Abstract
The aim of this experiment was to determine whether there is any response exhibited by planarian Dugesia sp. when presented with different levels of gravity. In addition, the study also sought to identify the response of the planarian Dugesia sp. when presented with lighted and non-lighted conditions. The alternate hypotheses were that planarian Dugesia sp. shows response to gravity and that planarian Dugesia sp. shows response to light. The Chi-Square statistics for both tests were significant (p <.001) and therefore the null hypotheses, the planarian Dugesia shows no response to gravity and that planarian Dugesia sp. shows no response to light, were rejected and the alternative hypothesis supported.
Introduction
Orientation behaviors (tropisms) are specific movements of the entire body so that an animal alters its position in space with respect to external stimulus (Barnard, 2004). Two types of behavioral responses which maintain animals in proper orientation to fundamental stimuli in their environment are recognized. These responses are
- kineses, i.e. undirected movements of the body in response to a stimulus, and
- taxes, i.e. directed movements of the body toward or away from a stimulus source (Barnard, 2004).
A behavioral response is positive if the animal moves towards the stimulus and negative if it moves away from the stimulus. Furthermore, responses are commonly named for the type of stimulus which evokes them. Thus, a phototaxis is a response to light, and a geotaxis is a response to gravity etc.
Planarian Dugesia sp. is an invertebrate belonging to the phylum platyhelminthes (Institute of Laboratory Animals (U.S.), 1966). Planarians are known to exhibit negative phototaxis and therefore it is common to find them move into directions where there is less light (Tsonis, 2008). Planarians exhibit positive geotaxis in a test tube which has water of constant light and temperature. Precisely, the planarians settle at the bottom of the test tube (Mandal, 2010).
The aim of this experiment is to determine whether there is any response exhibited by planarian Dugesia sp. when presented with different levels of gravity. In addition, the study also seeks to identify the response of the planarian Dugesia sp. when presented with lighted and non-lighted conditions. The null hypotheses are that planarian Dugesia sp. shows no response to gravity and planarian Dugesia sp. shows no response to light. Alternatively, it is hypothesized that planarian Dugesia sp. shows response to gravity and that planarian Dugesia sp. shows response to light.
Materials and Methods
Response to gravity
After marking the mid-point of the test tube, “conditioned” water i.e. water that had been standing in an open container for 24 hours to allow escape of chlorine was placed in the test tube but enough room was left for the stopper. A live planarian (Dugesia sp.) was then placed in the middle of a test tube using a pipette and a stopper was placed on one side of the test tube. With the stoppered end facing upwards, the test tube was placed in a holder in a position that allowed even illumination of the test tube.
A recording of whether the planarian was located in the upper or lower half of the test tube was made after 10 minutes. This procedure was repeated using a different planarian (Dugesia sp.) to allow for collection of independent observations. This time, the test tube’s position in the holder was reversed, with the stoppered end facing down but still ensuring that there was even illumination of the test tube. This was supposed to act as a control of the fact that only one end of the test tube was stoppered. The location of the planarian in the test tube was again recorded after 10 minutes.
Response to light
After marking the mid-point of the test tube, “conditioned” water i.e. water that had been standing in an open container for 24 hours to allow escape of chlorine was placed in the test tube but enough room was left for the stopper. To darken half of the test tube, an aluminum foil cap was moulded over the rounded end of the test tube. A planarian (Dugesia sp.) was then placed gently, using a pipette, in the middle of the test tube and a stopper was placed and the test tube placed horizontally on a paper towel on a bench.
Ensuring even illumination of the test tube, a source of light was placed 20cm away from the test tube and then a recording on the position (illuminated or darkened half of the test tube) of the planarian was made after 10 minutes. A different planarian (Dugesia sp.) was then used, after removing the previous one, and subjected to the same experimental conditions to facilitate collection of independent findings. However with the second planarian, the alfoil cap was placed over the half of the test tube with the stopper to control for the fact that only one side was illuminated.
Results
Response to gravity
At 1 degree of freedom, the Chi-Square value for response to gravity is 22.23 and this corresponds to a probability less than the critical value of.001. As such, the Chi-Square test is significant (Chi Square X2 = 22.23, df = 1, p <.001) (Appendix 1).
Response to light
The Chi Square value 38.34 for response to light is significant since the value corresponds to a probability less than.001 (Chi Square X2 = 38.34, df = 1, p <.001) (Appendix 2).
Discussion
Planarian Dugesia sp. showed response to both gravity and light. While planarian Dugesia sp. demonstrated negative phototaxis, they demonstrated positive geotaxis. The null hypotheses, the planarian Dugesia sp. shows no response to gravity and planarian Dugesia sp. shows no response to light, were therefore rejected. Instead, the hypotheses that planarian Dugesia sp. shows response to gravity and Dugesia sp. shows response to light, were supported.
When planarians are exposed to light, their activity tends to increase and this makes the resting planarians to start moving. In addition, already moving planarians usually increase their motion once presented with light. In essence, the planarians finally rest in the region of the container which is less illuminated (Khanna, 2004). Khanna (2004) explains that planarians are phobotactic and they avoid light by random movements, tending to react negatively when they are in more lighted areas and they fail to react once they come into a less lighted area. Even when planarians have their eyes removed, they still tend to avoid lighted areas in their environment and instead settle in darker parts. This response is exhibited even in decapitated planarians, only that the rate of movement slows down (Khanna, 2004).
Planaria exhibit positive geotaxis in a test tube which has water of constant light and temperature. In essence, the planaria settles at the bottom of the test tube (Mandal, 2010). The findings of this study are therefore in tandem with previous findings on response of planarians to gravity. According to Khanna (2004), there is tendency of planarians, when they are unfed, to settle at the bottom of a vessel (e.g. a test tube) if the water has enough oxygen. However, fed planarians seek the surface of the vessel in search of more oxygen for intracellular digestion. Since it is not identified whether the planarians were fed or unfed, it may be possible that the positive geotaxis exhibited by the planarians in this experiment may have been due to planarians probably being unfed.
It is important for further studies to focus more on how planarians respond to varying light intensity, as this would enhance the understanding of phototaxism in these platyhelminthes. Moreover, future studies need to control for confounding factors such as planarians being fed (or unfed) to provide more valid conclusions on geotaxis of the organisms.
Acknowledgements
I wish to extend my sincere gratitude to my laboratory partner for offering a helping hand in this experiment. I also acknowledge the input of my friend who proofread the report and also gave insights in statistical analyses.
References
Barnard, C. J. (2004). Animal Behaviour: Mechanism, development, function and evolution. Harlow: Pearson Education.
Khanna, D. R. (2004). Biology of platyhelminthes. Discovery Publishing House.
Institute of Laboratory Animal Resources (U.S.). (1966). Laboratory animals, Part 2. Revised edition. Washington, DC: National Academies.
Mandal, F. B. (2010). Textbook of animal behavior. New Delhi: PHI Learning Private Limited.
Tsonis, P. A. (2008). Animal models in eye research. San Diego, CA: Elsevier.
Appendix
Appendix 1. Calculation of Chi-Square to tests whether planarian Dugesia sp. shows response to gravity.
Null hypothesis (H0)
Planarian Dugesia sp. shows no response to gravity.
Alternative hypothesis (H1)
Planarian Dugesia sp. shows response to gravity.
Chi Square X2 formula for independent class (test tube half):
(O – E)2 /E
Where, O = observed frequency (observed number of planarians in the test tube halves)
E = expected frequency (expected number of planarians in test tube halves if null hypothesis is true)
Calculation of Chi-Square (X2) value
X2 = (9 – 26)2/26 + (43 – 26)2 /26 = 22.23
The Chi Square value (22.23) corresponds to a probability less than.001 (Chi Square X2 = 22.23, df = 1, p <.001) hence the null hypothesis, planarian Dugesia sp. shows no response to gravity, was rejected.
The alternative hypothesis, planarian Dugesia sp. shows response to gravity, was supported.
Appendix 2. Calculation of Chi-Square to tests whether planarian Dugesia sp. shows response to light.
Null hypothesis (H0)
Planarian Dugesia sp. shows no response to light.
Alternative hypothesis (H1)
Planarian Dugesia sp. shows response to light.
Chi Square X2 formula for independent class (test tube half):
(O – E)2/E
Where, O = observed frequency (observed number of planarians in the test tube halves)
E = expected frequency (expected number of planarians in test tube halves if null hypothesis is true)
Calculation of Chi-Square (X2) value:
X2 = (2 – 23)2/23 + (44 – 23)2 /23 = 38.34
Planarian Dugesia sp. showed response to light.
The Chi Square value 38.34 for response to light is significant since the value corresponds to a probability less than.001 (Chi Square X2 = 38.34, df = 1, p <.001).
The null hypothesis, planarian Dugesia sp. shows no response to light, was rejected.
The alternative hypothesis, planarian Dugesia sp. shows response to light, was supported.