexual maturity or their growth could have affected the increases in metabolism.
Conclusion:
In this experiment the conclusion is temperature of the water that daphnia are situated in does have a predominant affect on their heart rate; and the results prove that this prediction is correct. In the results, the rate of the daphnia’s heartbeat increased steadily up until 40oC, when all the daphnia at this temperature died – due to the enzyme that controls the metabolism was destroyed by the heat. As the enzyme had been denatured by the heat, leaving the daphnia to return to room temperature had no effect, the enzyme had lost its tertiary shape and cannot work again.
What is evident from this investigation is that daphnia can withstand a wide range in temperatures in which they are situated in from 0oC to 35oC.
Overall I think the results are fairly accurate, but there will always be some inaccuracy in the results because of the ‘human factor’, such as a delay in observing the daphnia under the microscope due to the fact it was difficult to focus, causing a rise in temperature from the light, also the use of a clicker counter would have helped a lot in recording the heart beats. Plus also daphnia do not always respond and cooperate, as they should. Possibly if we had made sure that all the daphnia tested were all the same size, in good health and that none had any eggs in their brood pouch could have made the results more precise.
Other significant investigations into factors that could affect daphnia’s heart rate and their general behaviour would be interesting experiments to do. I would suggest using further variables such as solutions of caffeine or alcohol, or possibly as daphnia are fresh water organisms, saline solutions or changes in pH should also produce an interesting investigation.
This has been a comprehensive experiment that demonstrates the association between temperature and the rate of metabolism in daphnia.
