Abstract The physiological processes of many organisms are sensitive to temperature. In order to see this effect of temperature, we examined the heart rate of a Daphnia magna over a range of different temperatures. Being an ectothermic animal, the Daphnia's body temperature is dependent on water temperature. It was hypothesized that since most physiological processes are faster at higher temperatures, the Daphnia's heart rate will be faster at higher temperatures and slower at low temperatures. This was, in fact, true and a pattern was evident which showed that heart rate increased as temperature increased. The Q10 was high at higher temperatures which show elevated sensitivity at higher temperatures. Clearly, Daphnia have an optimal temperature range outside which they do not function to their full potential. A Daphnia's heart rate, then, was proved to be dependent on temperature.
Introduction Daphnia magna is a widespread freshwater zooplankton. Since Daphnia are ectothermic animals, their body temperature fluctuates with environmental temperature. Hence, these animals are ideal to study the effects of temperature. Most such animals function well at certain specific temperatures. They have an optimal temperature range, outside which they are unable to perform physiological processes effectively (Lamkemeyer et al. 2003). It is believed that most physiological processes take place more rapidly at higher temperatures and that changes in temperature can influence physiological rates (Ziarek et al. 2010). In order to investigate this, we questioned whether the heart rate of a Daphnia is different at different temperatures. Q10, which is the temperature sensitivity of a reaction, was a useful tool. We hypothesized that the Daphnia will have different heart rates at different temperatures and hence that temperature will affect heart rate. It was also hypothesized that Q10 will differ at different temperatures. This hypothesis was tested by exposing the Daphnia to different water temperatures, letting it equilibrate to the water temperature and counting its heart beat in a systematic way. Since most physiological processes increase at higher temperatures, we predicted that if the temperature is higher (close to 35Â°C) then the heart rate of the Daphnia will be faster and if the temperature is low (close to 5Â°C) then it would be slower. In addition, we predicted that Q10 will be higher at low temperatures and lower at high temperatures. In view of the fact that Daphnia had an optimal temperature range, it would be understandable if the Daphnia was more sensitive to temperatures outside this range and consequently reacted by altering its heart rate.
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Methods A Daphnia was placed on a small smear of Vaseline on the bottom of a culture dish (Olaveson and Rush 2011). Aged water at room temperature was added to the dish. Five minutes were allowed for the Daphnia to adjust to the water temperature and the temperature of the water was measured and recorded. Under a dissecting microscope, the Daphnia was placed and the 4X lens were used to locate the heart and count the heartbeats. The number of beats was counted over a 10 second period which was followed by a 10 second pause in counting and then 10 seconds of counting again. In order to get 9 measurements of the heart rate, this pattern was repeated for 3 minutes. Then, ice and water were mixed in a beaker to make a water mixture between 5Â°C to 10Â°C. To replace the tap water in the culture dish with chilled water, a Pasteur pipette was used. Five minutes were allowed for the Daphnia to reach equilibrium and then the heart beat was counted to obtain 9 measures of heart rate (heartbeats/ 10 seconds). The values were recorded. The temperature was then increased in 5Â° increments till 35Â°C and heart rate was measured at each point. Small amounts of the colder water were replaced with the warmer water (obtained from a water bath) till the desired temperature had been reached. Five minutes were always allowed for equilibration and using the same method, 9 measures of heart rate were recorded. The 9 estimates of heart rate taken at each temperature were used to find the average heart rate at each temperature. These values were entered into an excel document by all students and later used for analysis.
Results Statistical analysis and data processing shed light upon the effect of temperature on the heart rate of a Daphnia. The Statistical t test analysis proved that the Ho could be rejected for all the three tests proving that temperature does have a significant effect on the heart rate of a Daphnia. The Q10 as well as the average heart rates at different temperatures provided evidence that supported the hypothesis that temperature would affect Daphnia heart rate too.
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At the temperature interval of 4Â°C to 14Â°C, the Q10 was found to be 1.31 (Table 1). Although this was not the highest Q10 value and hence not the most sensitive temperature interval, a decrease in heart rate was evident at the lower temperature of 4Â°C compared to other higher temperatures (figure 1). The heart rate at 4Â°C was found to be 106.74 beats per minute where as the heart rate at 14Â°C was 140.10 beats per minute. The significant decrease in heart rate at 4Â°C compared to heart rate at the ambient temperature (24Â°C) was supported by the t test analysis (sample t statistic: 14.3938; critical t statistic:1.978; df:136; p = 0.05). The temperature interval from 14Â°C to 24Â°C showed increased sensitivity (Q10:1.40). This indicated the increase in heart rate at 24Â°C compared to lower temperatures (figure 1) and was supported by the t test analysis as the Ho (hypothesis that no change in heart rate would be evident) was rejected (t statistic: 8.6519; critical t statistic:1.978; df:136; p = 0.05). During the temperature interval from 24Â°C to 34Â°C, the highest Q10 was noted (table 1).
Figure 1. The average heart rate (beats per minute) of a Daphnia over range of temperatures (Â°C)
Table 1. Q10 values of Daphnia heart rates across different temperature intervals
Temperature Interval (Â°C)
(High and Ambient Temperatures)
(Ambient and Moderate Temperatures)
(Moderate and Low Temperature)
This sensitivity to high temperatures was obvious when heart rates at the two temperatures were compared (heart rate at 24Â°C: 196.32 beats/min; at 34Â°C: 277.92 beats/ min). The H0 was hence rejected (t statistic: 9.7792; critical t statistic: 1.978; df: 136; p = 0.05).
All the three tests provided evidence that suggested that temperature had an effect on the Daphnia's heart rate. At higher temperatures, the heart rate was faster and at lower temperatures, it was slower. Generally, as temperature increased so did the Daphnia's heart rate (figure 1).
Discussion All organisms have an optimum temperature range over which they function best. Consequently, at certain temperatures, the physiological processes of a Daphnia magna are at its utmost potential. Some hypothesized that Daphnia optimize their fitness by allocating the time spent in the different habitats depending on the temperature gradient (Kessler & Lampert, 2004). Hence, evidently temperature has an effect on the performance of a Daphnia. Specifically, as hypothesized, temperature affected the heart rate of Daphnia. It was noticed that the Daphnia's heart rate increased at higher temperatures (close to 34Â°C) and decreased at lower temperatures (close to 5Â°C). Since Daphnia are ectothermic, their body temperature varies with environmental temperature. Since Daphnia cannot thermoregulate, their body temperature experiences variance following ï¬‚uctuations in the environment (Ziarek et al. 2010). Hence as the temperature of the water increased, so did the Daphnia's heart rate. The data supported this prediction. This is simply because most physiological processes take place more rapidly at higher temperatures. In addition, research has shown that increases of heart rate by significant values were measured in D. Magna as a function of temperature (Paul et al. 2004). One reason why the heart rate of Daphnia increases with temperature would be because less oxygen is present in the warmer water. Consequently, lack of oxygen could result in insufficient amount of oxygenated blood and hence the heart would have to work harder to pump blood around the body. For this purpose, it makes sense that the heart rate would increase. Very little research has been done about oxygen levels at different temperatures and Daphnia heart rates and perhaps more such research will provide more clarity on this topic. The little research that has been done, though, suggests that a reduced aerobic scope allows only time-limited survival at temperatures outside the optimal range (Lamkemeyer et al. 2003) in organism such as Daphnias. Other research has also shown that high temperatures increase a Daphnia's metabolic rates by increasing their heartbeat rates (MacArthur & Baittie, 1929), and consequently their oxygen demands (Ziarek et al. 2010).
It is also important to note that although it was predicted that Q10 would be higher at low temperatures and lower at high temperatures, the data did not support this prediction. The opposite, in fact, was evident. At higher temperatures the Q10 was higher and at lower temperatures it was lower. This could be possible merely because the Daphnia was more sensitive to changes in temperature at higher temperatures. It could also be possible that Q10 was higher at higher temperatures because of other errors. For instance, when the ice was added to the water in order to obtain low water temperatures (5Â°C), it was difficult to obtain the exact temperatures. Although the temperature obtained (about 8Â°C) was lower than room temperature (or ambient temperature), it is possible that it wasn't low enough to trigger major physiological changes. A significant fact to keep in mind is that since temperatures are increasing due to global warming, water temperatures are going to increase as well. This could lead to problems for organisms such as Daphnia which cannot function well outside of their optimal temperature range. It has been suggested that rising temperatures associated with global warming present a challenge to the fate of many aquatic organisms (Doorslaer et al. 2009)
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In conclusion, Daphnia heart rate is affected by temperature and tends to increase at high temperatures and decrease at low temperatures.